When the earth’s plates move around, they’re deformed. They might be squashed, or stretched, or twisted sideways. And when this compression, tension or shear happens, the energy in the rock builds up. When the energy density reaches the strength of the weakest point in the rocks – usually an existing fault – it breaks, and we have an earthquake.
The biggest earthquakes happen at the boundaries of the plates, where the stress builds up to the greatest levels. The most active plate boundary in the world is the northern part of the Australian plate, from Indonesia through Papua New Guinea to Fiji.
Earthquakes happen inside plates as well as at the plate boundaries, because of the movement of the plates. Australia is moving faster than any other plate and the stresses are high. We have quite a lot of earthquakes within Australia but nowhere near as many as at the plate boundaries.
In Victoria, the Strzelecki Ranges south-east of Melbourne are the most active area in south-east Australia. In 2009 we had 2 magnitude 4.6 earthquakes near Korumburra and other ones have occurred especially around Mirboo North and Boolarra in the Latrobe Valley.
Most earthquake damage occurs because the building’s been shaken. The ground moves, and the building on the top is shaken, and it resonates. The ground moves a certain amount; the building flops around even more, it’s deformed and breaks.
To minimise the damage in earthquakes, you have to design your buildings to be flexible so that they can move with the ground motion. Or alternatively, you can make them very rigid, so that they may be damaged, but they won’t collapse. In the recent Chile earthquake, we found that many buildings were very seriously damaged, but very, very few collapsed, and the casualties from the earthquake, were very few.
We can anticipate when future earthquakes will occur; but to say an earthquake will happen at a particular time and place or magnitude, say next Tuesday; I think that’s a long way off.