Just before 9pm last night (AEST) a magnitude-5.4 earthquake (initially measured as magnitude-5.3) rocked Victoria. The quake did minimal damage to property and no injuries have been reported, but it reminded us that the bit of the earth’s crust we live on isn’t as stable and quiet as we might often think.
The quake’s epicentre was located 10km southwest of Moe and occurred roughly 10km below the surface. It was the equal-largest earthquake to have occurred in Victoria since the magnitude-5.7 earthquake that occurred near Mount Hotham in 1966.
The public perception of earthquake hazards in Australia tends to be based largely on personal experience.
Many of us will have experienced small earthquakes or tremors but for the most part these are located far from built up areas and tend not to produce much more than a flurry of Facebook and Twitter updates, and perhaps the occasional website crash.
Many of you might remember the magnitude-5.6 Newcastle earthquake in 1989 which killed 13 people, left more than 100 injured and caused an estimated US$1.1 billion worth of damage. But unless you were directly affected by the tragedy of that event you probably don’t consider such an event likely to occur in Australia again.
Indeed, it’s easy to forget we live on a very dynamic continental plate. The Australian Plate (see image below) is actually one of the fastest moving on the planet, charging northwards at a rate of 7cm/year. Consequently, our continent is relatively highly stressed and more prone to earthquakes than we might expect.
One look at a map, or a drive along the coast, reveals evidence of large seismic events in our geological past. Not far from the recent Moe earthquake, the tilting of beach deposits at Warratah Bay suggests multiple, large earthquake rupture events have occurred in the region’s very recent geological past.
Further north, the development of a fault scarp – a cliff-like embankment – on the Cadell Fault has changed the course of the Murray River near Echuca several times in the past 100,000 years. These events must have been huge, on the order of magnitude 7 or greater.
More recently (geologically speaking), activity on an unnamed fault off the coast of South Australia near Beachport in 1897 caused a significant magnitude-6.5 earthquake. That earthquake is recorded as having rung church bells as far away as Bendigo.
Last night’s earthquake was what is known as an intraplate earthquake – that is, it occurred away from an active tectonic plate boundary.
Active plate boundary earthquakes, such as the Tōhoku-Oki earthquake of March 2011 (which led to the Fukushima nuclear disaster) and the April 2012 earthquake off the coast of the Indonesian province of Aceh are well known to be devastating but intraplate earthquakes can also be very damaging.
The magnitude-6.3 earthquake that hit Christchurch in February 2011 was about 150km east of the tectonic plate boundary, and was part of a sequence of shallow intraplate earthquakes. And yet this earthquake killed roughly 180 people and caused tens of billions of dollars damage.
The epicentre of the Christchurch event was characterised by a pattern of high-amplitude, high-frequency and short-duration “strong motion”. Strong motion implies that monitoring seismometers were overwhelmed in the event.
This pattern of “strong motion” is typical of that observed in relatively shallow intraplate Australian earthquakes and hints at what we could expect, should a quake of similar size occur here. With a similar magnitude event occurring, on average, once every five to ten years across Australia, that’s not an impossibility.
This potential is the driver behind Australian research attempting to better understand stress in the earth’s crust, and the resulting earthquake hazard. Scientists working on the current monitoring networks run by Geoscience Australia, Environmental Systems and Services (ES&S) and AuScope are seeking to improve instrumentation to more accurately locate seismic events and better understand the forces that resulted in those events. Further research is also needed into how continental earthquakes behave and how they are triggered on the Australian Plate.
Public and institutional perception of “low risk” is one of the reasons researchers are seeking an improved understanding of Australia’s crustal stability. Our lack of a collective experience of damaging earthquakes in Australia has meant that government and local authority planning – related to earthquake risk – is very poor compared with neighbouring regions of “higher risk” (such as New Zealand).
New hazard data and maps are currently being developed by a number of research groups (such as the Victorian Earthquake Hazard Map at the University of Melbourne, funded by the Natural Disaster Resilience Grant) to underpin the establishment of new planning codes and disaster management strategies that specifically consider seismic hazard.
These developing technologies typically involve injecting or removing fluids from the earths crust. So a detailed understanding of the natural background seismicity of prospective regions is critical to mitigating against anthropogenically triggered seismicity. It also reduces the potential for naturally occurring events to negatively impact public opinion on these important projects.
In the meantime, last night’s earthquake will probably serve to remind many of us that the earth beneath our feet isn’t as stable as we might think.