The earthquake that was felt across Victoria and other parts of the country is at the very least a reminder that we are living on a very thin crust that surrounds a very active planet.
For many people, their morning may have been interrupted by a slow rolling feeling followed by some alarming noises. Did your animals look around, wondering at the sound and did it seem as if the birds fell silent? Should we be concerned? Is this the start of something big?
The details first. On Wednesday 22nd September at 9.15 am, an earthquake of magnitude 5.9 was felt across most of Victoria as well as Sydney, Adelaide and Tasmania. Geoscience Australia says the focus was 10 km below the Eastern Highlands of Victoria, roughly halfway between Mansfield and the Latrobe Valley. Some damage was reported, though strangely, in Melbourne rather than in the immediate area.
People in our communities have asked if earthquakes are becoming more frequent. We do record more now, but in part, that is because there are more monitoring stations and more big cities and populations to feel and report them.
On average, 80 earthquakes of magnitude three or higher are recorded every year in Australia. Across the planet, an earthquake with a magnitude over six happens every three or four days, but in Victoria, this magnitude is rare. The last recorded one approaching this magnitude was in 1966, but we have no evidence of increasing frequency.
Part of the Geoscience program at Federation involves studying earthquakes, including why the number of earthquakes remains relatively constant. Earthquakes are a fascinating branch of geology to study.
Earthquakes release a lot of energy, and that energy is felt through vibrations in the ground and buildings.
This stored energy builds up over time and is released as faults — or fractures in the earth’s crust — move. In the case of our Victorian quake, the energy was probably released from a major fault located closest to the epicentre, perhaps the Fiddlers Green Fault. Faults in this area have moved before and will do so again.
In some parts of the world, where the faults are bigger or more complex, this release of energy can have catastrophic effects on humans and infrastructure. For example, the Christchurch quake, magnitude 7.1 in 2010, killed more than 180 people.
You might think 7.1 is not much more than 6.0, but the way earthquake intensity is measured means that a difference of one in magnitude represents 32 times the energy released — or about ten times the likely damage to infrastructure.
In Australia, our most lethal event was the 1989 Newcastle earthquake. It was slightly smaller than the Victorian quake but claimed 13 lives and led to much greater awareness of the earthquake hazard in Australia.
Unless we devote more effort to protecting communities, the number of earthquake disasters will grow, even if the number of these events stays the same.