tag:theconversation.com,2011:/us/topics/seismic-waves-16374/articlesSeismic waves – The Conversation2023-12-26T08:48:13Ztag:theconversation.com,2011:article/2165482023-12-26T08:48:13Z2023-12-26T08:48:13ZUnusual ancient elephant tracks had our team of fossil experts stumped – how we solved the mystery<figure><img src="https://images.theconversation.com/files/559289/original/file-20231114-17-wm62rc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Elephants communicate underground by generating seismic waves. </span> <span class="attribution"><span class="source">Anadolu Agency</span></span></figcaption></figure><p>Over the past 15 years, through our scientific study of tracks and traces, we have identified more than 350 <a href="https://doi.org/10.1016/j.quascirev.2019.07.039">fossil vertebrate tracksites</a> from South Africa’s Cape south coast. Most are found in cemented sand dunes, called aeolianites, and all are from the <a href="https://www.britannica.com/science/Pleistocene-Epoch">Pleistocene Epoch</a>, ranging in age from about 35,000 to 400,000 years. </p>
<p>During that time we have honed our identification skills and have become used to finding and interpreting tracksites – a field called ichnology. And yet, every once in a while, we encounter something we immediately realise is so novel that it has been found nowhere else on Earth.</p>
<p>Such a moment of unexpected discovery happened in 2019 along the coastline of the De Hoop Nature Reserve, about 200km east of Cape Town. Less than two metres away from a cluster of fossil elephant tracks was a round feature, 57cm in diameter, containing concentric ring features. Another layer was exposed about 7cm below this surface. It contained at least 14 parallel groove features. Where the grooves approached the rings, they made a slight curve towards them. The two findings, we hypothesised, were connected with each other and appeared to have a common origin.</p>
<p>Elephants are the largest, heaviest land animals. They leave large, deep, easily recognisable tracks. We’ve documented 35 fossilised elephant track sites in our study area, as well as the <a href="https://doi.org/10.1017/qua.2021.32">first evidence</a> of fossilised elephant trunk-drag impressions. </p>
<p>Elephants, like another group of massive land creatures, dinosaurs, can be viewed as geological engineers that create minor earth-moving forces on the ground they walk(ed) on. This can be related also to a remarkable ability that elephants possess: communicating by generating seismic waves. These are a form of energy that can travel under the surface of the Earth.</p>
<p>The feature we found in 2019 seemed to reflect just such a phenomenon: an elephant triggering waves that rippled outwards. After additional investigation and a thorough search for alternative explanations, we could report in a <a href="https://www.sciencedirect.com/science/article/pii/S0016787823000792">recently published study</a> that we believe we’ve found the world’s first trace fossil signature of seismic, underground communication between elephants. </p>
<h2>Elephant seismicity</h2>
<p>Since the 1980s, an ever-increasing body of literature has documented “elephant seismicity” and <a href="https://doi.org/10.1007/BF00300007">seismic communication through infrasound</a>. The lower threshold of human hearing is 20Hz; below that, low frequency sounds are known as infrasound. Elephant “rumbles”, originating in the larynx and transmitted into the ground through the limbs, fall within the infrasonic range. </p>
<p>Infrasound at high amplitude (it would seem very loud to us if at a slightly higher frequency) can travel further than high frequency sounds, <a href="https://doi.org/10.1016/j.cub.2018.03.062">over distances as great as 6km</a>. Elephants have an advantage here. Lighter creatures cannot <a href="https://doi.org/10.1152/physiol.00008.2007">generate low-frequency sound waves through vocalisation</a>. It is thought that long-distance seismic communication can allow elephant groups to interact over substantial distances, and it has <a href="https://doi.org/10.1016/j.cub.2018.03.062">been shown</a> that sandy terrain allows the communication to travel furthest.</p>
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Read more:
<a href="https://theconversation.com/fossil-tracks-and-trunk-marks-reveal-signs-of-ancient-elephants-on-south-africas-coast-164306">Fossil tracks and trunk marks reveal signs of ancient elephants on South Africa's coast</a>
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<p>Continuing the elephant-dinosaur analogy, we considered the multitude of publications on dinosaur tracks. We are aware of only a single example that exhibits possible concentric rings within a track, from Korea, and none that involve parallel grooves. This suggests something unique about elephants that generates concentric rings within tracks and leads to the associated groove features. Elephant rumbling provides a plausible explanation.</p>
<p>In our scenario at De Hoop Nature Reserve, we postulate that vibrations from rumbling travelled down the elephant limb and created the concentric ring features. They are reminiscent of some of the patterns that become evident when <a href="https://www.youtube.com/watch?v=tFAcYruShow">sprinkling sand onto a vibrating surface</a>. The surface on which the concentric rings appear must have been just below the dune surface at the time. The parallel grooves would then represent a trace fossil signature of subsurface communication. We’re not yet sure how old the trace fossil is; we’ve sent samples for testing.</p>
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<figcaption><span class="caption">A video showing sand vibrating when it’s exposed to sound.</span></figcaption>
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<h2>Rumblings in rock art</h2>
<p>Elephant seismicity is a relatively new field of study for scientists. However, those who have lived close to elephants won’t be surprised at the idea of the animals communicating through vibration. Indeed, vibrations from elephant rumblings can sometimes be felt (rather than heard) by the astute observer. And it appears that this knowledge is not just recent. </p>
<p>The rock art experts on our team have identified and interpreted rock art that suggests the indigenous San people appreciated and celebrated this knowledge in southern Africa thousands of years ago. Elephants were of profound importance to the San and were prominently featured in <a href="https://www.archaeology.org.za/sites/default/files/attachments/publications/2019/09/02/ds_2018_december.pdf">their works of art</a>. Several rock art sites appear to contain paintings of elephants in relation to sound or vibration.</p>
<p>For example, at the Monte Cristo site in the Cederberg the artist has painted 31 elephants, in several groups. They are in a realistic arrangement. Fine red lines surround each elephant; zigzag lines touch the abdomen, groin, throat, trunk, and specifically the feet. Many zigzag lines link the elephant to the ground. The finest lines are closest to the elephants, and every elephant is connected to this set of lines. These are in turn connected to broader lines surrounding the elephant group, which radiate out and away from the elephants as concentric rings. </p>
<p>This is interpreted as the San artist’s probable illustration of seismic communication between elephants. The feeling of shaking and vibration, which the San call <em>thara n|om</em>, is vital to the San healing dances, including the <a href="https://www.archaeology.org.za/sites/default/files/attachments/publications/2019/09/02/ds_2018_december.pdf">elephant song and elephant dance</a>. Lines of energy, called <em>n|om</em>, are regarded as a vibrant life-giving force that animates all living beings and is the source of <a href="https://www.spiritualityandpractice.com/book-reviews/view/28011/way-of-the-bushman">all inspired energy</a>.</p>
<p>We believe that an understanding of elephant seismicity requires the integration of three bodies of knowledge: research on extant elephant populations, ancestral knowledge (often manifested in rock art) and the trace fossil record. </p>
<p>That elephant seismic communication might leave a trace fossil record has never been reported before, or even postulated. Our findings may have the potential to stimulate multi-disciplinary research into this field. This could include a dedicated search for sub-surface patterns in the sand in the vicinity of modern rumbling elephants.</p><img src="https://counter.theconversation.com/content/216548/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Charles Helm does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Elephants can be viewed as geological engineers that create minor tectonic forces on the substrate they walk on.Charles Helm, Research Associate, African Centre for Coastal Palaeoscience, Nelson Mandela UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2020892023-04-04T12:17:54Z2023-04-04T12:17:54ZBuildings left standing in Turkey offer design guidance for future earthquake-resilient construction<figure><img src="https://images.theconversation.com/files/518309/original/file-20230329-28-3skd0d.jpg?ixlib=rb-1.1.0&rect=11%2C8%2C1905%2C1069&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Turkey's Adana Hospital survived February 2023 earthquakes with no damage because of its seismic isolation system. </span> <span class="attribution"><span class="source">Earthquake Protection Systems, Inc.</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>The Feb. 6, 2023, <a href="https://www.worldbank.org/en/news/press-release/2023/02/27/earthquake-damage-in-turkiye-estimated-to-exceed-34-billion-world-bank-disaster-assessment-report">earthquakes in Turkey and Syria</a> damaged over 100,000 buildings, caused more than 10,000 collapses and killed more than 50,000 people. These earthquakes also put to the test advanced building technologies that can minimize damage and keep buildings functioning after a quake.</p>
<p>Several hospitals built with <a href="https://www.hurriyetdailynews.com/seismic-isolation-devices-prevent-damage-in-four-hospitals-180830">one such technology</a> – called a seismic isolation system – <a href="https://localnewsmatters.org/2023/02/13/vallejo-companys-tech-keeps-turkey-hospital-operational-after-devastating-earthquakes/">survived the earthquakes</a> with almost no harm, according to local news reports, even while surrounding buildings sustained heavy damage. </p>
<p>Adana City Hospital was built to record both ground shaking and the building’s response. Thanks to its seismic isolation system, the building saw a 75% <a href="https://www.earthquakeprotection.com/">reduction in shaking</a>, according to the company that designed the isolation system, compared with neighboring structures. This system allowed the building to stay up and running after the earthquake.</p>
<p>Engineers aren’t surprised that the hospitals with seismic isolation systems survived with minimal damage, but through <a href="https://scholar.google.com/citations?user=VdoAeqAAAAAJ&hl=en">my work as a civil engineer</a>, I’ve been hearing people in Turkey and abroad ask why more buildings don’t use these smarter engineering technologies.</p>
<p>A year after the 1999 İzmit earthquake in Turkey killed over 17,000 people, I moved to Istanbul for a bachelor’s in civil engineering. I moved to the U.S. for my graduate studies in 2005, and since then, I have been working on advanced technologies and materials that can ensure rapid recovery and reoccupation of buildings <a href="https://engineering.virginia.edu/rail">after a strong earthquake</a>.</p>
<p>Although we’ve seen the effectiveness of seismic protection technologies during past major earthquakes, these technologies have been installed in only a tiny fraction of the places where they could potentially be useful.</p>
<h2>Earthquake-resilient building technology</h2>
<p>Engineers can control how structures respond to earthquakes in several ways.</p>
<p><a href="https://permanent.fdlp.gov/gpo15358/fema_p_749.pdf">Traditional approaches</a> rely on having certain components of the building, like columns or beams, absorb the earthquake’s energy. However, this method can lead to damage accumulating in these structural features that <a href="https://www.enr.com/articles/3447-engineers-surprised-by-damage-to-modern-buildings-in-christchurch">may render the building uninhabitable</a>.</p>
<p><a href="https://www.sciencelearn.org.nz/resources/1022-base-isolation-and-seismic-dampers">Earthquake-resilient systems</a> such as seismic isolation devices and seismic dampers minimize the seismic energy that goes into these columns or beams by either absorbing it or diverting it. As a result, the building experiences less motion and damage and is more likely to <a href="https://doi.org/10.3389/fbuil.2020.00126">remain functional</a> after an earthquake.</p>
<p><a href="https://www.hurriyetdailynews.com/seismic-isolation-devices-prevent-damage-in-four-hospitals-180830">Seismic isolation systems</a> prevent seismic energy from entering buildings in the first place by using devices made from rubber or steel plates coated with a friction-generating material that slide over one another to minimize an earthquake’s impact. These isolation devices are installed between the building’s foundation and the building itself. Alternatively, seismic dampers, installed in each story of a building, absorb earthquake energy the way shock absorbers work in a car and convert it into heat energy to <a href="https://buildcivil.wordpress.com/2013/11/25/passive-energy-dissipation-devices/">minimize damage</a>. </p>
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<a href="https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An illustration showing two side-by-side structures, the left with arrows denoting side-to-side motion. The right has small blocks at the building's foundation which absorb seismic energy and prevent motion." src="https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=391&fit=crop&dpr=1 600w, https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=391&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=391&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=491&fit=crop&dpr=1 754w, https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=491&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/518267/original/file-20230329-2631-bzokl7.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=491&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The left shows a building without seismic isolation, while the right image shows a building with a seismic isolation system, which minimizes how much damage the building sustains during an earthquake. The red lines denote how much motion the building could experience during an earthquake.</span>
<span class="attribution"><span class="source">Ozbulut Lab</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p>Both seismic isolation systems and seismic dampers can help a building achieve “<a href="https://www.nibs.org/blog/resilience-2021-importance-seismic-functional-recovery-and-community-resilience-built">functional recovery</a>” – a performance objective whereby buildings are constructed to prevent damage and enable reoccupancy. Designing such buildings will not only save people and buildings but also keep the earthquakes from collapsing communities and economies. </p>
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<p>While functional recovery is an emerging idea for building earthquake-resilient structures, global modern building codes stipulate that, at a minimum, structures must have measures in place to keep the building from collapsing – called the <a href="https://www.fema.gov/node/seismic-building-code-provisions-new-buildings-create-safer-communities">life safety objective</a>. Buildings following a life safety objective are engineered to sustain damage in a controlled way, to keep the building standing and protect those inside.</p>
<p>While these buildings likely won’t collapse, they may not be safe to use after a quake. While this is not the same as functional recovery, if more buildings had been built to a life safety threshold in Turkey and Syria, thousands of lives could have been saved.</p>
<h2>The case in Turkey</h2>
<p>Much of the damage in Turkey occurred in nonductile concrete buildings constructed under a pre-1998 Turkish building code. Ductile concrete building elements, required by newer building codes, are more flexible, thanks to steel reinforcing bars at critical locations. They can <a href="https://www.concreteconstruction.net/how-to/construction/earthquakes-and-reinforced-concrete_o">accommodate the building motions</a> induced by earthquakes. The older nonductile buildings also tended to have poorly arranged steel reinforcements, leaving them vulnerable to the sudden collapse of building columns.</p>
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<figcaption><span class="caption">This video, from The Associated Press, shows some of the buildings that collapsed in the aftermath of the Turkey earthquakes.</span></figcaption>
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<p>Similarly, many so-called soft-story buildings were damaged during these earthquakes. A soft story is a level that is significantly more vulnerable to lateral earthquake forces than the other stories in a multistory building. The first floor of these buildings – commonly used for commercial purposes like retail, garage or office space – tend to have more open areas and fewer structural components, like beams and columns, making them vulnerable to collapse.</p>
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<span class="caption">An example of a soft-story building, where the first story collapsed, leaving the rest of the floors relatively stable.</span>
<span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/TurkeySyriaEarthquake/48bbd617383140649e53dcb6fa509f79/photo?Query=turkey%20earthquake%20building%20collapse&mediaType=photo&sortBy=&dateRange=Anytime&totalCount=1042&currentItemNo=261">AP Photo/Emrah Gurel</a></span>
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<p>These types of buildings are found all over the world, including in highly populated, seismically at-risk areas like <a href="https://www.voanews.com/a/turkey-s-deadly-quake-renews-alarm-for-istanbul-/6968544.html#:%7E:text=Seismologists%20warn%20that%20a%20massive,to%2020%20million%20%E2%80%94%20by%202030.">Istanbul</a>, <a href="https://www.sfchronicle.com/sf/article/earthquake-building-risk-safety-17782287.php">San Francisco</a>, Los Angeles and Vancouver — all located near active fault lines.</p>
<p>Buildings designed under old codes can be strengthened to meet a life safety performance threshold. However, these upgrades can cost lots of money, and enforcing these upgrades, especially for private buildings, requires well-planned policies.</p>
<h2>Learning lessons</h2>
<p>While buildings designed for a life safety objective can protect thousands of lives, the February 2011 Christchurch earthquake in New Zealand revealed the limitations of modern seismic codes centered solely on this design goal. The damage to buildings designed under a life safety goal was so extensive that thousands had to be <a href="https://www.nist.gov/news-events/news/2021/01/new-report-charts-path-toward-superior-earthquake-recovery">demolished after the quake</a>. </p>
<p>It was this earthquake that led engineers to focus on “functional recovery” and to implement seismic protective technologies more widely. The <a href="https://thehill.com/opinion/energy-environment/504572-smarter-engineering-could-help-recovery-from-major-disasters/">additional cost</a> of such <a href="https://www.nytimes.com/1988/10/30/realestate/base-isolation-taking-the-shock-out-of-quakes.html">seismic protection technologies</a> is typically <a href="https://www.usrc.org/usrc-media-portfolio/#Papers">less than 5%</a> of the initial construction costs and pales in comparison to the cost of the social and economic disruptions caused by a major earthquake. In addition, securing lower insurance premiums may recoup most of these initial costs.</p>
<p>Total economic losses after the Christchurch earthquake was estimated at <a href="https://www.bloomberg.com/news/articles/2013-04-28/christchurch-quake-rebuild-soars-33-to-nz-40-billion-key-says#xj4y7vzkg">US$32 billion</a>, not accounting for inflation, of which $24 billion was construction costs. The cost of the recent earthquakes in Turkey is estimated to be more than <a href="https://www.reuters.com/world/middle-east/earthquake-could-cost-turkey-up-84-bln-business-group-2023-02-13/">$84 billion</a> and still counting.</p>
<p>The earthquakes in Turkey have shown that seismic protection technologies work. To avoid high economic and social consequences, local authorities can update the provisions and codes for designing new buildings to enable post-earthquake reoccupancy and functional recovery. Additionally, policies, financial incentives and tax benefits that promote enhanced building design could improve seismic safety on a larger scale.</p><img src="https://counter.theconversation.com/content/202089/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Osman Ozbulut receives funding from NSF. </span></em></p>February earthquakes wreaked havoc across Turkey and Syria, killing tens of thousands of people. An engineer originally from Turkey describes what kept some buildings functional while others collapsed.Osman Ozbulut, Associate Professor of Civil Engineering, University of VirginiaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2002582023-02-21T19:04:05Z2023-02-21T19:04:05ZIn a new study, we’ve observed clues that distinguish the very deepest part of Earth’s core<figure><img src="https://images.theconversation.com/files/511009/original/file-20230220-2192-e5uwge.png?ixlib=rb-1.1.0&rect=263%2C17%2C3544%2C1952&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Rost9/Shutterstock</span></span></figcaption></figure><p>Not so long ago, Earth’s interior was thought to be made up of four layers: the crust, mantle, (liquid) outer core and (solid) inner core.</p>
<p>In a new study <a href="https://doi.org/10.1038/s41467-023-36074-2">published today in Nature Communications</a>, we provide further evidence for the existence of an “innermost inner core” – a distinct internal metallic ball embedded in the inner core like the most petite Russian nesting doll. </p>
<p>Studying Earth’s centre is not just a topic of academic curiosity, but something that sheds light on the very evolution of life on our planet’s surface. </p>
<p>This is because the inner core grows outwards by solidifying materials from the liquid outer core. As these materials solidify, heat is released and causes upward movement in the liquid layer – what’s known as a convection current. In turn, this convection generates our planet’s geomagnetic field.</p>
<p>The magnetic field protects life on Earth from <a href="https://climate.nasa.gov/news/3105/earths-magnetosphere-protecting-our-planet-from-harmful-space-energy/">harmful cosmic radiation</a>. Without the shield it provides, life on Earth would not be possible in the form we know today. </p>
<p>So, understanding the evolutionary history of our planet’s inner core and its connection with the geomagnetic field is relevant to understanding the timeline of life’s evolution on Earth’s surface.</p>
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<a href="https://theconversation.com/what-makes-one-earth-like-planet-more-habitable-than-another-33479">What makes one Earth-like planet more habitable than another?</a>
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<h2>Studying the insides of the planet</h2>
<p>Like radiologists imaging a patient’s internal organs, seismologists use seismic waves from large earthquakes to study the deep interior of Earth. Earthquakes are our sources, and seismometers recording ground motions or vibrations that move through Earth are our receivers.</p>
<p>However, unlike medical imaging, we do not have the luxury of having sources and receivers equally distributed around the body. Large earthquakes useful for our probes are confined near tectonic margins, such as <a href="https://theconversation.com/five-active-volcanoes-on-my-asia-pacific-ring-of-fire-watch-list-right-now-90618">the Ring of Fire</a> surrounding the Pacific. Meanwhile, seismometers mainly exist on land. </p>
<p>Furthermore, the inner core, which is <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1365-246X.1995.tb03540.x/abstract">one-fifth of Earth’s radius</a>, accounts for less than 1% of Earth’s volume. To target this relatively small volume in the planet’s centre, seismometers often need to be positioned on the opposite side of the globe, the so-called antipode of an earthquake.</p>
<p>But that’s unlikely in practice because the antipodes of active earthquake zones are often in the ocean, where seismometers are expensive to install. </p>
<p>With the limited data we do have from such antipode measurements, an internal metallic ball within the inner core – the innermost inner core – <a href="http://www.pnas.org/content/99/22/14026">was hypothesised</a> about 20 years ago, with an estimated radius of about 300km.</p>
<p>Several <a href="http://www.sciencedirect.com/science/article/pii/S0031920118302395">lines of evidence</a> have confirmed its <a href="https://www.science.org/doi/10.1126/science.1078159">existence</a>, including <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JB020545">recent</a> <a href="https://onlinelibrary.wiley.com/doi/abs/10.1029/2021JB023540">studies</a> from our research group. </p>
<h2>Bouncing seismic waves</h2>
<p>Now, for the first time, we report observations of seismic waves originating from powerful earthquakes travelling back and forth from one side of the globe to the other up to five times like a ricochet. These new observations are exciting because they provide new probes from different angles of the centremost part of our planet. </p>
<p>A critical advantage of our study was getting data from dense continental-scale networks (consisting of several hundred seismometers) installed around some of the largest quakes.</p>
<p>It differs from previous studies because it uses seismic waves that bounce multiple times within Earth, along its diameter and through its centre. By capturing them, we obtain an unparalleled sampling of the innermost inner core.</p>
<h2>A ball in the centre</h2>
<p>The potential difference between the innermost metallic ball and the outer shell of the inner core is not in its chemical composition, like with some other Earth layers. Both are likely made of an iron-nickel alloy with small amounts of lighter chemical elements.</p>
<p>Additionally, the transition from the innermost (solid) ball to the outer shell of the inner core (also solid) seems gradual rather than sharp. That is why we can’t observe it via direct reflections of seismic waves from it. This differs from previous studies documenting sharp boundaries between the other layers inside Earth – from crust to mantle, for example.</p>
<p>So, what precisely did we observe that gives us clues about this innermost inner core?</p>
<p>The observed difference is in anisotropy – a material’s property to let (or propagate) seismic waves faster or slower through it depending on the direction in which they travel.</p>
<p>Different speeds could be caused by different arrangements of iron atoms at high temperatures and pressures, or by the arrangements of atoms when crystals grow.</p>
<p>There is <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JB020545">strong evidence</a> that the outer shell of the inner core is anisotropic. The slowest direction of seismic waves is in the equatorial plane (and the fastest is parallel to Earth’s spin axis).</p>
<p>By contrast, in the innermost part of the inner core – as our study of the ricochet waves shows – the slowest direction of propagation forms an oblique angle with the equatorial plane. This is critical, and this is why we can say we’ve detected “distinct” anisotropy in the innermost inner core. </p>
<p>Excitingly, while shallow structures within Earth’s crust and upper mantle are being mapped in incredible detail, we are still at the discovery stage regarding its deepest structures.</p>
<p>However, the image of Earth’s deep interior is getting sharper with the expansion of the dense continental networks, advanced data analysing techniques, and computational capacities.</p>
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Read more:
<a href="https://theconversation.com/just-add-mantle-water-new-research-cracks-the-mystery-of-how-the-first-continents-formed-156845">Just add (mantle) water: new research cracks the mystery of how the first continents formed</a>
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<img src="https://counter.theconversation.com/content/200258/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Hrvoje Tkalčić receives funding from The Australian Research Council. </span></em></p><p class="fine-print"><em><span>Thanh-Son Pham does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Earth doesn’t just have an inner core. It also has an innermost inner core, a solid ball within the solid ball in the very middle of the planet.Thanh-Son Pham, Postdoctoral Fellow in Geophysics, Australian National UniversityHrvoje Tkalčić, Professor, Head of Geophysics, Director of Warramunga Array, Australian National UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1942972022-11-18T16:46:55Z2022-11-18T16:46:55ZMars: how we discovered two huge, unusual impact craters – and the secrets they unveil<figure><img src="https://images.theconversation.com/files/494464/original/file-20221109-24-4801be.png?ixlib=rb-1.1.0&rect=0%2C9%2C1024%2C1013&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">InSight's dusty solar panel.</span> <span class="attribution"><span class="source">NASA/JPL</span></span></figcaption></figure><p>Most of the worlds of our Solar System are pockmarked with impact craters. These bear testament to the violence of the early days of the Sun, when asteroids, comets and entire planets routinely collided with and annihilated each other. </p>
<p>Our own Moon was most likely formed by one of these collisions, and is itself home to the largest impact feature in the Solar System – the <a href="https://www.nasa.gov/mission_pages/LRO/multimedia/lro-20100709-basin.html">South Pole/Aitken Basin</a>, some 2,500km across. Mars’ vast, flat northern deserts may too have formed during a gigantic collision some 4 billion years ago. </p>
<p>Today’s Solar System is a much more peaceful place. But impacts from meteorites are still one of the dominant processes shaping planetary landscapes on most worlds other than the Earth. Now our new study of the largest recent impact craters on Mars, <a href="https://www.science.org/doi/10.1126/science.abq7704">published in Science</a>, sheds new light on the red planet’s interior.</p>
<p>Examining impact craters can teach us a lot – from understanding the composition and size of the asteroids or comets which created them, through to unearthing the properties of planetary surfaces and interiors. The interiors of craters can in fact be used to study otherwise inaccessible underground geology. The degree of cratering on a surface can also be used to estimate its age: the older it is, the more craters (usually). </p>
<p>Late last year, Nasa’s InSight spacecraft, which is <a href="https://theconversation.com/mars-insight-here-is-whats-next-after-the-tricky-landing-107749">on the surface of Mars</a> “listening” to seismic waves in the planet’s interior, detected two enormous “<a href="https://theconversation.com/mars-quakes-the-insight-lander-shows-active-faults-in-the-planets-crust-132315">marsquakes</a>” around 90 days apart – among the largest we have seen so far during our research. </p>
<p>These marsquakes were rather different to previous ones recorded by InSight. For example, they seemed to be what we call “surface waves” – that is, seismic waves propagating in the outermost layers of the martian crust (its surface layer). </p>
<p>These sorts of waves are rare. They are also particularly exciting because they allow us to “map” the structure of Mars’ highly unusual crust, which is much flatter in the northern hemisphere and thicker and more mountainous in the southern. </p>
<h2>Martian detective work</h2>
<p>We could tell the marsquakes probably had a shallow origin – potentially produced by an enormous impact event rather than originating from processes deeper within the planet’s interior. By analysing the seismic waves that InSight recorded, we were also able to work out the marsquakes’ approximate epicentre, or point of origin. Because these two quakes were so unusual, we requested follow-up observations from the <a href="https://mars.nasa.gov/mro/">Mars Reconnaissance Orbiter</a> spacecraft which orbits the planet. </p>
<p>The results were quite remarkable. The epicentres of both marsquakes were found to correlate with the positions of enormous black smudges on the planet’s surface – the blast zones of new impact craters. Looking back at older, low resolution images allowed the imaging team to pin down the exact dates for the formation of the craters, which coincided exactly with when the marsquakes were detected by InSight. </p>
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<img alt="Image showing Mars insight landing site." src="https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=605&fit=crop&dpr=1 600w, https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=605&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=605&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=761&fit=crop&dpr=1 754w, https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=761&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/494462/original/file-20221109-13740-a4qjc0.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=761&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Mars insight landing site.</span>
<span class="attribution"><span class="source">Doyeon Kim, Martin van Driel, Christian Böhm</span></span>
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<p>The craters themselves were enormous – around 130m and 150m in diameter respectively. The “blast zones”, created by the shockwaves from the meteors entering the atmosphere and impacting the surface, extended out for dozens of kilometres. These were by far the biggest fresh craters we had ever seen form anywhere in the Solar System. </p>
<p>The larger of the two craters was only around thirty degrees north of Mars’ equator – by martian standards, a semi-tropical latitude. At the bottom of the crater were chunks of what was identified as ice (from water), excavated by the impacting body as it broke through into an underground frozen layer. This was the closest to the equator that we’d ever seen ice, and means that there is likely more water on Mars (albeit frozen) than previously thought. This is particularly important if humans are to one day settle there. </p>
<p>As it turned out, the surface waves from one of the events were so strong that they had actually been recorded by InSight after going both ways around the planet – a first for seismology. </p>
<p>By analysing the surface waves, we were also able to create an image of the structure of the crust. Initial results suggested that the differences between the northern and the southern hemisphere might be more superficial than previously believed. Specifically, it looked like some of the differences in the crust were confined to the area very near the surface rather than extending deeper down. Why the northern and southern hemispheres look so different, despite being very similar at even shallow depths, remains a bit of a mystery.</p>
<p>We also don’t know why these two craters formed so close to each other in time – much closer together than random statistics would suggest is likely. One theory that we explored was whether an asteroid may have broken up in orbit around Mars and the fragments slowly re-entered the atmosphere over a period of several months, creating different craters. But the lack of any other similarly sized craters or direct evidence for this makes it challenging to prove. </p>
<p>Sadly, the detection of these impact events is likely to have been one of the last results of the InSight mission. The spacecraft’s solar panels are now so dusty that it is becoming impossible to keep the batteries charged enough to remain operational. Although we will keep listening for as long as we can, it may be only after the next set of seismometers are sent to Mars that we can explore some of these unanswered questions about impact events on the red planet.</p><img src="https://counter.theconversation.com/content/194297/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The author received some funding for the work preliminary to this effort from the UK Space Agency. </span></em></p>A team of scientists have found a surprising amount of water ice on Mars.Benjamin Fernando, Junior fellow, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1907552022-09-19T20:14:45Z2022-09-19T20:14:45ZFor the first time, robots on Mars found meteorite impact craters by sensing seismic shock waves<figure><img src="https://images.theconversation.com/files/485209/original/file-20220919-60301-kjcm6t.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2000%2C1994&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://mars.nasa.gov/insight/multimedia/raw-images/?order=sol+desc%2Cdate_taken+desc&per_page=50&page=2&mission=insight">NASA / JPL-Caltech</a></span></figcaption></figure><p>Since 2018, NASA’s <a href="https://mars.nasa.gov/insight/mission/overview/">InSight mission</a> to Mars has recorded seismic waves from more than <a href="https://www.essoar.org/doi/10.1002/essoar.10512017.1">1,300 marsquakes</a> in its quest to probe the internal structure of the red planet. The solar panels of the car-sized robotic lander have become caked with Martian dust, and NASA scientists <a href="https://mars.nasa.gov/news/9191/nasas-insight-still-hunting-marsquakes-as-power-levels-diminish/?site=insight">expect</a> it will completely power down by the end of 2022.</p>
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Read more:
<a href="https://theconversation.com/first-recorded-marsquakes-reveal-the-red-planets-rumbling-guts-132091">First recorded 'marsquakes' reveal the red planet's rumbling guts</a>
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<p>But the internal rumblings of our planetary neighbour aren’t the only things that InSight’s seismometers detect: they also pick up the thuds of space rocks crashing into the Martian soil.</p>
<p>In <a href="https://www.nature.com/articles/s41561-022-01014-0">new research</a> published in Nature Geoscience, we used data from InSight to detect and locate four high-speed meteoroid collisions, and then tracked down the resulting craters in satellite images from NASA’s Mars Reconnaissance Orbiter.</p>
<h2>Rocks from space</h2>
<p>The Solar System is full of relatively small rocks called meteoroids, and it’s common for them to collide with planets. When a meteoroid encounters a planet with an atmosphere, it heats up due to friction – and may burn up entirely before reaching the ground.</p>
<p>On Earth, we know these incoming meteoroids as shooting stars, or meteors: beautiful events to observe in the night sky. Sometimes a meteoroid explodes when it reaches the thicker atmosphere closer to the ground, creating a spectacular airburst.</p>
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Read more:
<a href="https://theconversation.com/where-do-meteorites-come-from-we-tracked-hundreds-of-fireballs-streaking-through-the-sky-to-find-out-160096">Where do meteorites come from? We tracked hundreds of fireballs streaking through the sky to find out</a>
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<p>Occasionally, a space rock survives its fiery path through the air and drops to the ground, where it is known as a meteorite. </p>
<p>A few of these meteorites hit the surface at such speed they blast a hole in the ground called an impact crater. Compared to a human lifetime, these events are very rare on Earth. </p>
<h2>Recording space rock impacts</h2>
<p>Scientists have detected the vibrations from meteoroid airbursts using seismic detectors numerous times, including <a href="https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/abs/statistical-analysis-of-fireballs-seismic-signature-survey/1683309FE1240CFCE1460AD3A11776BC">a recent survey</a> of bright meteors above Australia. </p>
<p>However, only once has a high-speed space rock crashing into the ground been observed both visually and with modern seismic equipment. This was an impact crater that <a href="https://en.wikipedia.org/wiki/2007_Carancas_impact_event">formed in 2007</a> near the village of Carancas in Peru. </p>
<p>Numerous impacts were detected on the Moon by the network of seismic sensors set up during the US Apollo missions of the 1960s and ’70s. However, there was no recording of a natural impact associated with visual detection of a new crater. </p>
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Read more:
<a href="https://theconversation.com/the-moon-is-still-geologically-active-study-suggests-116768">The moon is still geologically active, study suggests</a>
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<p>The closest things to such an observation were artificial impacts: the crash-landings of the booster rockets of the ascent modules that lifted Apollo astronauts off the Moon. </p>
<p>These human-made impacts on the Moon were recorded both in seismic data and visual imagery from orbit. These data were <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021EA001887">recently used</a> to test simulations of how impacts produce seismic waves.</p>
<h2>Martian meteorites</h2>
<p>Incoming meteoroids make waves in the atmosphere and also the ground. The atmosphere of Mars is equivalent to 1% of the Earth’s, and has a different chemical composition. This means meteor events on Mars take a different form.</p>
<p>For meteor events large enough to drop a meteorite, the fate of the meteorite and any resulting crater is <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021JE007149">different</a> from what we have come to expect on our home planet. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=445&fit=crop&dpr=1 600w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=445&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=445&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=560&fit=crop&dpr=1 754w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=560&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/485232/original/file-20220919-49069-ivyabg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=560&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Many craters on Mars come in clusters, because meteoroids often explode into fragments not long before they hit the surface.</span>
<span class="attribution"><a class="source" href="https://www.uahirise.org/ESP_028444_2040">MRO / HiRISE / NASA / JPL-Caltech / UArizona</a></span>
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<p>Here on Earth, or on the Moon, single craters are the norm. On Mars, however, about half the time a high-speed space rock will burst in the atmosphere shortly before impact, resulting in a tightly grouped cluster of craters.</p>
<p>The separation of these individual fragments remains close at ground level, forming a <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2021JE007145">cluster of small impacts</a>.</p>
<h2>From vibrations to craters</h2>
<p>Recently, the InSight mission has observed acoustic and seismic waves from four meteoroid impact events. These waves travel at different speeds, and comparing their different arrival times and other properties allowed us to estimate the location of the impacts.</p>
<p>These impact locations were then confirmed with satellite imaging from the Mars Reconnaissance Orbiter.</p>
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<img alt="" src="https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=538&fit=crop&dpr=1 600w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=538&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=538&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=676&fit=crop&dpr=1 754w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=676&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/485216/original/file-20220919-65079-nfw0w4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=676&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">A sketch of how an incoming space rock makes waves that InSight can detect and interpret.</span>
<span class="attribution"><a class="source" href="https://www.nature.com/articles/s41561-022-01014-0">Garcia et al. / Nature Geoscience</a></span>
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<p>Knowing the size and exact location of these impact craters helps us calculate the size and speed of the incoming space rock and how much energy the impact released.</p>
<p>Once we are confident we know something about the impact that created the seismic waves we detected, we can use the waves to learn about the interior of Mars. What’s more, when we compare seismic observations on Mars with observations from Earth and the Moon, we can learn more about how the planets formed and how the Solar System evolved.</p><img src="https://counter.theconversation.com/content/190755/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr Katarina Miljkovic works for Curtin University and is fully funded by the Australian Research Council. She is a science collaborator for the NASA InSight mission.</span></em></p>In an extraterrestrial first, scientists have linked seismic waves on Mars to meteorite impact craters spotted via satellite.Katarina Miljkovic, ARC Future Fellow, School of Earth and Planetary Sciences, Curtin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1643682021-07-14T12:52:22Z2021-07-14T12:52:22ZRumble in the jungle: an ear to the ground can tell us how elephants are faring in the wild<figure><img src="https://images.theconversation.com/files/411108/original/file-20210713-23-mlrrqx.png?ixlib=rb-1.1.0&rect=0%2C0%2C2830%2C1888&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Rumbles elephants make travel through the air and the ground.</span> <span class="attribution"><span class="source">Beth Mortimer</span>, <span class="license">Author provided</span></span></figcaption></figure><p>African elephants can be found roaming the forests and grasslands of 37 countries across the continent. But sadly, these sentient and intelligent animals are rapidly declining, and were <a href="https://www.iucn.org/news/species/202103/african-elephant-species-now-endangered-and-critically-endangered-iucn-red-list">recently declared endangered</a>. </p>
<p>For these remaining elephants to find each other, they make a variety of vocal noises to greet and warn each other, or to woo potential mates. Some of their vocalisations, which are called rumbles, are very low-pitched. So low in fact that humans can barely hear them. Due to the firm stance and weight of the elephants (which can reach 6,000kg), these waves travel not only through the air but also into the ground.</p>
<p>Elephants are thought to communicate over large distances – <a href="https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.530.8940&rep=rep1&type=pdf">up to several kilometres</a> in some cases. But it hasn’t been clear to scientists how important the seismic vibrations of their rumbles are in these long-distance chats. Is it just a coincidence that these sounds travel so far through the ground? Or is it something elephants actively exploit to stay in touch? </p>
<p><a href="http://rsif.royalsocietypublishing.org/doi/10.1098/rsif.2021.0264">We wanted to</a> find out. By deciphering the hidden information in these rumbles, we hoped they might also help us study and track elephants in future.</p>
<h2>Pinpointing elephants</h2>
<p>Working at Mpala Research Center in Kenya with computer scientists, earth scientists, conservationists and biologists, we set up microphones and seismometers around a watering hole known to be frequented by elephants. Seismometers pick up small underground vibrations and are typically used to measure earthquakes and explosions, some of which can be detected on the other side of the globe.</p>
<figure class="align-center ">
<img alt="A herd of elephants departing a lake." src="https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/411211/original/file-20210714-21-hb6rlw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">On the move: a herd of elephants leaving the watering hole.</span>
<span class="attribution"><span class="source">Beth Mortimer</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>For the first time ever, we found it was possible to accurately locate elephants by measuring vibrations in the ground caused by their low-pitched rumbles. Our devices recorded the rumbles they made at distances of up to 500 metres. </p>
<p>Comparing the signals at all the seismometers, we could estimate the animal’s location with an average accuracy of just a few metres. Surprisingly, the seismic sensors were slightly more accurate in pinpointing the elephants than the microphones. It’s quite common for scientists to track species using acoustic technology, but getting better results with signals through the ground could open a new way of monitoring wildlife.</p>
<h2>The future of wildlife monitoring</h2>
<p>It’s unknown how exactly elephants pick up these vibrations and how they decipher their meaning. But our study suggests that the seismic rumbles could announce the location of the vocalising animal to other elephants far away, despite the thick layers of earth that these waves pass through. </p>
<p>Whereas sound recordings can be interrupted by rain, wind and trees, seismic monitoring is relatively free of these types of interference. Where acoustic monitoring fails or gives bad results, seismic monitoring could be used instead. </p>
<p>This is helpful, as it’s important to know whether or not elephants are present in protected areas, or if they’ve wandered into places where they might be in danger, such as unguarded territories or towns and villages. Knowing this could allow rangers to respond quicker and prevent poaching, as well as prevent other kinds of conflict erupting between elephants and people.</p>
<figure class="align-center ">
<img alt="A large elephant amid African scrubland." src="https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/411212/original/file-20210714-27-1i8szsp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Elephants can sometimes wander into trouble.</span>
<span class="attribution"><span class="source">Beth Mortimer</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Seismic monitoring could even be used to monitor hoofed animals like endangered species of giraffe and zebras. The seismic vibrations of their footsteps could help scientists study their group behaviour and how these species interact with their environment, without needing to tag them.</p>
<p><a href="https://doi.org/10.1016/j.cub.2018.03.062">Previous work</a> has shown that seismic recordings can accurately differentiate between the sounds made by elephants walking and their vocalisations. In future work, we hope to use this to develop AI algorithms that can detect what kind of wildlife is passing close to these sensors and what they are up to. This could help us monitor threatened or endangered species, count populations, and learn more about their movement and fascinating social behaviours.</p><img src="https://counter.theconversation.com/content/164368/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>African elephants stay in touch over large distances. We found out how.Michael Reinwald, Postdoctoral Research Associate in Zoology, University of OxfordBeth Mortimer, Royal Society University Research Fellow of Zoology, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1536722021-02-15T11:18:44Z2021-02-15T11:18:44ZMars InSight: why we’ll be listening to the landing of the Perseverance rover<figure><img src="https://images.theconversation.com/files/384085/original/file-20210213-23-er0yjs.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C3840%2C2160&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist impression of Mars Insight.</span> <span class="attribution"><span class="source">NASA/JPL-Caltech</span></span></figcaption></figure><p>As I write this, Nasa’s <a href="https://mars.nasa.gov/mars2020/">Perseverance Rover</a> is hurtling toward Mars at a speed of more than 80,000 km/h and is set to land at the Red Planet on February 18. Landing on Mars is no mean feat - the spacecraft will have to decelerate from more than 30 times the speed of sound to stationary in a matter of minutes, while it’s heated to more than 1,000°C by friction with the atmosphere. </p>
<p>Just over 3,000kms away from Perseverance’s landing site, another Nasa mission, <a href="https://theconversation.com/mars-insight-here-is-whats-next-after-the-tricky-landing-107749">InSight</a>, will be keeping an ear out. InSight’s primary goal is the seismic exploration of Mars, and it has been listening for “marsquakes” since November 2018. </p>
<p>Two years ago, the InSight team announced <a href="https://theconversation.com/mars-quakes-the-insight-lander-shows-active-faults-in-the-planets-crust-132315">the first successful detection</a> of a quake on another planet - 30 years after this had first been attempted. The quakes look very different to the ones here on Earth, but exactly why this is the case remains a mystery. </p>
<p>On Earth, a network of thousands of seismic stations record vibrations from earthquakes which travel through the interior of the planet. Using data from the global seismic network, we can rapidly work out how big a quake was, and where and when it happened. As different types of seismic waves propagate in different directions and at different speeds, this also allows us to build up a picture of the Earth’s interior. </p>
<p>On Mars, where we have only a single seismic station, this is much more challenging to do. InSight recently celebrated its first Martian birthday (which actually amounts to 687 Earth days). In that time it has detected hundreds of quake events. But we’ve been unable to work out the parameters for the source of any of these – such as how large they were or where they occurred. It would be invaluable to have a source with known properties, which would offer us a chance to “calibrate” the apparatus so that we have a point of reference for other measurements. This can then help us to create precise models of the Martian interior.</p>
<h2>Calibration experiment</h2>
<p>So because earthquakes are common and we have plenty of seismic station it is relatively straightforward to set up a calibration experiment on Earth. But on Mars, seismic events are much rarer. Luckily, the entry, descent and landing of the Perseverance rover is so energetic that it produces signals that are detectable by seismometers. And we will know the parameters for it, such as where the rover will land, how much it weighs, its landing velocity and so forth. So could we use it as a source for calibration? We <a href="http://bit.ly/m2020-paper">recently evaluated</a> this.</p>
<figure class="align-center ">
<img alt="Artist's image of the Perseverance rover landing." src="https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/384086/original/file-20210213-21-k811n0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Will the landing be loud enough?</span>
<span class="attribution"><span class="source">NASA</span></span>
</figcaption>
</figure>
<p>There are three types of seismic signals produced by landing events. First, the rapid deceleration of the spacecraft produces a sonic boom. This shockwave propagates down toward the ground, where there are two processes which may produce detectable signals. Some of the energy from the sonic boom can get trapped by wind layers in the atmosphere, which channel it in a very efficient manner over great distances. The sonic boom may also strike the surface, creating seismic waves which propagate in the ground. We evaluated the likely amplitudes of both of these signals at the location of InSight, but found that they were unlikely to be detectable. </p>
<p>But the third type of signal is more promising. During the landing sequence, Perseverance will eject two large weights. These <a href="https://mars.nasa.gov/mars2020/timeline/landing/entry-descent-landing/">Cruise Mass Balance Devices</a> (CMBDs) are used to control the spacecraft’s trajectory on its journey from Earth to Mars, but are jettisoned more than a thousand kilometres above the surface. Because of this, they are not slowed down by the deceleration of the rest of the spacecraft, but are expected to hit the surface at thousands of kilometres per hour, producing craters several metres in radius.</p>
<p>This will transmit a huge amount of energy into the ground, which will produce seismic waves. We estimated that these signals will be “loud” enough to be detected by InSight’s seismometers about 40% of the time in the best-case scenario. The uncertainties of our estimates are significant, mainly because no one has ever tried to detect an impact event at these distances before (though fans of the Netflix series Away will recall a similar storyline using InSight in that programme). </p>
<p>Our results will nevertheless help work out how Mars’ interior is structured and how seismic waves propagate through it. This is the first time that anyone has tried using a spacecraft on the surface of another planet to detect another spacecraft arriving - so it really is unprecedented. Wish us luck in keeping an ear to the ground, as it were. </p>
<p><em>You can hear more about the three Mars missions arriving at the red planet in February in the first episode of our new podcast, <a href="https://theconversation.com/uk/topics/the-conversation-weekly-98901">The Conversation Weekly</a> – the world explained by experts. Subscribe wherever you get your podcasts.</em></p>
<iframe src="https://player.acast.com/60087127b9687759d637bade/episodes/a-big-month-for-mars?theme=default&cover=1&latest=1" frameborder="0" width="100%" height="110px" allow="autoplay"></iframe><img src="https://counter.theconversation.com/content/153672/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Benjamin Fernando receives funding from The Natural Environment Research Council (NERC) and is supported by the UK Space Agency. Outside of his doctoral work he chairs the Labour Party's Science Affiliate group, Scientists for Labour. </span></em></p>The Perseverance rover’s landing could help reveal secrets of Mars’ interior.Benjamin Fernando, PhD candidate and college lecturer, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1390192020-05-27T19:09:29Z2020-05-27T19:09:29ZNew Zealand sits on top of the remains of a giant ancient volcanic plume<figure><img src="https://images.theconversation.com/files/336579/original/file-20200520-152320-84uoa2.jpeg?ixlib=rb-1.1.0&rect=23%2C40%2C1270%2C619&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ewing</span> <span class="attribution"><span class="license">Author provided</span></span></figcaption></figure><p>Back in the 1970s, scientists came up with a revolutionary idea about how Earth’s deep interior works. They proposed it is slowly churning like a lava lamp, with buoyant blobs rising as plumes of hot mantle rock from near Earth’s core, where rocks are so hot they move like a fluid.</p>
<p>According to the theory, as these plumes approach the surface they begin to melt, triggering massive volcanic eruptions. But evidence for the existence of such plumes proved elusive and geologists had all but rejected the idea. </p>
<p>Yet in a <a href="https://advances.sciencemag.org/content/6/22/eaba7118/tab-article-info">paper published today</a>, we can now provide this evidence. Our results show that New Zealand sits atop the remains of such an ancient giant volcanic plume. We show how this process causes volcanic activity and plays a key role in the workings of the planet.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/satellites-reveal-melting-of-rocks-under-volcanic-zone-deep-in-earths-mantle-80452">Satellites reveal melting of rocks under volcanic zone, deep in Earth's mantle</a>
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</em>
</p>
<hr>
<h2>Unusual vibrations</h2>
<p>About 120 million years ago - during the time of dinosaurs in the <a href="https://www.livescience.com/29231-cretaceous-period.html">Cretaceous period</a> - vast volcanic eruptions under the ocean created an underwater plateau about the size of India. Over time, it was broken up by the movements of tectonic plates. One fragment now lies beneath New Zealand and forms the <a href="https://en.wikipedia.org/wiki/Hikurangi_Plateau">Hikurangi Plateau</a>. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=675&fit=crop&dpr=1 600w, https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=675&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=675&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=848&fit=crop&dpr=1 754w, https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=848&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/336350/original/file-20200520-152349-mtsh6d.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=848&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This map of the southwest Pacific and New Zealand shows the dispersed fragments of a once giant oceanic plateau. Red arrows show the directions of seafloor spreading. Straight black lines show the areas measured in our study.</span>
<span class="attribution"><span class="source">Simon Lamb</span></span>
</figcaption>
</figure>
<p>We measured the speed of seismic pressure waves - effectively soundwaves - and how they travel through mantle rocks beneath the Hikurangi Plateau. These vibrations were triggered either by earthquakes or deliberate explosions and reached speeds of 9 kilometres per second. </p>
<p>It’s well known these waves, known as P-waves, travel in the uppermost mantle of the Earth at a remarkably constant speed: around 8.1km per second (about 30,000km per hour). Even small deviations from this constant speed reveal important information about the state of the mantle rocks. </p>
<p>Since the late 1970s, fast P-wave speeds (8.7-9.0km/s) had been reported from a depth of about 30km under New Zealand’s eastern North Island. The seismic vibrations recorded in these early data were only travelling in one direction through a small part of the mantle, and the significance of the high speed was unclear. </p>
<p>Our new data is much more extensive, from a <a href="https://www.rnz.co.nz/national/programmes/ourchangingworld/audio/2490512/scanning-a-plate-boundary">major seismic experiment</a> in 2012 that spanned the southern North Island and offshore regions, including the Hikurangi Plateau. </p>
<p>It shows the speed of P-waves reached 9km/s, regardless of the horizontal direction in which they travelled. But a careful analysis of vibrations triggered by deep earthquakes showed unusually low speeds for vibrations travelling in the vertical direction. </p>
<p>This reveals crucial information about how the mantle rocks have been stretched or squeezed by the huge forces inside the Earth, and this turns out to confirm the existence of the elusive plumes.</p>
<h2>A seismic pancake</h2>
<p>The pattern of seismic speeds we observed requires the mantle rocks beneath the Hikurangi Plateau to have been stretched and squeezed in much the same way as one might produce a pancake shape by flattening a rubber ball.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=416&fit=crop&dpr=1 600w, https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=416&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=416&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=523&fit=crop&dpr=1 754w, https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=523&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/337152/original/file-20200523-124845-10usggx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=523&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Computer simulations of a plume of buoyant hot rock in the Earth’s mantle rising up towards the surface from the core-mantle boundary. In the later stages, the plume head collapses under gravity to form a pancake shape.</span>
<span class="attribution"><span class="source">James Moore</span></span>
</figcaption>
</figure>
<p>When we carried out computer simulations of rising plumes in the mantle, we found they reproduced exactly this pancake flattening pattern, as the mushroom-shaped head of the plume spreads sideways and collapses near the surface.</p>
<p>We also looked at data from seismic experiments by international teams on other oceanic plateaux in the south-west Pacific region. Remarkably, both the Manihiki and Ontong-Java plateaux showed the same pattern as we observed beneath the Hikurangi Plateau. P-waves travelled at the same high speeds regardless of the horizontal direction, but at significantly slower speeds in the vertical direction.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/parts-of-the-pacific-northwests-cascadia-fault-are-more-seismically-active-than-others-imaging-data-suggests-why-100631">Parts of the Pacific Northwest's Cascadia fault are more seismically active than others – imaging data suggests why</a>
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</p>
<hr>
<h2>Reconstructing an ancient superplume</h2>
<p>The major oceanic plateaux of the southwest Pacific are now dispersed, but we know how they once fitted together, about 120 million years ago. They formed a region underlain by a thick layer of volcanic rock, thousands of kilometres across. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=323&fit=crop&dpr=1 600w, https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=323&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=323&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=407&fit=crop&dpr=1 754w, https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=407&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/337842/original/file-20200527-141312-1bmtk8z.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=407&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This reconstruction of oceanic plateaux at 120 million years ago shows how they fitted together above the pancake-shaped head of a superplume.</span>
<span class="attribution"><span class="source">Simon Lamb</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Our analysis shows this entire region lay above the single head of a giant plume – a superplume – which melted to produce massive lava outbursts over a geologically brief period of a few million years.</p>
<p>Siberia is the only other place on Earth where this pattern of P-wave speeds has been observed in the upper mantle. And it turns out this was also the scene of widespread volcanic eruptions about 250 million years ago, thought to be caused by the rise of a superplume. </p>
<p>This volcanic activity may have changed Earth’s climate and triggered a mass extinction that affected the evolution of life. </p>
<p>New Zealand and some scattered islands in the southwest Pacific are perched on the remains of what was once an immensely powerful geological force. We don’t know whether this process is still ongoing today, but our new seismic technique for finding these superplume remnants may help us discover more - providing further insight into the many connections between the deep interior of our planet and what happens at the surface.</p><img src="https://counter.theconversation.com/content/139019/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Simon Lamb receives funding from New Zealand Marsden Fund, New Zealand Earthquake Commission, Victoria University of Wellington University Research Fund.</span></em></p><p class="fine-print"><em><span>Timothy Stern receives funding from. New Zealand Marsden Fund, NZ Earthquake Commission, NZ Royal Society Endeavour Fund.</span></em></p>New research confirms that massive plumes of buoyant hot rock once rose from near the Earth’s core to the surface and triggered vast volcanic eruptions - and that New Zealand sits on top of one.Simon Lamb, Associate Professor in Geophysics, Te Herenga Waka — Victoria University of WellingtonTimothy Stern, Professor of Geophysics, Te Herenga Waka — Victoria University of WellingtonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1006312018-08-01T10:38:21Z2018-08-01T10:38:21ZParts of the Pacific Northwest’s Cascadia fault are more seismically active than others – imaging data suggests why<figure><img src="https://images.theconversation.com/files/229901/original/file-20180730-106514-1rqf4bf.jpg?ixlib=rb-1.1.0&rect=114%2C2%2C1652%2C1092&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What's going on 150 kilometers below the Earth's surface?</span> <span class="attribution"><a class="source" href="https://www.goodfreephotos.com">Good Free Photos</a></span></figcaption></figure><p>The Pacific Northwest is known for many things – its beer, its music, its mythical large-footed creatures. Most people don’t associate it with earthquakes, but they should. It’s home to the <a href="http://discovermagazine.com/2012/extreme-earth/01-big-one-earthquake-could-devastate-pacific-northwest">Cascadia megathrust fault</a> that runs 600 miles from Northern California up to Vancouver Island in Canada, spanning several major metropolitan areas including Seattle and Portland, Oregon.</p>
<p>This geologic fault has been relatively quiet in recent memory. There haven’t been many widely felt quakes along the Cascadia megathrust, certainly nothing that would rival a catastrophic event like the 1989 <a href="https://earthquake.usgs.gov/earthquakes/events/1989lomaprieta/">Loma Prieta earthquake</a> along the active San Andreas in California. That doesn’t mean it will stay quiet, though. Scientists know it has the potential for large earthquakes – as big as <a href="https://www.newyorker.com/magazine/2015/07/20/the-really-big-one">magnitude 9</a>.</p>
<p>Geophysicists have known for over a decade that not all portions of the Cascadia megathrust fault behave the same. The northern and southern sections are much more seismically active than the central section – with frequent small earthquakes and ground deformations that residents don’t often notice. But why do these variations exist and what gives rise to them?</p>
<p><a href="https://scholar.google.com/citations?user=67KN5e4AAAAJ&hl=en&oi=ao">Our</a> <a href="https://scholar.google.com/citations?user=SXGw77gAAAAJ&hl=en&oi=sra">research</a> tries to answer these questions by <a href="https://doi.org/10.1029/2018GL078700">constructing images of what’s happening deep within the Earth</a>, more than 100 kilometers below the fault. We’ve identified regions that are rising up beneath these active sections which we think are leading to the observable differences along the Cascadia fault.</p>
<h2>Cascadia and the ‘Really Big One’</h2>
<p>The Cascadia <a href="https://www.livescience.com/43220-subduction-zone-definition.html">subduction zone</a> is a region where two tectonic plates are colliding. The <a href="https://americastectonics.weebly.com/juan-de-fuca-explorer-and-gorda-plates.html">Juan de Fuca</a>, a small oceanic plate, is being driven under the North American plate, atop which the continental U.S. sits.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=363&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=363&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=363&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=456&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=456&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229912/original/file-20180731-102467-1oh0x8m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=456&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Juan de Fuca plate meets the North American plate beneath the Cascadia fault.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Cascadia_earthquake_sources.png">USGS</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Subduction systems – where one tectonic plate slides over another – are capable of producing the world’s largest known earthquakes. A prime example is the <a href="https://www.livescience.com/39110-japan-2011-earthquake-tsunami-facts.html">2011 Tohoku earthquake</a> that rocked Japan.</p>
<p>Cascadia is seismically very quiet compared to other subduction zones – but it’s not completely inactive. Research indicates the fault ruptured in a <a href="https://www.opb.org/news/series/unprepared/jan-26-1700-how-scientists-know-when-the-last-big-earthquake-happened-here/">magnitude 9.0 event in 1700</a>. That’s roughly 30 times more powerful than the largest predicted San Andreas earthquake. Researchers suggest that we are within the roughly <a href="https://projects.oregonlive.com/maps/earthquakes/timeline">300- to 500-year window</a> during which <a href="https://www.newyorker.com/magazine/2015/07/20/the-really-big-one">another large Cascadia event may occur</a>.</p>
<p>Many smaller undamaging and unfelt events take place in northern and southern Cascadia every year. However, in central Cascadia, underlying most of Oregon, there is very little seismicity. Why would the same fault behave differently in different regions? </p>
<p>Over the last decade, scientists have made several additional observations that highlight variations along the fault.</p>
<p>One has to do with <a href="https://www.ldeo.columbia.edu/%7Edjs/aleut/info_for_public.html">plate locking</a>, which tells us where stress is accumulating along the fault. If the tectonic plates are locked – that is, really stuck together and unable to move past each other – stress builds. Eventually that stress can be released rapidly as an earthquake, with the magnitude depending on how large the patch of fault that ruptures is.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=769&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=769&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=769&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=966&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=966&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229900/original/file-20180730-106521-cc0xx3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=966&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A GPS geosensor in Washington.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:EarthScope-geosensor.jpg">Bdelisle</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Geologists have recently been able to deploy <a href="https://www.unavco.org/projects/major-projects/pbo/pbo.html">hundreds of GPS</a> monitors across Cascadia to record the subtle ground deformations that result from the plates’ inability to slide past each other. Just like historic seismicity, plate locking is more common in the <a href="http://geodesygina.com/Cascadia.html">northern and southern parts of Cascadia</a>.</p>
<p>Geologists are also now able to observe difficult-to-detect seismic rumblings known as <a href="https://pnsn.org/tremor">tremor</a>. These events occur over the time span of several minutes up to weeks, taking much longer than a typical earthquake. They don’t cause large ground motions even though they can release significant amounts of energy. Researchers <a href="https://doi.org/10.1126/science.1084783">have only discovered</a> <a href="https://doi.org/10.1126/science.1060152">these signals</a> in the <a href="https://doi.org/10.1126/science.1070378">last 15 years</a>, but permanent seismic stations have helped build a robust catalog of events. Tremor, too, seems to be more concentrated along the <a href="https://doi.org/10.1130/G23740A.1">northern and southern parts</a> of the fault. </p>
<p>What would cause this situation, with the area beneath Oregon relatively less active by all these measures? To explain we had to look deep, over 100 kilometers below the surface, into the Earth’s mantle.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=588&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=588&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=588&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=738&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=738&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229896/original/file-20180730-106524-1kc0lc8.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=738&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Green dots and blue triangles show locations of seismic monitoring stations.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.1029/2018GL078700">Bodmer et al., 2018, Geophysical Research Letters</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Imaging the Earth using distant quakes</h2>
<p>Physicians use electromagnetic waves to “see” internal structures like bones without needing to open up a human patient to view them directly. Geologists <a href="https://www.iris.edu/hq/inclass/animation/seismic_tomography_ct_scan_as_analogy">image the Earth</a> in much the same way. Instead of X-rays, we use seismic energy radiating out from distant magnitude 6.0-plus earthquakes to help us “see” features we physically just can’t get to. This energy travels like sound waves through the structures of the Earth. When rock is hotter or partially molten by even a tiny amount, seismic waves slow down. By measuring the arrival times of seismic waves, we create 3D images showing how fast or slow the seismic waves travel through specific parts of the Earth. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=557&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=557&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=557&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=699&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=699&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229895/original/file-20180730-106499-xy3gha.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=699&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ocean bottom seismometers waiting to be deployed during the Cascadia Inititive.</span>
<span class="attribution"><span class="source">Emilie Hooft</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>To see these signals, we need records from seismic monitoring stations. More sensors provide better resolution and a clearer image – but gathering more data can be problematic when half the area you’re interested in is underwater. To address this challenge, we were part of a team of scientists that deployed hundreds of seismometers on the ocean floor off the western U.S. over the span of four years, starting in 2011. This experiment, the <a href="https://cascadia.uoregon.edu/">Cascadia Initiative</a>, was the first ever to cover an entire tectonic plate with instruments at a spacing of roughly 50 kilometers.</p>
<p><a href="https://doi.org/10.1029/2018GL078700">What we found are two anomalous regions</a> beneath the fault where seismic waves travel slower than expected. These anomalies are large, about 150 kilometers in diameter, and show up beneath the northern and southern sections of the fault. Remember, that’s where researchers have already observed increased activity: the seismicity, locking, and tremor. Interestingly, the anomalies are not present beneath the central part of the fault, under Oregon, where we see a decrease in activity.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=446&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=446&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=446&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=561&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=561&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229899/original/file-20180730-106505-ah3ah8.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=561&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Regions where seismic waves moved more slowly, on average, are redder, while the areas where they moved more quickly are bluer. The slower anomalous areas 150 km beneath the Earth’s surface corresponded to where the colliding plates are more locked and where tremor is more common.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.1029/2018GL078700">Bodmer et al., 2018, Geophysical Research Letters</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
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<p>So what exactly are these anomalies?</p>
<p>The tectonic plates float on the Earth’s rocky mantle layer. Where the mantle is slowly rising over millions of years, the rock decompresses. Since it’s at such high temperatures, nearly 1500 degrees Celsius at 100 km depth, it can <a href="https://www.wired.com/2012/12/why-do-rocks-melt-volcano/">melt ever so slightly</a>.</p>
<p>These physical changes cause the anomalous regions to be more buoyant – melted hot rock is less dense than solid cooler rock. It’s this buoyancy that we believe is affecting how the fault above behaves. The hot, partially molten region pushes upwards on what’s above, similar to how a helium balloon might rise up against a sheet draped over it. We believe this increases the forces between the two plates, causing them to be more strongly coupled and thus more fully locked.</p>
<h2>A general prediction for where, but not when</h2>
<p>Our results provide new insights into how this subduction zone, and possibly others, behaves over geologic time frames of millions of years. Unfortunately our results can’t predict when the next large Cascadia megathrust earthquake will occur. This will require more research and dense active monitoring of the subduction zone, both onshore and offshore, using seismic and GPS-like stations to capture short-term phenomena. </p>
<p>Our work does suggest that a large event is more likely to start in either the northern or southern sections of the fault, where the plates are more fully locked, and gives a possible reason for why that may be the case.</p>
<p>It remains important for the public and policymakers to stay informed about the potential risk involved in <a href="https://www.seattletimes.com/seattle-news/science/californias-celeb-quake-expert-says-preventing-damage-is-key-to-quick-recovery/">cohabiting with a subduction zone fault</a> and to support programs such as <a href="https://earthquake.usgs.gov/research/earlywarning/">Earthquake Early Warning</a> that seek to expand our monitoring capabilities and mitigate loss in the event of a large rupture.</p><img src="https://counter.theconversation.com/content/100631/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Doug Toomey receives funding from National Science Foundation and the United States Geological Survey. </span></em></p><p class="fine-print"><em><span>Miles Bodmer does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A new array of seismometers provides a glimpse of what’s happening deep beneath this geologic fault. New data help explain why the north and south of the region are more seismically active than the middle.Miles Bodmer, PhD Student in Earth Sciences, University of OregonDoug Toomey, Professor of Earth Sciences, University of OregonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/910802018-04-29T20:14:32Z2018-04-29T20:14:32ZLaunching in May, the InSight mission will measure marsquakes to explore the interior of Mars<figure><img src="https://images.theconversation.com/files/213532/original/file-20180406-125187-1k9h41.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">InSight aims to figure out just how tectonically active Mars is, and how often meteorites impact it. </span> <span class="attribution"><a class="source" href="https://mars.nasa.gov/insight/multimedia/images/2018/measuring-the-pulse-of-mars">NASA </a></span></figcaption></figure><p>When we look up at Mars in the night sky we see a red planet - largely due to its <a href="https://www.space.com/16999-mars-red-planet.html">rusty surface</a>. But what’s on the inside?</p>
<p>Launching in May, the next NASA space mission will study the interior of Mars. </p>
<p>The InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) spacecraft will be a stationary lander mission that measures seismic activity on Mars (often referred to as marsquakes) as well as interior heat flow.</p>
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Read more:
<a href="https://theconversation.com/a-brief-history-of-martian-exploration-as-the-insight-lander-prepares-to-launch-91313">A brief history of Martian exploration – as the InSight Lander prepares to launch</a>
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<p>By listening to and probing the Martian crust and interior, the project aims to understand the formation and evolution of Mars.</p>
<p>The InSight mission is scheduled to launch from California in early May, with landing on Mars planned for November. The expected lifetime of the mission is at least two years. </p>
<h2>Origins of marsquakes</h2>
<p>The payload on board InSight includes the seismic instrument SEIS (Seismic Experiment for Interior Structure). Its task is to record seismic activity, or vibrations, of the planet. </p>
<p>Apart from shaking the ground while passing, seismic waves can be extremely useful in telling us about the structure of planetary interiors. Seismic waves travel at different speeds when passing through different materials. Processing their arrival time and strength via recorded seismographs is a clever way to learn about the interior structure of a planetary body - such as the crust, the next layer down (the mantle), and the core.</p>
<p>Seismic activity on Mars could be caused by a number of processes. For example, shallow marsquakes could originate from meteoroid strikes, and deep marsquakes could come from martian tectonic activity (the movement of tectonic plates at the surface of the planet). </p>
<p>It is generally believed that tectonic processes could have shaped Mars in its early evolution, similar to the Earth. However, unlike the Earth in younger ages, Mars has become largely tectonically dormant. </p>
<h2>We think lots of meteoroids hit Mars</h2>
<p>Considering that tectonics on Mars may not be reminiscent of what we see on our planet, we suspect that meteoroid strikes will play a major role in causing marsquakes. </p>
<p>On Earth, frequent and small meteoroids most often burn up in the atmosphere and appear to us as a form of “shooting star”. When a rock from space moving at supersonic speed encounters the terrestrial atmosphere, the air in front of it gets compressed extremely quickly. Temperature rises and heat builds up, so the rock starts to shine bright under the process of its destruction. </p>
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Read more:
<a href="https://theconversation.com/look-up-your-guide-to-some-of-the-best-meteor-showers-for-2018-86053">Look up! Your guide to some of the best meteor showers for 2018</a>
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<p>However, on Mars we think that meteoroids may not necessarily burn up entirely upon encountering the martian atmosphere. This is simply because Mars has a less dense atmosphere than the Earth – so incoming meteoroids have a higher penetrating power. These impact events would produce seismic disturbance in the atmosphere, and also likely in the ground. </p>
<p>Detecting meteoroid strikes on planetary bodies began with the lunar Apollo program. Apollo missions carried seismometers to the Moon, and as a result we had a network of seismometers that operated on the Moon from 1969-77. </p>
<p>During its lifetime, the Apollo seismic network recorded shallow quakes produced by frequent meteoroid bombardment. Considering that the Moon does not have an atmosphere to protect its surface from the incoming meteoroids, the Apollo seismic network provided heaps of seismic data from the Moon. These impact-induced seismic moonquakes provided the first constraints about the thickness of the lunar crust as well as structure of crust and deep interior. </p>
<h2>We’ve tried to measure Mars seismic activity before</h2>
<p>During the lunar exploratory boom with the Apollo program, NASA also launched Vikings 1 and 2 to Mars in 1975. These became the first missions to land on Mars, and each Viking mission carried a seismometer.</p>
<p>While instruments on Viking have collected more data than expected, the seismometer on Viking Lander 1 did not work after landing. The seismometer on Viking Lander 2 demonstrated poor detection rates, with no quakes coming off the ground (as it had remained on the Lander).</p>
<p>To date, we have had no other seismic station on any extraterrestrial planetary body. This makes InSight the first-of-its-kind mission to be placed on Mars. While its design relies on proven technologies from past missions, it is ground-breaking in terms of expected science goals. </p>
<p>Instead of making orbital remote sensing surveys or roving on the surface similar to other rovers, InSight has a different goal to previous Martian missions. </p>
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Read more:
<a href="https://theconversation.com/take-it-from-me-im-not-signing-up-to-become-a-space-tourist-just-yet-92680">Take it from me: I'm not signing up to become a space tourist just yet...</a>
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<h2>Why are we so interested in the subsurface of Mars?</h2>
<p>Mars and Earth differ in size, temperature, and atmospheric composition. But similar geological features such as craters, volcanoes, or canyons can be observed on both planets. This implies that the interior of Mars may be similar to Earth’s. </p>
<p>It is also quite likely that there was liquid water on the surface of ancient Mars, which was the time Mars could have been very similar to Earth. So Mars could answer questions about the ancient habitability of our solar system. </p>
<p>Unlike potentially habitable planets orbiting distant stars, Mars is reachable within our lifetime. Discovering martian crustal properties is of great importance when it comes to planning landing missions and investigating signs of extraterrestrial habitability. </p>
<p>My role in the InSight mission is to work with the science team in analysing the data (impact-induced seismograms and any respective orbital imagery) to work out what kind of impacts had occurred during the mission lifetime.</p><img src="https://counter.theconversation.com/content/91080/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dr Katarina Miljkovic is an ARC DECRA fellow at Curtin University. She receives funding from the Australian Research Council. She is a collaborator on the InSight mission to Mars. </span></em></p>What is Mars made of? We hear from a scientist who will be part of the team analysing ‘marsquake’ seismic data and orbital imagery from the InSight mission to the red planet.Katarina Miljkovic, ARC DECRA fellow, Curtin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/910532018-02-07T19:00:46Z2018-02-07T19:00:46ZMore bad news for dinosaurs: Chicxulub meteorite impact triggered global volcanic eruptions on the ocean floor<figure><img src="https://images.theconversation.com/files/205347/original/file-20180207-74479-1ragczb.jpg?ixlib=rb-1.1.0&rect=229%2C0%2C4290%2C3149&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Seismic shockwaves after a meteorite’s collision could affect systems all over the planet.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/large-meteor-burning-glowing-hits-earths-488993764">solarseven/Shutterstock.com</a></span></figcaption></figure><p>The end of the Cretaceous period 66 million years ago was a rough time to be living on Earth.</p>
<p>Three global catastrophes occurred nearly simultaneously: The <a href="https://doi.org/10.1126/science.208.4448.1095">Chicxulub meteorite slammed</a> into what is now Mexico’s Yucatan Peninsula, the massive <a href="https://doi.org/10.1038/333843a0">Deccan Traps volcanic province in modern-day India erupted</a>, and some three-quarters of Earth’s plants and animals, including all non-avian dinosaurs, went extinct. The occurrence of these three events at the same time in our planet’s history has fueled a decades-long <a href="https://doi.org/10.1016/S1631-0713(03)00006-3">debate about causal links</a>. Either a large sequence of volcanic eruptions or an extraterrestrial impact could conceivably cause a mass extinction – but were they all somehow connected?</p>
<p>As Earth scientists, we have reason to believe that there may be another event to add to the list. <a href="http://advances.sciencemag.org/content/4/2/eaao2994">Our new research</a>, published in Science Advances, shows that the Chicxulub impact may have triggered additional volcanic activity far from the Deccan Traps – along tens of thousands of miles of undersea volcanic ridges that lie at the edges of tectonic plates. The meteorite impact caused large seismic waves that traveled around the globe and were apparently capable of flushing magma out of the mantle and into the oceanic crust. This would presumably be more bad news for the dinosaurs and other flora and fauna of the time.</p>
<h2>Ripple effects of seismic activity</h2>
<p>It is well known that seismic activity can trigger a variety of hydrologic phenomena, and sometimes even volcanic eruptions. In the aftermath of nearby large earthquakes, <a href="https://doi.org/10.1038/ncomms8597">dry streams can start flowing</a>, well levels can go up or down, and <a href="https://doi.org/10.1146/annurev.earth.34.031405.125125">geysers sometimes erupt</a>. Seismicity also sets off volcanic activity, but only when conditions are just right – it’s only about <a href="https://doi.org/10.1146/annurev.earth.34.031405.125125">0.4 percent of explosive volcanic eruptions</a> that might be triggered by large earthquakes.</p>
<p>So could the massive earthquake generated when the Chicxulub meteorite crashed into Earth be related to the ongoing eruptions in the Deccan Traps? This volcanic province covered much of India with lava flows in less than a million years. A University of California, Berkeley-led team of researchers (including one of us, Leif Karlstrom) <a href="https://doi.org/10.1130/B31167.1">revisited the possibility of a connection</a> between these two events.</p>
<p>The most recent efforts to date these eruptions have clearly shown that the <a href="https://doi.org/10.1126/science.aaa0118">Deccan Traps began spewing lava</a> before the meteorite impact and the mass extinction occurred. But the Berkeley-led study suggested that the <a href="https://doi.org/10.1126/science.aac7549">Chicxulub impact triggered a rapid increase in their eruption rate</a>. If true, all three events could conceivably be connected: The impact would be followed by accelerated volcanic activity that could contribute to the mass extinction.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/205141/original/file-20180206-88775-1vu003i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Underwater lava flows ooze out between tectonic plates, as at Axial Seamount, where it lies on top of older lavas.</span>
<span class="attribution"><span class="source">Bill Chadwick, Oregon State University, and ROV Jason, Woods Hole Oceanographic Institution</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Looking to the ocean floor</h2>
<p>If the triggering-by-impact hypothesis is right, we’d expect that other volcanic systems would have been set off as well.</p>
<p>At any given time, the vast majority of the volcanic activity on Earth isn’t occurring in continent-covering floods of magma or in explosions like at Mount St. Helens. It’s on the seafloor, where the tectonic plates are spreading apart. As the Earth’s crust splits, the mostly solid mantle layer rises to fill the space created. It melts as it decompresses on the way up.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=203&fit=crop&dpr=1 600w, https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=203&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=203&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=255&fit=crop&dpr=1 754w, https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=255&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/205132/original/file-20180206-88799-1rj8m9l.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=255&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Illustration of a mid-ocean ridge, with magma rising from the mantle and erupting through the crust at the boundary between tectonic plates.</span>
<span class="attribution"><span class="source">Background, E. Paul Oberlander, WHOI Graphic Services. Inset, Bill Chadwick, Oregon State University, and ROV Jason, Woods Hole Oceanographic Institution. Modified by Joseph Byrnes</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
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<p>This new magma percolates its way to the surface and fuels nearly continuous volcanic activity along what are known as <a href="https://en.wikipedia.org/wiki/Mid-ocean_ridge">mid-ocean ridges</a>. This process creates practically all of the crust on the bottom of the ocean. Since the <a href="http://www.earthbyte.org/Resources/agegrid2008.html">ages of the seafloor are relatively well-known</a>, it preserves a record of oceanic volcanic activity stretching back over 100 million years. This remarkable record of volcanic activity creates an opportunity to test the triggering hypothesis.</p>
<p><a href="http://advances.sciencemag.org/content/4/2/eaao2994">In our new study</a>, we used publicly available data sets to make a record of the structure of the seafloor stretching back 100 million years. Since better topographic maps exist for <a href="https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA02820">Mars</a> and <a href="https://sos.noaa.gov/datasets/venus-topography/">Venus</a> than do for the <a href="http://topex.ucsd.edu/marine_topo/">Earth’s seafloor on a global scale</a>, we were forced to use indirect methods to look for variations in seafloor structures.</p>
<p>Minute variations in the strength of gravity at different locations as measured by satellites <a href="http://topex.ucsd.edu/marine_grav/mar_grav.html">provide the requisite mapping tool</a>. Spots that have an excess amount of rock sitting on the seafloor, as you’d expect to result from accelerated volcanic activity, will have a slightly stronger measurement for Earth’s gravitational field.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=298&fit=crop&dpr=1 600w, https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=298&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=298&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=375&fit=crop&dpr=1 754w, https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=375&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/205133/original/file-20180206-88775-1s17005.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=375&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The time with the most small structural anomalies on the sea floor – indicating 8 percent more mass anomalies than on average – occurs at 66 million years ago and coincides with the age of the Chicxulub meteorite impact.</span>
<span class="attribution"><span class="source">Byrnes and Karlstrom, Sci. Adv. 2018;4: eaao2994</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
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<p>We then inspected the record of these “gravity anomalies” to look for any changes to the structure of the seafloor that happened quickly. We found an unusual abundance of these small structural anomalies on the seafloor happened within 1 million years of the Chicxulub impact. The gravity anomalies are consistent with roughly 650 foot high piles of excess material lying on 66-million-year-old seafloor in the Indian and Pacific Oceans.</p>
<p>The total volume of excess material is difficult to pin down, because a large amount of magma could have been injected into the lower crust where it would have a weaker gravitational signature. But we estimate that around the time of the Chicxulub impact, on the order of 23,000 to 230,000 cubic miles of magma erupted out of the mid-ocean ridges, all over the globe. This is on par with the largest eruptive events in Earth’s 4.5-billion-year history, including the Deccan Traps.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=261&fit=crop&dpr=1 600w, https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=261&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=261&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=327&fit=crop&dpr=1 754w, https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=327&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/205136/original/file-20180206-88775-1h6n02.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=327&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Dots mark areas on the seafloor that show high rates of spreading at the time of the Chicxulub impact 66 million years ago. Colors indicate the maximum gravity anomaly within 2 degrees.</span>
<span class="attribution"><span class="source">Byrnes and Karlstrom, Sci. Adv. 2018;4: eaao2994</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Refining the picture</h2>
<p>Our observations suggest the following sequence of events at the end of the Cretaceous period. Just over 66 million years ago, the Deccan Traps start erupting – likely initiated by a plume of hot rock rising from the Earth’s core, similar in some ways to what’s happening beneath Hawaii or Yellowstone today, that impinged on the side of India’s tectonic plate. The mid-ocean ridges and dinosaurs continue their normal activity.</p>
<p>About 250,000 years later, Chicxulub hits off the coast of what will become Mexico. The impact causes a massive disruption to the Earth’s climate, injecting particles into the atmosphere that will eventually settle into <a href="https://doi.org/10.1126/science.1177265">a layer of clay found across the planet</a>. In the aftermath of impact, volcanic activity accelerates for perhaps tens to hundreds of thousands of years. The mid-ocean ridges erupt large volumes of magma, while the Deccan Traps eruptions flood lava across much of the Indian subcontinent. In the end, three-quarters of the Earth’s plant and animal species have disappeared; the only remaining dinosaurs are the feathered, flying variety, normally referred to as birds. </p>
<p>Now, the goal is to further refine our understanding of each event and their interactions. Was there enough mid-ocean ridge activity to contribute to the mass extinction, or was the triggered submarine volcanism merely a symptom of some more significant planetary ailment? Were other volcanic systems triggered by the Chicxulub impact? Which played a larger role in driving the extinction: the volcanism or the meteor?</p>
<p>What is clear is that this new research points to global-scale connections between catastrophes, a good reminder that events happening on the other side of the planet can have effects felt everywhere.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/8wy33t0U1DE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Massive eruption of magma may have contributed to mass extinction at the end of the Cretaceous.</span></figcaption>
</figure><img src="https://counter.theconversation.com/content/91053/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Leif Karlstrom receives funding from the National Science Foundation.</span></em></p><p class="fine-print"><em><span>Joseph Byrnes does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Research suggests a new threat to life on Earth from the meteorite’s crash: Via seismic waves, the impact triggered massive undersea eruptions, as big as any ever seen in our planet’s history.Leif Karlstrom, Assistant Professor of Earth Sciences, University of OregonJoseph Byrnes, Postdoctoral Associate of Earth Sciences, University of MinnesotaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/874802017-11-16T01:40:45Z2017-11-16T01:40:45ZAfter Iran-Iraq earthquake, seismologists work to fill in fault map of the region<figure><img src="https://images.theconversation.com/files/194935/original/file-20171116-19823-1vceqr2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Earthquake survivors are living in tents in western Iran.</span> <span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/Iran-Iraq-Earthquake/8d1afd6e4be24f2ba87f19b3e9a4997e/2/0">AP Photo/Vahid Salemi</a></span></figcaption></figure><p>With a <a href="https://earthquake.usgs.gov/earthquakes/eventpage/us2000bmcg#executive">magnitude of 7.3</a>, the <a href="https://www.nytimes.com/2017/11/13/world/middleeast/iran-iraq-earthquake.html">Nov. 12, 2017 earthquake</a> that shook the border region between Iran and Iraq is among the largest ever recorded in this area. Seismologists know it resulted from the pressure built up between the colliding Arabian and Eurasian plates of the Earth’s crust. But there’s still a lot for researchers to uncover about seismic activity in the region.</p>
<p><iframe id="tc-infographic-126" class="tc-infographic" height="400px" src="https://cdn.theconversation.com/infographics/126/cf6b3e9d59efdac5c40fa6c375504b22d941c754/site/index.html" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>Originally from Iran, I’m a seismologist who studies earthquakes, tsunamis and landslides. I’ve been thinking a lot about potential seismic activity and the consequent hazard in this area. My earth sciences colleagues have been <a href="https://doi.org/10.1111/j.1365-246X.1990.tb06579.x">examining these faults</a> for years in order to better understand the <a href="https://doi.org/10.1016/0040-1951(94)00185-C">fault systems in the region</a>. However, the Earth sometimes surprises us, and this time the rupture did not happen on a previously known major fault.</p>
<p>Our lack of knowledge about the specific fault causing this earthquake is mainly because seismologists know only about faults that have already caused earthquakes. Only after new earthquakes can we update our fault maps to be more complete. It’s learning from past earthquakes that lets us better understand and prepare for future seismic hazards.</p>
<h2>Tectonic plates in motion</h2>
<p>The outer rigid surface of the Earth is divided into chunks known as tectonic plates. These plates move around at the rate of a few centimeters per year – by coincidence, the same rate at which your fingernails grow. The Arabian Peninsula and Iran are on separate adjacent plates in this region.</p>
<p>The mostly northward continental collision between the Arabian plate and Eurasia (which includes Iran) has created the Zagros mountains as the plates crash together in slow motion. Collision energy is also released in the form of earthquakes at fault lines along or close to these boundaries. Many researchers are studying what portions of this region’s collision energy are <a href="https://doi.org/10.1111/j.1365-246X.1990.tb06579.x">spent building mountains versus causing earthquakes</a>.</p>
<p>Seismologists do know the Zagros mountains host many active fault lines, and the tectonic wiggles on these faults cause a significant number of earthquakes in Iran and Iraq. In fact, about <a href="http://irsc.ut.ac.ir/">25,000 earthquakes have been recorded</a> in the Zagros mountains over just the past 11 years. Although these earthquakes are usually small in size, the data show that every now and then moderate to large events also occur; these can result in significant destruction.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=426&fit=crop&dpr=1 600w, https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=426&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=426&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=535&fit=crop&dpr=1 754w, https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=535&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/194692/original/file-20171114-30038-iyiuuq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=535&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Recorded earthquakes in the region are marked with gray circles. Major fault lines are in blue, with the Nov. 12 epicenter marked by a star.</span>
<span class="attribution"><span class="source">Amir Salaree</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The main fault responsible for the Nov. 12 earthquake has yet to be identified. As located by the Iranian Seismological Center, the quake took place in a zone between two major known faults: the High Zagros Fault and the Mountain Front Fault.</p>
<p>One good thing that comes from a big earthquake is more data about the structure of tectonic plates and therefore the seismic potential in the area. Researchers and planners can in turn use this information to prepare for future events. As the saying goes, we cannot predict earthquakes, but we can anticipate them.</p>
<h2>What was different about this quake</h2>
<p>Large earthquakes in Iran have typically caused a high number of fatalities. The <a href="https://doi.org/10.1093/gji/ggv044">1990 Rudbar</a> (magnitude 7.4) and <a href="http://www.iranicaonline.org/articles/bam-earthquake-2003">2003 Bam</a> (magnitude 6.6) earthquakes resulted in a total of around 55,000 deaths and as much as US$9 billion of economic loss.</p>
<p>According to the Iranian state-run news agency, the Nov. 12 earthquake <a href="http://english.alarabiya.net/en/News/middle-east/2017/11/15/Survivors-of-Iran-quake-await-badly-needed-aid-3-days-later.html">killed over 500 people</a>, as of this publication, with thousands injured, mostly on the Iranian side of the border. Registering a magnitude of 7.3, the quake was comparable in size to its 1990 and 2003 counterparts, but produced a relatively low number of casualties. This was due to several important factors.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/194936/original/file-20171116-19823-1d1rh84.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Even with a smaller death toll, thousands were left homeless, in need of aid and mourning those who were killed.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/APTOPIX-Iran-Iraq-Earthquake/bc828132f6e943338fce3dde21c3ce7b/5/0">AP Photo/Vahid Salemi</a></span>
</figcaption>
</figure>
<p>First, this latest earthquake was preceded by a much smaller magnitude 4.4 foreshock – a relatively smaller earthquake that precedes the largest earthquake <a href="https://earthquake.usgs.gov/learn/animations/aftershocks.php">in a series</a>. The foreshock caused many people to leave their homes and, in effect, escape the subsequent destruction. As a seismologist would tell you, earthquakes don’t kill people; buildings do.</p>
<p>Secondly, it occurred on much more rigid ground cover – mostly rocks instead of thick layers of unconsolidated soil compared to the other two events. <a href="https://earthquake.usgs.gov/hazards/urban/sfbay/soiltype/">These geological conditions</a> mean the seismic waves from the earthquake were less amplified, and so less shaking was observed on the surface.</p>
<p>Also, following the previous recent destructive earthquakes in Iran, the Iranian government <a href="http://iisee.kenken.go.jp/worldlist/26_Iran/Iran%20National%20Seismic%20Code_2007_3rd%20Version_English.pdf">passed new construction regulations</a> for more earthquake-safe buildings, calling for such things as concrete and steel frames and detailed study of the base soil prior to the construction. Considering the alarming foreshock, the smaller population in the affected towns (compared to the former two destructive earthquakes) and the unknown extent of enforcement of the building codes, it is difficult to estimate how the number of casualties would have increased in the absence of these laws.</p>
<p>For a more complete picture of this earthquake, we still need more data that are yet to be collected and documented both from field surveys and the study of seismic waves recorded by seismometers throughout the world. <a href="http://www.gsi.go.jp/cais/topic171115-index-e.html">Seismologists are looking</a> for further evidence about the propagation of the earthquake rupture to learn more about the internal characteristics of the fault as well as the properties of the convergence between the Arabian and Eurasian plates. They’ll also use seismic waves recorded from this earthquake to image the structure of Earth’s crust in the region – just like an ultrasound that provides a picture of your internal organs. </p>
<p>The aftermath of a seismic event like this one is an excellent opportunity to evaluate our understanding of earthquakes and their hazards in Iran and Iraq as well as elsewhere around the world.</p><img src="https://counter.theconversation.com/content/87480/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Amir Salaree does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The Nov. 12 earthquake wasn’t centered on any known major faults in the Earth’s crust. In its wake, scientists will collect data to add detail to what they know about seismic activity in the area.Amir Salaree, Ph.D. Candidate in Earth and Planetary Sciences, Northwestern UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/642812016-08-25T18:08:40Z2016-08-25T18:08:40ZHow a ‘weather bomb’ shook the Earth – and why that’s not an earthquake<figure><img src="https://images.theconversation.com/files/135529/original/image-20160825-6618-1r6itf3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>The Earth beneath your feet is “humming” all the time. Typically these vibrations are too faint and low-frequency for your ears to hear. But they can be detected by seismometers, the instruments designed to study the generally more powerful vibrations that emanate from earthquakes.</p>
<p>Now researchers have used an array of seismometers in Japan to show that a group of tremors they detected had their origin in a violent “<a href="http://science.sciencemag.org/content/353/6302/869">weather bomb</a>” storm on the other side of the planet off the coast of Greenland. There’s a danger that this research could be misreported as an Atlantic storm causing an earthquake in Japan. The reality is that the Japanese scientists detected an intensification of the usual background hum. But these vibrations could prove useful in helping us to study the structure of the planet.</p>
<p>The Earth typically hums slowly, with most energy transmitted at about ten seconds per vibration. But this is mixed up in a noisy continuum of overlapping “<a href="https://en.wikipedia.org/wiki/Seismic_noise">background noise</a>”, vibrations lasting from less than a second to half a minute each. These come from many sources including ocean waves in general, weak earthquakes deep within the Earth, and the planet creaking as it is deformed by tides.</p>
<p>When researchers Kiwamu Nishida and Ryota Takagi analysed the Earth’s hum on December 9-11 2014, as recorded by Japan’s <a href="http://www.hinet.bosai.go.jp/summary/?LANG=en">“hi-net” array of seismometers</a>, <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aaf7573">they realised</a> they were picking up some unusual signals. By working out the direction and distance the vibrations had travelled, the researchers back-tracked them to their source and showed that they came from storm waves shaking the shallow, sloping sea-floor off the south-east coast of Greenland. These waves were particularly violent because the local atmospheric pressure at the time was plummeting, creating a so-called “weather bomb”.</p>
<p>This was the perfect storm to set up pressure waves that resonated between the sea surface and the sea floor, passing their energy to corresponding vibrations in the bedrock that could be detected as far away as Japan. Nishida and Takagi do not say they had detected earthquakes that were caused by the storm. They were well aware that this was just an intensification of the usual background hum. </p>
<h2>Why not an earthquake?</h2>
<p>The vibrations caused by the Greenland storm are not an earthquake. <a href="http://earthquake.usgs.gov/earthquakes/map/">Most naturally-occurring earthquakes</a> happen near the boundaries between the tectonic plates into which the Earth’s rigid outer layer is divided. The plates are moving relative to each other at speeds of a few centimetres per year, but at the fault surfaces where one plate grinds past another the motion is not smooth. Friction and irregularities bind the two sides together until enough strain has been built up to overcome the resistance. The fault then gives way in a near-instantaneous jolt – much more powerful than the humming caused by storm waves that the Japanese researchers found. </p>
<p>Tectonic earthquakes are not restricted to plate boundaries, however. They can occur, usually less powerfully, when ancient faults move a little, or the Earth’s crust adjusts to the changing load of sediment upon it. A recent example was the <a href="https://www.theguardian.com/uk-news/2015/may/22/earthquake-hits-kent-shaking-houses-waking-residents">magnitude 4.2 earthquake</a> in Kent, England in 2015.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/wF7eaEAtjFc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The May 2015 Kent earthquake compared with the Nepal earthquake.</span></figcaption>
</figure>
<p>There are also some Earth tremors caused by human activity. This includes tremors caused by the ground shifting in <a href="http://earthquakes.bgs.ac.uk/earthquakes/recent_events/ollerton_earthquakes.html">former coal mining areas</a>, and efforts to pump water into the ground to heat it for <a href="https://www.theguardian.com/world/2009/dec/15/swiss-geothermal-power-earthquakes-basel">electricity generation</a>. </p>
<p>And then there is fracking. Here, deep layers of shale are artificially fractured to liberate trapped reserves of natural gas. This is a potentially vital source of gas for the UK if it wishes to free itself from <a href="http://uk.reuters.com/article/uk-centrica-gas-deals-idUKKBN0NY1FH20150513">dependence on Russian gas</a>, but it has had a bad press since fracking below Morecambe bay triggered a (non-damaging) magnitude 2.3 earthquake in 2011. This was a result of water pumped into the well <a href="https://www.newscientist.com/article/dn21120-how-fracking-caused-earthquakes-in-the-uk/">lubricating a previously jammed fault</a> rather than of the fracking process itself. Similarly, Oklahoma has seen a dramatic rise in magnitude 2 and 3 earthquakes since gas extraction from fracked shale began. These have caused mostly only minor damage, but it seems that the lesson is, if we want natural gas, then the fracking wells need to be situated well away from fault zones.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/F6PHVhjaNRI?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The link between earthquakes and fracking in Oklahoma.</span></figcaption>
</figure>
<p>The tremors picked up in Japan might not have counted as an earthquake, but we may be able to use these kind of vibrations in a similar way as we do earthquakes <a href="http://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/structureOfEarth.html">to study the internal structure</a> of the planet. For example, the speed at which waves travel through the Earth can reveal how dense the rock is that they are passing through. Knowing we can isolate the signals from storms could be particularly useful because the region where the “weather bomb” occurred almost never experiences earthquakes. So storms elsewhere may in time prove equally useful.</p><img src="https://counter.theconversation.com/content/64281/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>David Rothery is Professor of Planetary Geosciences at the Open University, where he chairs the introductory science short module S186 'Volcanoes, Earthquakes and Tsunamis'. Among his books are 'Volcanoes, Earthquakes and Tsunamis: A complete introduction'' (Hodder & Stoughton, 2015).</span></em></p>Scientists in Japan have discovered a way to ‘hear’ storms on the other side of the planet and use them to study the Earth’s crust.David Rothery, Professor of Planetary Geosciences, The Open UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/545552016-03-01T19:04:18Z2016-03-01T19:04:18ZExplainer: making waves in science<figure><img src="https://images.theconversation.com/files/111888/original/image-20160218-1243-ky6bkp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Making waves.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/_imax/2644301036/">Flickr/Max Nathan</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>We see them at the beach. They’re behind every sound and light show and the miracle of Wi-Fi. And now, thanks to what’s being called the discovery of the century, they have opened a way of detecting distant black-hole collisions.</p>
<p>I’m talking, of course, about waves.</p>
<p>We wouldn’t have speech or ultrasound imaging without sound waves. Water waves are a surfer’s paradise. Electromagnetic waves make both vision and television possible, as well as Wi-Fi, chest X-rays and microwave ovens.</p>
<p>It is electrical waves, not electrons, that sweep down our wires and power lines at close to the speed of light (the actual electrons drift along behind, <a href="http://sciencequestionswithsurprisinganswers.org/2014/02/19/what-is-the-speed-of-electricity/">at less than a snail’s pace</a>!). </p>
<p>And the recent <a href="https://theconversation.com/gravitational-waves-discovered-the-universe-has-spoken-54237">discovery of gravitational waves</a> will open up a new frontier in astronomy.</p>
<h2>What’s in a wave?</h2>
<p>Waves are very different from particles. Waves have energy, but not mass. They love to diffract or spread out, not stay in fixed lumps.</p>
<p>When two waves meet they don’t bounce off each other: they just add and subtract as they pass through each other, and then carry on their ways as if they’d never met. This is called interference, and it makes waves highly unsuitable for snooker, but it is what lets many people use their mobile phones at the same time.</p>
<p>Water is a good example for thinking about the difference between waves and particles. Water can carry energy in two different ways. </p>
<p>First, it can flow from one place to another, such as from the river to the sea. In such a flow, each water molecule starts upstream and moves downstream. The flow is made up of particles.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/112121/original/image-20160219-21502-12gsfdc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The ripples are waves in the water.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/scott-s_photos/7904846012/">Flickr/Scott Cresswell</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>But imagine the ripples spreading out from a dropped pebble in a pond, or watching the waves spread out from the bow of a passing boat. These waves also carry energy as, for example, they rock floating sticks and even push them along a little. </p>
<p>But the water molecules that make up the shape of the ripple just after the pebble is dropped are completely different to the ones that make up the ever-spreading ripple five seconds later. Each water molecule stays roughly where it is, barring some jiggling, while the wave moves on. So water waves are not a flow of particles.</p>
<h2>So how do waves move?</h2>
<p>When that pebble is dropped in the pond, it pushes water out of the way. The water has nowhere to go but to the side and up, creating a circular peak around the drop point. This peak falls again, under the forces of gravity and surface tension, pushing the water beneath it out of the way.</p>
<p>On the inside of the circle, this newly pushed water fills the hole left by the pebble passing through. But on the outside, it creates a new circular peak, just a little further out.</p>
<p>So a ripple spreads out from the drop point even though the individual water molecules are mostly just moving up and down in place.</p>
<p>More generally, waves need something to wave in: a medium. Water, air, power lines and the electromagnetic field are all suitable media. Even spacetime itself will do, in the case of gravitational waves.</p>
<p>Waves are simply distortions moving through the medium. These distortions can be started off by many means: a dropped pebble, a shout, a radio transmitter or colliding black holes.</p>
<p>In each case, the medium has some degree of elasticity and responds to a distortion by trying to snap back into shape. But this distorts the neighbouring region, and so on, and so a wave is born. </p>
<p>The strength of the distortions is called the amplitude of the wave, and is closely related to its energy.</p>
<h2>Catching the perfect wave</h2>
<p>All waves, whether in water, air or spacetime, can come either in pulses, such as a sharp sound, or as a collection of ripples, such as at the beach. But no matter what shape and size, any wave can be thought of as made up of many perfect waves added together. </p>
<p>A perfect wave is what we hear when a singer holds a single beautiful note. It is a smooth series of peaks and troughs in the strength of the wave, with successive peaks all separated by the same distance: the wavelength. The number of peaks passing a given point every second is called the frequency.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=509&fit=crop&dpr=1 600w, https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=509&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=509&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=640&fit=crop&dpr=1 754w, https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=640&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/112479/original/image-20160223-16455-1h8xsog.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=640&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p>Every wave is a combination of interfering perfect waves, and so has a spectrum of different frequencies. Visible light waves, for example, have a spectrum of colours, with each colour corresponding to a different frequency.</p>
<p>They can actually be separated out into their spectrum by a prism, as Isaac Newton famously showed to develop his theory of colours. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/69610/original/image-20150121-29731-1ciiybx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A prism reveals the many colours of visible light.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/23629083@N03/14200678625">Flickr/final gather</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Different radio and television stations transmit their signals on waves made up of different frequency bands, so that we can tune into the frequency we want. </p>
<p>The distortions of perfect waves, at any given point in the medium, fluctuate up and down in strength either along the same direction the wave is moving (longitudinal waves), or at right angles (transverse waves). These choices depend on the medium, and are called polarisations.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=741&fit=crop&dpr=1 600w, https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=741&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=741&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=931&fit=crop&dpr=1 754w, https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=931&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/113173/original/image-20160229-27003-1gyu6cp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=931&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Longitudinal and transverse waves.</span>
</figcaption>
</figure>
<p>Sound waves in air are longitudinally polarised, light and gravitational waves are transversely polarised, while the seismic waves causing earthquakes come in both varieties. As do <a href="https://www.youtube.com/watch?v=ilZj8JUTvy8">slinky waves</a>!</p>
<p>Perfect transverse waves have a further choice of the different directions at right angles to the direction the wave is moving in. Polaroid sunglasses take advantage of this, blocking the glare that comes from horizontal fluctuations, while letting through vertically polarised waves.</p>
<h2>Measuring waves</h2>
<p>Measuring waves is important in many parts of science, whether it gives us information about the source of the waves, or about the medium that they have travelled through.</p>
<p>For example, light waves emitted from the sun give us information about its temperature and composition, while light waves passing through a microscope slide can tell us whether someone needs medical treatment or not.</p>
<p>In all cases, the wave must be detected by some means, such as an eye or a camera. </p>
<p>Significant progress in science is made every time we learn how to generate or control or detect a new type of wave. Electromagnetic waves were only <a href="https://theconversation.com/let-there-be-light-celebrating-the-theory-of-electromagnetism-35723">discovered 150 years ago</a> and look at the use we make of them now, as mentioned before: radio, television and microwaves, to name just a few.</p>
<p>Gravitational wave detection is the most recent example, providing a unique window on those events strong enough to shake space and time themselves.</p>
<h2>A quantum twist</h2>
<p>At atomic scales and smaller, the distinction between waves and particles becomes somewhat blurred.</p>
<p>Sufficiently chilled-out atoms can start behaving as if they are spread out and <a href="https://www.learner.org/courses/physics/visual/visual.html?shortname=route_to_bec">overlapping each other</a>, rather like waves. And if the intensity of a light beam is dialled down enough, it is found to only illuminate <a href="https://www.youtube.com/watch?v=GzbKb59my3U">a single camera pixel at a time</a>, as if the beam was made up of particles.</p>
<p>Quantum mechanics tells us that waves and particles are fundamentally two sides of the same coin: different kinds of distortions in a medium. But the nature of the quantum medium is a profound mystery that drives the research of many scientists around the world (including my own).</p>
<p>It is only with its solution that we will finally understand just what waves are.</p><img src="https://counter.theconversation.com/content/54555/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Hall receives funding from the Australian Research Council and the Foundational Questions Institute.</span></em></p>We find them at the beach, in every sound and light show, the miracle of wi-fi and now in the fabric of space-time itself. But what exactly is a wave?Michael Hall, Senior Research Fellow, Griffith UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/440282015-07-02T05:14:10Z2015-07-02T05:14:10ZOur new anti-earthquake technology could protect cities from destruction<p>Protecting cities from earthquakes is still a grand challenge that needs addressing, as recent disasters in <a href="https://theconversation.com/many-feared-dead-as-second-quake-hits-devastated-nepal-41712">Nepal</a>, <a href="http://www.theguardian.com/world/2015/may/30/japan-earthquake-85-magnitude-ogasawara-islands">Japan</a>, <a href="https://theconversation.com/natural-disasters-put-haiti-and-philippines-on-the-map-20879">Haiti</a>, and <a href="http://edition.cnn.com/2014/04/01/world/americas/chile-earthquake/">Chile</a> confirm. Although significant progress has been made in understanding <a href="https://theconversation.com/rumbling-from-ocean-trenches-could-be-sign-that-japan-faces-mega-earthquake-41464">seismic activity</a> and developing building technology, we still don’t have a satisfactory way of protecting buildings on a large scale. </p>
<p>For new buildings, anti-seismic technology is today considered quite advanced and it is possible to build individual structures that can withstand the vast majority of recorded earthquakes. Devices such as isolation systems and dampers, which are designed to reduce the vibrations (and as a consequence the damage) of structures induced by earthquakes, are successfully employed in the design of new buildings.</p>
<p>But large numbers of buildings exist in earthquake zones that don’t have built-in protection, particularly in developing countries where replacing them or introducing stricter – and more expensive – building codes aren’t seen as an option. More than <a href="http://www.un.org/press/en/2015/db150430.doc.htm">130,000 houses</a> were destroyed by the earthquake in Nepal in April 2015.</p>
<p>What’s more, these technologies are <a href="https://theconversation.com/how-earthquake-safety-measures-could-have-saved-thousands-of-lives-in-nepal-40907">rarely used</a> for protecting existing buildings, as they generally require substantial alteration of the original structure. In the case of heritage buildings, critical facilities or urban housing especially in developing countries, traditional localised solutions might be impractical. </p>
<p>This means there is a need for alternative solutions that protect multiple existing buildings without altering them using a single device. At the University of Brighton, we have designed a novel <a href="http://rspa.royalsocietypublishing.org/content/471/2179/20150075">vibrating barrier</a> (ViBa) to reduce the vibrations of nearby structures caused by an earthquake’s ground waves. The device would be buried in the soil and detached from surrounding buildings, and should be able to absorb a significant portion of the dynamic energy arising from the ground motion with a consequent reduction of seismic response (between 40-80%).</p>
<p>The idea behind this is to look at buildings as an integral part of a city model, which also includes the soil underneath and the interaction between each element, rather than as independent structures. Each ViBa can be designed to protect one or more buildings from an earthquake but also it forms part of a network of devices placed at strategic locations in order to protect entire cities.</p>
<p>The ViBa itself is essentially a box containing a solid central mass held in place by springs. These allow the mass to move back and forth and absorb the vibrations created by seismic waves. The entire structure is connected to the foundations of buildings through the soil to absorb vibrations from them. The box’s position underground would depend on how deep the surrounding foundations went and could even be placed on the surface.</p>
<p>As the ViBa is designed to reduce all vibrations in the soil, it could also be used to insulate buildings against ground waves from human activities such as road traffic, high-speed trains, large machinery, rock drilling and blasting. In this way, the technology would be able to absorb a larger quantity of energy than traditional measures used to insulate railways such as trenches or buried sheet-pile walls.</p>
<h2>Starting construction</h2>
<p>The problem with the ViBa is its size – it would need to be at least 50% of the mass of the average building it was protecting – and how much money it would cost to build and install as a result. So compared to current technologies to protect single buildings it would likely come with a much higher price tag. But as the ViBa can be designed to reduce the vibrations of more than one building or for buildings of historical importance for which current technologies are impractical, it can still be considered as a viable solution.</p>
<p>So far we have only modelled how the ViBa would work, using computers and prototypes in the lab. To be deployed in the real world we would need to do a lot more experimenting to understand exactly how it would work and to make sure it didn’t produce any damaging side-effects on the surrounding buildings. We would also need to work with industry to work out how to build and install it in the most cost-effective way.</p>
<p>But our <a href="http://rspa.royalsocietypublishing.org/content/471/2179/20150075">latest research</a> suggests the ViBa is a viable alternative strategy for protecting buildings from earthquakes. In the long term, it could lead to safer cities that are better equipped to deal with disasters and ultimately save lives.</p><img src="https://counter.theconversation.com/content/44028/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Pierfrancesco Cacciola receives funding from EPSRC.</span></em></p>Retrofitting old or cheap houses with earthquake protection is often expensive and laborious. What if we could save whole streets at a time?Pierfrancesco Cacciola, Assistant head, School of Environment and Technology, University of BrightonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/414642015-05-11T05:25:07Z2015-05-11T05:25:07ZRumbling from ocean trenches could be sign that Japan faces mega earthquake<figure><img src="https://images.theconversation.com/files/80877/original/image-20150507-1224-1cb9xsh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ocean bottom seismometer floating after releasing its anchor on the seafloor.</span> <span class="attribution"><a class="source" href="http://www.eurekalert.org/jrnls/sci/pages/yamashita-05-08-15.html">Yusuke Yamashita, ERI, Univ. of Tokyo, Japan</a></span></figcaption></figure><p>Researchers in Japan have for the first time<a href="http://www.sciencemag.org/lookup/doi/10.1126/science.aaa4242"> detected and traced shallow tremors</a> under the ocean that could be a sign that the country is heading towards a huge earthquake. But the technique itself may one day help us predict exactly when such an event would take place, which could save thousands of lives.</p>
<p>Japan still has the devastating 9.0 magnitude, megathrust earthquake in Tohoku in fresh memory, which produced a powerful tsunami and <a href="http://www.npa.go.jp/archive/keibi/biki/higaijokyo_e.pdf">killed nearly 16,000 people</a> when it hit in 2011. It is therefore no wonder that Japanese researchers are the first to detect weak signals of seismic activity.</p>
<p>Japan already has the most powerful seismic network in the world – and research institutions in the country are constantly growing it. Ocean Bottom Seismometers, which measure motion under the sea, have greatly facilitated these efforts by listening to the “rumbling” that is created when <a href="http://www.bbc.co.uk/science/earth/surface_and_interior/plate_tectonics">two tectonic plates</a> meet. Such instruments have helped detect low-energy, “<a href="http://www.scec.org/news/00news/feature000401.html">slow earthquakes</a>” along oceanic trenches that we otherwise wouldn’t notice.</p>
<p>These earthquakes, which <a href="http://www.sciencemag.org/content/307/5708/389">we know are produced</a> deep under the famous San Andreas fault, <a href="http://www.sciencemag.org/content/335/6069/705">preceded the Tohoku Earthquake</a>. They occur much more slowly than standard earthquakes. If they are associated with the underground movement of magma and hot water but they are not related to volcanoes, they are knows as “<a href="http://www.sciencemag.org/content/296/5573/1679">non-volcanic tremors</a>”. By comparison, big earthquakes are caused by the rupture of faults and give rise to short-lived, high-energy seismic waves.</p>
<p>Slow-slip earthquakes and tremors don’t cause any damage on their own. However, if they coincide with very-low-frequency earthquakes they can. These are another type of slow earthquake that is caused by processes deeper down under ground than tremors and usually indicate fault motions near the dangerous area where the tectonic plates meet. If all these types of slow earthquakes take place, along the faulted zone at different depths, they could be a sign we are near to a mega-thrust earthquake.</p>
<p>The researchers – who investigated the <a href="http://www.geographic.org/geographic_names/name.php?uni=-239293&fid=6441&c=undersea_features">Kyushu Palau Ridge</a>, southeast of Kyushu – have, for the first time, been able to detect and map shallow tremors in correlation with the other kinds of slow earthquakes. Even more importantly, they have showed what direction all these events are moving in. This kind of detailed knowledge of seismic activity is considered one of the most reliable ways of predicting big earthquakes. </p>
<h2>Warning signs</h2>
<p>What the study found out is that the waves produced by all these quiet earthquakes consistently migrated north along the ridge. The movement abruptly ended at the limit of the trench, where it was blocked by a so-called <a href="http://earthquake.usgs.gov/learn/glossary/?term=locked%20fault">locked zone</a> – where friction keeps the two plates together so they can’t slip – where previous mega-thrust events have occurred. After this, the waves travelled east.</p>
<p>This does not look promising, as to avoid a mega-thrust earthquake you’d prefer the slow quakes to stay in a locked zone, where the stress caused by them can be released and the movement can fizzle out. In this case, however, they are probably causing the coupling between the two plates to weaken, which is expected before a mega-thrust event.</p>
<p>The study, which was published in Science on May 7, shows that shallow slow earthquakes may therefore become a reliable way of detecting when and where the next mega earthquake will strike. This can be done by deploying ocean bottom seismometers along different trenches. In that way, we could detect the pattern of earthquakes in various places so that they would become an exact marker of when any mega-thrust earthquake strikes under the ocean, often causing a tsunami as well.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=451&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=451&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=451&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=567&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=567&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80878/original/image-20150507-1215-1bvooqf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=567&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Deploying an ocean bottom seismometer.</span>
<span class="attribution"><a class="source" href="http://media.eurekalert.org/scipak/gallery/images/2015-05/yamashita2HR.jpg">Yusuke Yamashita, ERI, Univ. of Tokyo, Japan</a></span>
</figcaption>
</figure>
<p>The next such earthquake could strike the coast of Kyushu, a region well known for its dangerous volcanoes. Let’s hope that, by then, we have come far enough to prevent the same devastation as we saw in 2011. No place is better than Japan to drive such technological progress.</p><img src="https://counter.theconversation.com/content/41464/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Luca De Siena does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Japan has the most powerful seismic network in the world. And this network is throwing out some warning signs.Luca De Siena, Lecturer in Geophysics, University of AberdeenLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/344722015-05-01T17:54:20Z2015-05-01T17:54:20ZSeismologists deploy after a quake to learn more, so we can prepare for the next one<figure><img src="https://images.theconversation.com/files/79990/original/image-20150430-30705-10ausqo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Surface measurements hint at what's going on within.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/ctbto/15825401591">CTBTO</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><blockquote>
<p>The past is never dead. It’s not even past.
– William Faulkner</p>
</blockquote>
<p>When disasters like the <a href="http://ds.iris.edu/ds/nodes/dmc/specialevents/2015/04/25/nepal/">Nepal earthquake</a> strike, seemingly out of the blue, one can’t help but feel anguish at the mismatch between the capacity of human memory and the tenacity of denial. The simple truth about great earthquakes, and the miserable cascade of events they often trigger, is this: if an earthquake has affected a region, recently or in historical records, then future earthquakes in that region are inevitable. But, if no damaging earthquake has happened in recent memory, it’s easy to ignore the need to prepare for a future event of uncertain magnitude and proximity. The earthquake cycle is long relative to the terms of a city council, a state legislature, and even a national government.</p>
<p>As a practicing seismologist, the political questions implicit in a discussion of how much risk a society is prepared to assume relative to the costs of mitigation are largely beyond my influence. On the other hand, seismologists like me can help address the question of where earthquakes have occurred in the past – and where they will occur again in the future.</p>
<p>We can estimate how large a magnitude earthquake can be expected in a given region. We can determine <a href="http://earthquake.usgs.gov/regional/nca/soiltype/">how different substrates</a> – soils, sand, fill, bedrock – will affect ground shaking, and we can <a href="http://www.gsi.go.jp/ENGLISH/page_e30085.html">map the distribution</a> of these foundational materials on a building-by-building scale, if necessary. We can assess the propensity for <a href="http://landslides.usgs.gov">slope failure</a>, which leads to landslides. And, for some regions, we can come up with ballpark <a href="http://serc.carleton.edu/quantskills/methods/quantlit/RInt.html">estimates of the average time</a> between large-magnitude earthquakes.</p>
<p>Even after a major quake, there’s much seismologists can learn that can hopefully help people prepare for the next one. </p>
<h2>What do we want to know?</h2>
<p>Scientists and policymakers ideally want to forecast the time, place and magnitude of a future earthquake. Knowing that information well in advance, we could issue a region-specific targeted alert, complete with estimates of expected shaking. Such knowledge would allow for the maximum safeguarding of populace and infrastructure. Perfect forecasting would also mean no disastrous <a href="http://www.nytimes.com/2012/10/27/opinion/a-failed-earthquake-prediction-a-crime.html?_r=0">failures-to-predict</a> and no false alarms.</p>
<p>So what can seismologists do to get closer to this goal?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/79991/original/image-20150430-30696-1v9qkjq.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Quakes happen along the edges of the planet’s tectonic plates.</span>
<span class="attribution"><a class="source" href="http://commons.wikimedia.org/wiki/File:Quake_epicenters_1963-98.png">NASA, DTAM project team</a></span>
</figcaption>
</figure>
<h2>It all comes down to plate tectonics</h2>
<p>In seismology, our framework for understanding earthquakes begins with <a href="http://www.livescience.com/37706-what-is-plate-tectonics.html">plate tectonics</a> theory. The Earth’s surface is divided into around 12 major shell-like plates that move relative to one another. Earthquakes happen when the plates rub against each other or collide. We’ve observed that the vast majority of earthquakes occur within the wide (60-600 miles; 100-1,000 km) boundary zones at the edges of the slowly, continuously moving plates. Within these boundaries, plate motions are typically distributed on many active faults that sometimes slip – benignly! – slowly and continuously like the plates. But far more often the plate boundaries stick and are motionless for long periods before suddenly rupturing and producing catastrophic large-magnitude earthquakes.</p>
<p>Given the slow, steady motion of the plates, you might think that earthquakes on plate boundary faults would rupture periodically, say every few decades or centuries, when the stresses that build up on the faults due to the steady motions become greater than the frictional strength holding the fault still. Seismologists have been looking for such nicely behaved faults since the first precision-instrument recordings of earthquakes in 1889, but to no avail. We’ve yet to discover a predictable fault that has a quake right on schedule every 80 years, for example.</p>
<h2>Recording at the surface for hints from within</h2>
<p>We already know a lot about most major faults – where they are, their extents and depths, and at least their recent destructive histories. But there are many crucial things about these faults we don’t understand. The best-studied faults are basically covered with various instruments recording seismic phenomena, and I do mean covered: these observations are made only at the Earth’s surface, or very shallow depths. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80015/original/image-20150501-30721-2zs6m3.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Setting up surface sensors to record seismic waves after 2010 earthquake in Chile.</span>
<span class="attribution"><span class="source">Ray Russo</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>We rely on <a href="http://crack.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html">seismic waves</a> generated by earthquakes themselves to characterize the faults and their behavior. These waves of energy spread out from a rupturing fault and are recorded on seismometers and other geophysical instruments. Their characteristics, recognizable to seismologists, tell us about the type of earthquake rupture and the extent of the faulting. But, because these waves travel through complex materials on their way to the Earth’s surface, our ability to ‘see’ details of what happens at depth is inevitably compromised.</p>
<p>Seismic recordings have taught us that major fault zones are complex, typically involving multiple surfaces on which slip can and does occur. These surfaces are usually not continuous, but rather indicate that the major faults are segmented - planes of slightly different orientations juxtaposing <a href="http://nsf.gov/news/news_summ.jsp?cntn_id=110106">potentially very different materials</a>. Different segments of the fault zone can slip apparently independently, although they do influence each other.</p>
<p>Fault surfaces are rough, not smooth, and marked by asperities: sharp bumps, knobs and ridges on the walls of the fault that jab from one side into the other, creating locked points or patches. Stronger patches are more likely to remain locked until the steady plate motions build up enough to break them, while weaker patches slip slowly and steadily. <a href="http://earthquake.usgs.gov/research/parkfield/fluids.php">Groundwater flow</a> may both weaken fault rocks by dissolving minerals, or strengthen a patch of fault through precipitation of new minerals.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=376&fit=crop&dpr=1 600w, https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=376&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=376&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=473&fit=crop&dpr=1 754w, https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=473&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/79994/original/image-20150430-30726-1hmm6ue.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=473&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Japan’s 2011 Tōhoku earthquake registered on seismograms in Hawaii.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/parksjd/7245859020">Joe Parks</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>For every large-magnitude earthquake that occurs on a fault system, thousands or even tens of thousands of little earthquakes will occur. These low-magnitude events can be triggered by small changes in stress on the fault. For example, when seismic waves from a large-magnitude quake somewhere else in the world pass by segments of California’s San Andreas fault, the fault lights up with <a href="http://www.sciencedaily.com/releases/2009/07/090709140817.htm">lots of little tremors</a>. So we infer that many faults are near ‘criticality’ – at least some patches of the fault segments are ready to slip at any time, just waiting for a minuscule amount of stress to be applied.</p>
<p>If the faults are actually moving, just a little bit, essentially all the time, what has to happen for these little motions to coalesce into the big slip over a large area that would be a huge quake? Seismologists have been looking for consistently observed precursory phenomena – some change in fault behavior or structure that always, reliably, occurs before or even during the cascading of little earthquakes into a monster earthquake. So far, we haven’t found it.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=385&fit=crop&dpr=1 600w, https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=385&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=385&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=483&fit=crop&dpr=1 754w, https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=483&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/79992/original/image-20150430-30721-bl45jv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=483&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Unloading seismic monitoring equipment in the field.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/ctbto/15642118967">CTBTO</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Lots of science to be done after a big quake</h2>
<p>Ironically, large-magnitude earthquakes like the Nepal event provide some of the most useful information for seismic hazard mitigation: the thousands of aftershocks in the following days and months occur all along the surface of the fault segments that ruptured. Seismologists usually rush to <a href="http://www.earthscope.org/assets/uploads/pages/Wi11_MauleAftershockDeploy.pdf">deploy many temporary seismic stations</a> in the rupture region to record these aftershocks and then locate them with high precision – thus defining the fault’s slip surface accurately.</p>
<p>To do this well, we need to surround the rupture region with sensors that turn shaking due to seismic waves into electrical signals that are then recorded on a weather-proofed computer hard disk. The seismograms they record show the ground moving up and down and side-to-side systematically as the waves travel past the sensor.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=373&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=373&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=373&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=468&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=468&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80014/original/image-20150501-30716-1k3ycn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=468&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Example of a seismic microzonation map for the city of Bangkok.</span>
<span class="attribution"><a class="source" href="http://en.wikipedia.org/wiki/File:Bangkok_microzonation_map.jpg">Tuladhar, R., Yamazaki, F., Warnitchai, P & Saita, J., Seismic Microzonation of the Greater Bangkok area using Microtremor Observations, Earthquake Engineering and Structural Dynamics,v33, 2004: 211-225</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The aftershock team’s work affords us an accurate measure of these parameters. Then we can make a firm estimate of the largest magnitude earthquake a particular cascading sequence of rupturing fault segments can produce. The upper magnitude limit for the region can then be used to estimate the maximum expected shaking, and, in combination with studies of substrate materials, <a href="http://earthquake.usgs.gov/hazards/designmaps/">expected hazard maps</a> can be produced, building codes updated based on realistic expectations, and civil defense planning focused to mitigate specific disaster scenarios.</p>
<h2>How to protect against future quake disasters?</h2>
<p>The <a href="http://earthquake.usgs.gov/earthquakes/eventpage/us20002926#general_summary">Nepal earthquake</a> was long expected. A <a href="http://www.rediff.com/news/report/nepal-earthquake-is-an-eerie-reminder-of-1934-tragedy/20150425.htm">predecessor event in 1934</a> ruptured an even greater area, yielding a higher magnitude quake. And if earthquake preparedness there received less-than effective attention given this clear warning, imagine how much more difficult it is to motivate preparation in places that are susceptible to huge earthquakes, but whose most recent big quake occurred long before any of us were born, even before written history…. The past is never truly past, indeed!</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/79993/original/image-20150430-30709-z5suvr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Nepal’s earthquake caused countless buildings to crumble.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/unitednationsdevelopmentprogramme/17250166966">United Nations Development Programme</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>Globally, we need a program of identification and characterization of potentially hazardous faults in urban areas. From those studies, site-specific expected seismic shaking maps can be developed and construction codes and engineering design specifications for infrastructure enacted, mitigating hazard to new and future construction.</p>
<p>Then urban political leaders and civil defense agencies must collaborate to lead local populations in an open and honest dialog to identify both irreplaceable cultural heritage, and also infrastructure that must survive natural disasters intact in order to prevent an earthquake from triggering a series of consequent catastrophes – fires, water and food shortages and disease outbreaks. These structures should be retrofitted to survive the predicted shaking from the maximum expected magnitude earthquake for the given area. A number of different mechanisms to pay for this costly preventive engineering are almost certainly needed, tailored to local conditions.</p>
<p>It’s clear the Earth has moved before and will move again, but will we move to do what’s necessary to mitigate preventable disasters?</p><img src="https://counter.theconversation.com/content/34472/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ray Russo receives funding from the US National Science Foundation.</span></em></p>For seismologists, there’s much to be learned after a major earthquake, as aftershocks help them map out the fault with high precision. More data now can prepare a region for its next big one.Ray Russo, Associate Professor of Geophysics, University of FloridaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/407412015-04-24T13:28:39Z2015-04-24T13:28:39ZYellowstone earthquakes reveal a volcanic system six times bigger than we thought<figure><img src="https://images.theconversation.com/files/79151/original/image-20150423-25553-ug9ubp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">I've been underestimated for too long.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/allan_harris/4835019324">alh1/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>Seismologists have discovered a massive magma reservoir beneath the <a href="http://www.volcano.si.edu/volcano.cfm?vn=325010">Yellowstone supervolcano</a> in Wyoming, US, that suggests its volcanic system could be more than 5.6 times larger than was previously thought. </p>
<p>Although it was already known that Yellowstone had one magma reservoir, located about 5-16km (3-10 miles) below the surface, <a href="http://www.sciencemag.org/content/early/2015/04/22/science.aaa5648.abstract">the new study</a>, published in Science, has revealed another, much larger reservoir sitting directly below the first, located around 20-50km (12-30 miles) below the surface. This reservoir is thought to have a volume of around 46,000 cubic km – compared to a volume of around 10,000 cubic km for the shallow reservoir.</p>
<p>To make their discovery scientists analysed the vibrations made by earthquakes that passed beneath the volcano. The technique not only sheds light on this volcano’s <a href="https://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=130898">potentially life-threatening eruptions</a> but it could also help us understand other volcanoes such as <a href="https://theconversation.com/calbuco-volcano-evacuations-and-air-traffic-disruption-follow-eruption-40720">Calbuco</a>, which is currently erupting in Chile. </p>
<h2>Sleeping beauty</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/79248/original/image-20150424-14585-5igqfm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Luckily no sign of an eruption anytime soon.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/98674575@N02/9270994938">gcnmrk5ii/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Yellowstone volcano is composed of an immense volcanic crater - known as a caldera - more more than 70km (44 miles) in length, most of which lies within Yellowstone National Park. The volcano rarely erupts lava (it last did so about <a href="http://pubs.usgs.gov/fs/2005/3024/">70,000 years ago</a>, but the magma lying beneath the surface gives rise to spectacular geothermal features, such as geysers and colourful hot springs. </p>
<p>The last large eruption at Yellowstone <a href="http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_54.html">was 640,000 years ago</a>, and ejected around 1,000 cubic kilometres (240 cubic miles) of volcanic material. This cataclysm created the Yellowstone caldera. To get an idea of the scale of this, the largest eruption in recorded history, <a href="http://www.sciencedirect.com/science/article/pii/037702738690079X">Mount Tambora in 1815</a>, erupted about a sixth of that. </p>
<p>Magma reservoirs are thought to occur beneath most volcanoes, and play a crucial role in the dynamics of eruptions. However, they are too deep, and conditions within them too extreme, to be measured directly so volcanologists have to infer information about them using other means, such as measuring seismic waves. These waves travel more slowly when they pass through molten rock, and accordingly the group were able to use the velocities of the earthquake waves to infer the presence of a large, deep zone of partially molten material. </p>
<h2>Carbon footprint explained</h2>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1131&fit=crop&dpr=1 754w, https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1131&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/79243/original/image-20150424-14549-oz0jhj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1131&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Powerful fissures.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/jurvetson/4780005143">Steve Jurvetson/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The magma stored in the deeper reservoir probably doesn’t cause eruptions at Yellowstone directly. Instead, it likely acts as a “feeder” for the smaller, shallower reservoir – which is the ultimate source of the volcano’s catastrophic eruptions. Scientists had suspected the existence of a second magma reservoir at Yellowstone for some time, but this new evidence is among the strongest support of the theory to date. </p>
<p>The discovery of this second magma reservoir may also help to explain a mysterious feature of the Yellowstone volcano: its carbon footprint. <a href="http://volcanoes.usgs.gov/hazards/gas/climate.php">Carbon dioxide gas</a> is commonplace at volcanoes (it is given off by rising magma), but Yellowstone’s output, which is around <a href="http://adsabs.harvard.edu/abs/2006AGUFM.V33C0696E">45,000 tonnes</a> of CO<sub>2</sub> each day, was too high to be explained by a single magma reservoir. But according to the study’s authors, the presence of the new reservoir is enough to account for the volcano’s CO<sub>2</sub> flux. </p>
<p>If the high-resolution seismic imaging technique used in the study could be repeated at other volcanoes whose deep structure is poorly understood – such as Calbuco volcano in Chile – volcanologists might eventually be able to understand how such eruptions take place. The first stirrings of volcanic eruptions happen far below the surface. If researchers can emulate the findings at Yellowstone at other volcanoes, it can only tell us more about the risks they pose.</p><img src="https://counter.theconversation.com/content/40741/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Robin Wylie does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Earthquake analysis could help us understand the deep structure of volcanoes.Robin Wylie, PhD researcher in Volcanology, UCLLicensed as Creative Commons – attribution, no derivatives.