tag:theconversation.com,2011:/us/topics/italy-earthquake-30618/articlesItaly earthquake – The Conversation2016-08-31T20:09:09Ztag:theconversation.com,2011:article/645482016-08-31T20:09:09Z2016-08-31T20:09:09ZStill standing: how an ancient clock tower survived Italy’s deadly earthquake<p>Of the many devastating pictures to come out of central Italy after <a href="https://theconversation.com/italys-deadly-earthquake-is-the-latest-in-a-history-of-destruction-64384">last week’s deadly earthquake</a>, the clock tower of Amatrice standing defiantly amid the rubble of the town has become an iconic image. </p>
<p>The clock tower was reportedly built in the 13th century and its solid stance defies us to understand how this remarkable structure has evaded destruction at least twice in the past 800 years.</p>
<p>But perhaps surprisingly, it’s not unusual for tall, ancient structures to survive earthquakes.</p>
<h2>Unlikely survivors</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1052&fit=crop&dpr=1 600w, https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1052&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1052&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1322&fit=crop&dpr=1 754w, https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1322&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/136013/original/image-20160831-789-pegxwp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1322&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">Nepal’s Dharawara tower in 2013, before it was destroyed in the 2015 earthquake.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:KATHMANDU_NEPAL_FEB_2013_(8581665041).jpg">KATHMANDU NEPAL FEB 2013</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
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<p>Similar towers are relatively commonplace in Italy and part of the country’s charm. The town of San Gimignano, about 200km from the centre of the Amatrice earthquake, has <a href="http://www.atlasobscura.com/places/towers-san-gimignano">14 towers</a> that date as far back as the 12th century – and have consequently survived many earthquakes big and small. Other towers can be seen in <a href="https://hal.archives-ouvertes.fr/file/index/docid/299422/filename/nhess-7-251-2007.pdf">Alba in northern Italy</a>.</p>
<p>Further afield, a memorable image of the Izmit earthquake in Turkey in 1999 was of the tower of the <a href="http://earthquake.usgs.gov/earthquakes/events/1999izmit/">Golcuk Mosque</a> standing forlornly among the ruins. </p>
<p>Photos from the 1906 San Francisco earthquake show a <a href="http://c8.alamy.com/comp/FFT0G5/san-francisco-earthquake-na-man-and-woman-standing-amid-the-rubble-FFT0G5.jpg">slender tower</a> and an <a href="http://www.gettyimages.com.au/detail/photo/chimneys-tower-above-the-rubble-after-the-high-res-stock-photography/128594760">array of chimneys</a> standing in the rubble of the city.</p>
<p>In many instances, however, the towers fall, as happened to the <a href="https://en.wikipedia.org/wiki/Dharahara">Dharahara</a> tower during the magnitude-7.8 <a href="https://theconversation.com/the-science-behind-the-nepal-earthquake-40835">Nepal earthquake</a> in April 2015.</p>
<p>Why do some of these slender icons survive repeated earthquakes and others fall? An article in <a href="http://www.economist.com/news/europe/21705981-government-offers-incentives-earthquake-proof-homes-few-take-them-up-quake-prone">The Economist</a> suggested that the clock tower was better constructed than the surrounding buildings, pointing out that it even survived better than a modern school and hospital. The <a href="https://en.wikipedia.org/wiki/2009_L%27Aquila_earthquake">L'Aquila experience</a> suggests that this is probably one part of the story. </p>
<p>However, the reality is more complex. Other factors can and do contribute to the resilience of buildings.</p>
<h2>On shaky ground</h2>
<p>It is very likely that the clock tower’s survival was influenced by the relationship between the frequency of the earthquake waves and the <a href="https://www.iris.edu/hq/inclass/animation/building_resonance_the_resonant_frequency_of_different_seismic_waves">natural resonance</a> of the building. To understand why, we have to consider how earthquakes interact with buildings.</p>
<p>Earthquakes generate seismic waves that pass through the ground. Like ocean waves, they have peaks and troughs. The frequency of the wave is related to its “period” – the time taken for one complete waveform (including a peak and a trough) to pass.</p>
<p>A building has a natural period that causes it to vibrate back and forth. Think of a child on a swing – a swing with short ropes will complete a full cycle much more quickly than a long swing. </p>
<p>The same is true of buildings with different heights. A building is effectively an <a href="http://bssa.geoscienceworld.org/content/53/2/403">upside-down pendulum</a> and <a href="http://blogs.agu.org/tremblingearth/2011/05/05/swaying-high-rises-and-resonance-frequencies/">taller buildings</a> have longer natural periods of oscillation (swinging back and forth).</p>
<p>The ground also has a preferred period at which it oscillates. Soft sediment in a river valley will oscillate over longer periods, and hard bedrock over shorter ones. </p>
<p>High-frequency (short period) earthquake waves are therefore amplified in bedrock, such as the site of Amatrice, and are the dominant frequency radiated by small to moderate and shallow earthquakes such as last week’s. </p>
<p>Low-frequency (long period) earthquake waves are amplified in sediment and form a greater part of the seismic energy radiated by larger earthquakes, such as the <a href="https://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami">Tohuku earthquake</a> in Japan and the Nepal quake that felled the Dharahara tower.</p>
<p>When the resonant frequency of the ground coincides with the resonant frequency of the building, the structure will undergo its largest possible oscillations and suffer the greatest damage. The rigidity and distribution of mass along the height of a building also have a big effect on the likely damage sustained in a given earthquake, as this governs the way the induced forces are distributed.</p>
<p>You can try this for yourself by experimenting with a broom handle and a 30cm ruler. Held vertically, the top of the broom handle will do little if you vigorously shake its base with small movements, whereas the ruler will oscillate under the same shaking. </p>
<p>Slow the shaking down and the handle will begin to whip back and forth while the ruler settles down. Place a large mass on the end of either the ruler or the broom handle and the characteristics will change. </p>
<p>The concept is beautifully demonstrated in a <a href="https://www.youtube.com/watch?v=uFlIbujTuIY">video by Robert Butler of the University of Oregon</a>.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/uFlIbujTuIY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
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<h2>A resonant problem</h2>
<p>Of course, real structures and real earthquakes are far more complex. Real structures have many natural frequencies, and earthquakes vibrate across a spread (or <a href="http://earthquake.usgs.gov/learn/glossary/?term=spectrum">spectrum</a>) of frequencies. </p>
<p>Destruction occurs when any of a buildings’s natural frequencies coincide with any of the dominant frequencies of the earthquake. In some situations, there may be just a few structures that avoid this dangerous combination, such as the clock tower at Amatrice, or the chimneys of San Francisco.</p>
<p>The characteristics of shaking at Amatrice have not yet been published, but it is highly likely that the tower is standing not only because it was built well in the first instance, but also because it is just the right size and shape to survive the frequency of shaking that occurs during Italy’s moderate-magnitude earthquakes.</p>
<p>This process is equally important in other regions. The magnitude-6.8 <a href="http://earthquake.usgs.gov/earthquakes/eventpage/us10006gbf#executive">Myanmar earthquake</a> on August 24 damaged many historic temples in the Irrawaddy Valley, but none appears to have collapsed. These high-but-squat structures are susceptible to high-frequency shaking, whereas the passage of earthquake waves through alluvium is likely to have amplified mainly low-frequency earthquake waves. </p>
<p>Notably, much of the damage to the temples seems to have occurred as a result of the <a href="http://phys.org/news/2016-08-quake-scores-myanmar-heritage-bagan.html">collapse</a> of <a href="http://www.tandfonline.com/doi/abs/10.1080/00438240802453195">recent cheap “restorations”</a>.</p>
<p>Building practices are extremely important in mitigating the effect of shaking on buildings. Modern buildings are commonly fitted with <a href="http://gizmodo.com/5833664/how-buildings-stay-up-when-the-earth-shakes">devices</a> to reduce the effects of resonance. Engineered solutions <a href="https://en.wikipedia.org/wiki/Seismic_retrofit">are available</a> to retrospectively enhance the performance of unreinforced masonry buildings, with little impact on their aesthetics. </p>
<p>In Italy, this retrofitting needs to be done as quickly as possible before the next earthquake. This will be a costly exercise. Even apparently resilient medieval towers may require retrofits, because they have commonly <a href="http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/39/016/39016707.pdf">accumulated a degree of damage</a>. </p>
<p>However, Italy is a globally important cultural and tourism hub, and her earthquake-prone buildings, like those in Myanmar, are part of our collective heritage. Italy should not be left to struggle alone with the management of earthquake-prone building hazards.</p><img src="https://counter.theconversation.com/content/64548/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>Amatrice’s still-standing ancient clocktower has become an iconic image from last week’s deadly earthquake. But it is not the only unusual survivor.Brendan Duffy, Lecturer in Applied Geoscience, The University of MelbourneColin Caprani, Lecturer, Structural Engineering, Monash UniversityMark Quigley, Associate professor, The University of MelbourneLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/644432016-08-26T01:37:33Z2016-08-26T01:37:33ZEarthquakes don’t kill, our collapsing structures do. So how can we build them to stay up?<p>The magnitude 6.2 earthquake that <a href="https://theconversation.com/italys-deadly-earthquake-is-the-latest-in-a-history-of-destruction-64384">struck central Italy</a> this week has so far resulted in the deaths of <a href="http://www.abc.net.au/news/2016-08-25/italys-earthquake-toll-rises-to-247/7785244">at least 250 people</a>. Victims range in age from infants to pensioners and it is a tragedy that we unfortunately witness too often. </p>
<p>However, as this is written, there are stories of survivors being pulled from the rubble nearly 24 hours after the main quake. </p>
<p>And this is the point: it is not the earthquake that claims victims, but the collapse of human-built structures: buildings, bridges, retaining walls, embankments, and so on.</p>
<p>In the February 2011 <a href="https://theconversation.com/au/topics/christchurch-earthquake-13854">Christchurch earthquake</a>, for example, 115 of the 185 victims died in <a href="http://canterbury.royalcommission.govt.nz/Final-Report---Part-Three">one building</a>. Meanwhile, people ran from city-centre buildings, collecting in Hagley Park – the nearest open space – recognising that most danger lay with the surrounding structures.</p>
<p>Already, there are <a href="https://www.theguardian.com/world/2016/aug/24/italy-earthquake-throws-spotlight-on-lax-construction-laws">stories</a> about “lax construction laws” being at fault in Italy. But Italy is a modern, developed country, in which some of the world’s latest building design standards, the <a href="http://eurocodes.jrc.ec.europa.eu/">Eurocodes</a>, apply. </p>
<p>If Europe can’t get it right, can anyone? </p>
<p>But this is the 21st century, so where are the earthquake-proof buildings?</p>
<h2>Making progress</h2>
<p>It’s not that the structural engineering profession is not trying. Recent decades have seen tremendous improvement in knowledge and approaches in earthquake engineering. </p>
<p>Massive <a href="https://en.wikipedia.org/wiki/Earthquake_shaking_table">shake tables</a> exist for testing full-scale structures; we have complex computer models of structural behaviour; and novel testing approaches, including <a href="https://nees.org/resources/5693/download/Methods_for_assessing_the_stability_of_slopes_during_earthquakes-NEES.pdf">pseudo-static</a> and <a href="http://www.sciencedirect.com/science/article/pii/S0141029611004032">real-time hybrid simulation</a>. The profession has made great strides in developing energy-absorbing structural connections, such as replaceable “<a href="http://www.slideshare.net/manishnoida/earthquake-fuse">seismic fuses</a>”.</p>
<p>Potentially, the most far-reaching recent development is a fundamental change in the philosophy of designing structures for extreme events such as earthquakes. In the past, and still in some codes of practice, structural engineers were involved in the prediction of earthquake frequency and intensity. </p>
<p>They would design for a “one in 500 year” event, for example, and infer an absolute statement of structural safety for this hypothetical and unobserved event. </p>
<p>Of course, such an earthquake is rather meaningless, as it is merely an extrapolation from what has already happened. Consider that in the 2011 Christchurch earthquake, vertical accelerations of over 1.0g (buildings in free-fall) were <a href="http://ir.canterbury.ac.nz/handle/10092/11982">widely observed</a> and rarely before seen and that the <a href="http://www.ga.gov.au/scientific-topics/hazards/earthquake/basics/historic#heading-7">Newcastle earthquake</a> of 1989, led to significant <a href="http://www.ejse.org/Archives/Fulltext/2008/Special1/200802.pdf">improvement in seismic design</a> in Australia. It is fairly clear that extrapolating from what has already been observed is insufficient evidence for what may yet come.</p>
<p>More recently, “performance-based design” has emerged as a result of this evolving understanding of the nature of the threat. In this approach, the structural engineer makes statements roughly like “the probability of failure given a magnitude 7 earthquake is 5%”. If the client, regulations, or society more generally, requires a lower probability of failure, then the design is altered accordingly.</p>
<p>Such simple statements belie the enormous implications of this change in design philosophy. The structural engineer is removed from making decisions about the chances of some earthquake event, and instead rightly focuses on the structural response. </p>
<p>The client or other agent takes ownership of what might be called the design earthquake, consulting seismologists for advice, and abiding by regulations of what is socially desired.</p>
<h2>Progress bottlenecks</h2>
<p>In spite of the advances already made, the evidence from Italy and other earthquakes is damning: structures are not generally earthquake-proof and people are dying. Why is this still the case?</p>
<p>Firstly, and most obviously, the structures that already exist were not built with the benefit of the latest techniques or knowledge. Strengthening existing structures is an exciting area of research, and many significant pieces of infrastructure have undergone “seismic retrofit”. </p>
<p>In spite of this, though, there is a massive amount of building stock that is brittle, such as masonry structures, and very seismically sensitive. Is the cost-benefit ratio of widespread seismic retrofitting acceptable to society? Currently, the answer seems to be: “no”.</p>
<p>Secondly, there remains a fundamental lack of knowledge about the behaviour of structures in earthquakes. Computer models are at best approximations to reality and experimental testing is extremely expensive and so usually restricted to small-scales or just components. </p>
<p>Even the few largest shake tables, costing millions of dollars and capable of shaking full-scale multi-storey buildings – such as the <a href="http://www.eucentre.it/trees-lab-experimental-methods/high-performance-uniaxial-shake-table/?lang=en">EUCENTRE</a> in Europe, <a href="http://nees.ucsd.edu/">UC San Diego</a> in the US, and <a href="http://www.bosai.go.jp/hyogo/ehyogo/">E-Defence</a> in Japan – do not fully account for the full six possible motions (translation and rotations about the three axes) or consider the significant influence of soil-structure interaction (although <a href="http://nees.ucsd.edu/facilities/soil-shear-box.shtml">progress is being made</a>).</p>
<p>Thirdly, competition for limited research funds is fierce, and academics are just one grant cycle away from not being able to continue their work. This contributes to “bite-sized” research projects with limited scope and potential. </p>
<p>Government funding sources are also diminishing in many countries, with the expectation that private industry will fund research. But this mainly focuses on projects that lead to “jobs and growth”. </p>
<p>So the current situation is insufficient to tackle the research mega-project that the earthquake-proof structures challenge represents. Rather, a major international effort akin to the Large Hadron Collider is needed. Happily, there is a glimmer of such an effort emerging in the <a href="https://www.globalquakemodel.org/">Global Earthquake Model</a> project.</p>
<p>The latest research findings often take several decades to make their way to practice. After (eventually) taking root in the academic community, an idea <em>might</em> make its way to the relevant standardisation committee. </p>
<p>Such committees usually consist of key stakeholders: government, private industry, and academics (if their key performance indicators allow). Some may benefit from the idea, some may not, but if all of the stakeholders (eventually) agree, the idea becomes standardised when the next publication of the code of practice is (eventually) made. </p>
<p>Where public safety is concerned, such conservatism is understandable, but not the delays.</p>
<p>All of this is surely of little consolation to the people of Amatrice and surrounds in Italy tonight. But for them, and the others who unfortunately will follow, our research and profession must strive to do better. </p>
<p>We must commit ourselves to removing any artificial barriers and facilitate fast-track research-informed improvements in structural and public safety.</p><img src="https://counter.theconversation.com/content/64443/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Colin Caprani 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>Yesterday’s earthquake in central Italy has resulted in many deaths. But it is not the earthquake that claims victims but our built infrastructure. Why is this so?Colin Caprani, Lecturer, Structural Engineering, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/644082016-08-25T11:56:17Z2016-08-25T11:56:17ZCan we get better at predicting earthquakes?<p>An earthquake measuring 6.2 rocked central Italy in the early hours of Aug. 24, leaving more than 200 dead and hundreds missing in the rubble of the disasters. </p>
<p>Given the devastation earthquakes cause, seismologists and public officials have long wanted to know when earthquakes will happen, and after the powerful <a href="http://earthquake.usgs.gov/earthquakes/events/alaska1964/">1964 Alaska earthquake</a>, U.S. scientists proposed a worldwide research program on earthquake prediction. </p>
<p>In covering the initiative, <a href="http://science.sciencemag.org/content/150/3694/321">Science magazine emphasized</a> that “carrying out the proposal (i) would offer a fair chance to develop a method of giving warnings ‘hours to days’ in advance of major earthquakes and (ii) would, through engineering research, provide means of minimizing loss of life and property damage, even if a warning system were not achieved.” </p>
<p>The public has been interested more in prediction than in mitigation, however. And in spite of major progress among scientists in understanding the earthquake process and what causes disastrous shaking, it seems there is substantial <a href="https://www.washingtonpost.com/national/health-science/when-an-earthquake-is-coming-how-can-you-get-even-a-little-warning/2015/10/30/df12c640-63c1-11e5-9757-e49273f05f65_story.html">disappointment</a> in the apparent failure of “earthquake prediction.”</p>
<p>To some extent, this is an issue of semantics and objectives. Is the goal to predict an earthquake occurrence, predict ground motion due to an earthquake, or predict a disaster? Considering all of these, what is it that seismologists can and cannot do when it comes to predicting earthquakes?</p>
<h2>Early warning</h2>
<p>Fortunately, all earthquakes do not lead to disasters and, therefore, understanding where and why disasters are produced is the first goal of earthquake seismology. </p>
<p>But in our efforts to better predict earthquakes, we have to be precise about the timescale: is it a prediction that an earthquake is imminent – that is, within seconds, hours, or even days before the shaking? Or that it is likely to happen within years or tens of years?</p>
<p>Each of these predictions could be useful, and the type of action this information would lead to depends on the location of the earthquake and the sociogeographic and economic circumstances. </p>
<p>For example, even if one cannot predict earthquakes themselves, the ability to predict ground motion shortly after the onset of an earthquake could allow one to send warnings seconds or minutes before the main shaking is expected to occur. This means that some critical infrastructure could be automatically switched off safely, such as trains and nuclear power plants, and the public could be alerted. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/WWl3m4OyU44?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">How the an experimental earthquake early warning system called ShakeAlert works.</span></figcaption>
</figure>
<p>This form of early warning, produced after the earthquake (or at least after the beginning of the earthquake), is <a href="http://science.sciencemag.org/content/150/3694/321">already in practice</a> in several of the most populated and earthquake-prone places in the world. </p>
<h2>Predicting vulnerability</h2>
<p>The vibrations we feel during earthquakes, and which could destroy buildings and other infrastructure, are the effect of the <a href="http://allshookup.org/quakes/wavetype.htm">elastic waves</a> that propagate from the earthquake source through the rocks in the Earth’s interior and along its surface. Earthquakes produce different types of waves, some more destructive than others.</p>
<p>These waves propagate with velocities of the order of kilometers per second, but the most damaging waves often arrive after the first waves. This means there is a time lag between the electronic signals sent by our instruments and the most damaging waves, creating opportunities for action. This field is called <a href="http://www.nature.com/nature/journal/v390/n6659/full/390461a0.html">real-time seismology</a>, and operational systems have been set in different regions of the world, including Japan, Italy, Mexico, and California. These systems do not predict the time of occurrence but act as a warning of activity.</p>
<p>For long-term intervals (years to decades), geophysics provides the basis for rational management of affected resources and seismic safety. The quantitative analysis of actual earthquakes allows one to decipher the local conditions prone to disastrous shaking for specific buildings. Therefore, it provides the basis for including land management in seismic risk mitigation plans. In that sense, seismologists already provide the information required to adopt development policies oriented toward public safety.</p>
<p>Large improvements have been and will continue to be made, but it is difficult for the general public to appreciate the effectiveness of this approach (known as probabilistic seismic hazard analysis) because this type of “prediction” is expressed in terms of long-term probabilities, whereas the visible parts of earthquakes are the dramatic images of disasters in the news. </p>
<p>The public view of earthquake prediction typically concerns a deterministic (yes/no) assessment of whether or not an earthquake will happen on the intermediate timescale (hours to days). And it is precisely this field for which seismologists have not yet made definitive operational progress. The reasons for this are related to the complexity of the physical processes that cause earthquakes. </p>
<h2>Thousand-year buildup</h2>
<p>Earthquakes result from instabilities at locations where the resistance to slipping (the friction) of a fault is close to the forces that come from the slow plate movements of the Earth. </p>
<p>Although not felt in our everyday experience, the solid Earth is an evolving system. Its inner parts are subject to thermochemical convection. That is, cold and heavy materials at the surface tend to move downward and, conversely, hot material from the deep interiors moves upwards, and these motions result in plate tectonics. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=381&fit=crop&dpr=1 600w, https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=381&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=381&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=479&fit=crop&dpr=1 754w, https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=479&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/127739/original/image-20160622-7188-11dvyl3.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=479&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The location of global tectonic plates is well-known and can indicate which parts of the world are vulnerable to earthquakes.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Tectonic_plates-fr.png">USGS</a></span>
</figcaption>
</figure>
<p>Typical velocity of plate motion is of the order of centimeters per year. During an earthquake, the two sides of a fault slide at velocity of the order of a meter per second – about a billion times (!) faster than the steady-state background motion. </p>
<p>In other words, damaging earthquakes occur in seconds but have typically been in the making for tens, hundreds or sometimes even thousands of years. The time for us to observe the Earth is so short that we have no hope of assessing with fine precision when a critical state will be reached. </p>
<p>Nevertheless, it has been recognized for a long time in experimental and theoretical studies that instabilities could be preceded by a change in a short time period relative to the long tectonic/geologic preparation. Such a preparatory phase, often referred to as <a href="http://www.nature.com/nature/journal/v438/n7065/edsumm/e051110-12.html">initiation</a>, is not systematically observed with present day geophysical methods, and one would be forgiven to think that the case is hopeless given the uncertainties about duration and amplitude of the initiation process. However, discoveries in recent years are giving a more encouraging prospective.</p>
<h2>‘Slow earthquakes’</h2>
<p><a href="http://oceanservice.noaa.gov/facts/geodesy.html">Geodesy</a> and seismology have seen substantial progress in detecting subtle changes in rocks below the surface. Continuous GPS recorders and advanced processing techniques allow for the detection of smaller and smaller motions. Both GPS and seismometers are increasingly deployed in large dense arrays, producing antennas with unprecedented detection capabilities. </p>
<p>In the past few years, these efforts have led to the discovery of new types of <a href="http://crack.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html">deformation processes</a>. The most spectacular of these concerns the widespread observation of so-called <a href="http://www.geodesy.cwu.edu/instruments/tilt/explanation_ETS.html">slow earthquakes</a>. Slow earthquakes are slipping episodes at depth with deformation velocities that are in between plate tectonic motion and regular earthquake slip. </p>
<p>The largest known slow earthquakes have magnitude of more than 7.5 when measured as regular earthquakes, and there is increasing evidence that, just as for regular earthquakes, slow earthquakes occur on a wide range of magnitudes. This suggests that deformation of the Earth occurs on a broad spectrum of timescales. That is, times in between the extremes of slow motions from plate tectonics and mantle convection and the ultrafast and disastrous seismic ruptures. </p>
<p>The newly discovered <a href="http://www.nature.com/nature/journal/v427/n6975/full/nature02335.html">transient deformations</a> can be studied with geodetic measurements and by a refined analysis of seismic records. These have indicated that slow deformation is accompanied by a characteristic weak grind. </p>
<p>There is, therefore, hope that one day we could detect and monitor extremely slight changes in the rocks that would precede earthquakes. This is, indeed, a long way from “prediction” of precisely when and where a disaster will occur, but geophysicists will persevere and continue to make new discoveries about the changing Earth. </p>
<p>For now, knowing earthquakes is one way to live with them, to be prepared, to know the vulnerability of our communities and to adopt sound policies for earthquake-safe environments.</p><img src="https://counter.theconversation.com/content/64408/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Les auteurs ne travaillent pas, ne conseillent pas, ne possèdent pas de parts, ne reçoivent pas de fonds d'une organisation qui pourrait tirer profit de cet article, et n'ont déclaré aucune autre affiliation que leur organisme de recherche.</span></em></p>There are already early warning systems for earthquakes, but advances in seismology provide hope that experts will be able to predict when new ones will occur.Michel Campillo, Sismologue, professeur à l'Université Grenoble-Alpes, Institut universitaire de France, Université Grenoble Alpes (UGA)Rob van der Hilst, Sismologue, professeur au Massachusetts Institute of Technology, Massachusetts Institute of Technology (MIT)Licensed as Creative Commons – attribution, no derivatives.