Canberra is the capital city at highest risk of an earthquake, because of its proximity to one of Australia’s most active faults, at Lake George.
But while a quake is unlikely in our lifetime, senior seismologist at Geoscience Australia Trevor Allen said it always paid to be prepared.
Geoscience has just released the first update to the National Seismic Hazard Assessment for Australia since 2012, with updated data incorporating potential ground-shaking hazards due to active faults.
Dr Allen said that outside of regional Australia, Canberra was now the capital city with the “highest estimated seismic hazard”.
He said Lake George, site of a quake fault driven past by hundreds of commuters between NSW and the ACT each day, was “a relatively fast-moving fault” when compared to quake-prone regions like New Zealand and California.
“It's still moving relatively slowly, but we think on average it's slipping around about 100 metres every million years,” he said.
“For most people, that would seem quite slow, but in terms of a stable tectonic plate ... this is actually a relatively fast-moving fault. We know it’s roughly 75-km long, and based on our understanding from other regions around the world, a fault that long can actually generate an earthquake as big as a magnitude 7.4.”
He said while the likelihood of such an event in Australia was low, it was important to take earthquakes into consideration when installing major infrastructure in our cities.
“We haven't actually seen an earthquake this large in Australia in historic times, and even in our lifetimes, we're probably unlikely to experience an earthquake this large, but it's important for us to actually consider these earthquakes when we're planning and designing our future communities,” he said.
“The infrastructure that we're designing at the moment, today, is likely to be around for the next 100-200 years, and these earthquakes that occur on these faults do have the potential to impact our communities in the future.”
He said although Australia was not usually associated with large, damaging earthquakes - the last was a magnitude 5.4 in Newcastle in 1989 - there were still around 100 earthquakes of magnitude 3.0 or large detected across the country every year.
“One of the things with the Newcastle earthquake is we still don't really have a good understanding of the faults on which that occurred,” he said.
“For most earthquakes across Australia it's very difficult to assign a particular fault to it.
"But what we do know is, looking at the geology and looking at the landscape and how faults have changed the landscape, we can see that these large earthquakes have occurred, such as the Lake George fault.”
He said in terms of the relative energy being released from a Newcastle-sized earthquake, a quake on the Lake George fault could be roughly 1000 times greater.
But such a seismic event could be hundreds of thousands of years away.
Then again, it could happen tomorrow.
Dr Allen said research being done by Geoscience Australia and the Australian National University shows that there has been ‘episode recurrence” on the Lake George fault.
“Large earthquakes appear to occur in clusters, so there might be a really active phase that maybe lasts for 20,000 years, and then the fault essentially goes to sleep for about a million years,” he said.
“That last activity that we can actually find on this particular structure on the Lake George fault was around about a million years ago.”
He said Canberra was also close to another active fault to the south-west, at Murrumbidgee.
“We think whilst it's not as obvious as the Lake George fault, that this probably has around about the same activity rate.
"So both of these structures are contributing to our higher or elevated earthquake hazard estimates for the ACT region.”