AUSTRALIA is an ancient continent that has been unchanged by seismic activity for millennia 鈥� according to the textbooks. But a study of its geological past suggests that this dogma should be discarded. 鈥淭he continent is stirring. There are clear signs of increased tectonic activity over at least the past 6 million years,鈥� says Mike Sandiford of the University of Melbourne.
Because there are few obvious features in the landscape suggesting ancient seismic activity, geologists generally assume that the earthquakes that occasionally rattle south-eastern Australia, such as the Newcastle earthquake that killed 13 people in 1989, are a recent, rare phenomenon. But according to Sandiford, the continent is well into a period of tectonic activity. In the past few million years alone, he says, earthquakes have doubled the size of mountains in the Flinders and Mount Lofty ranges in South Australia, and in the Otway ranges in Victoria. And he expects the activity to continue.
Records of seismic activity in Australia only go back about 150 years. So to find out what has happened before then, Sandiford used a variety of techniques, including quartz sampling, to identify fault lines less than 6 million years old across south-eastern Australia, where most of the quakes occur. He then measured the slip rates. From that information he built up a tectonic map of the region, showing how the Earth has buckled and folded. 鈥淚n some cases the faults are slipping 50 metres per million years, equivalent to maybe 50 big earthquakes in a million years,鈥� Sandiford says.
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He reports in a special publication of the Geological Society of Australia that the quakes Australia experiences today are not a new phenomenon. The regions of ancient activity coincide perfectly with today鈥檚 seismic hot spots, suggesting that quakes started hitting at least 6 million years ago and have continued unabated to the present day. 鈥淭his should convince geologists that there has been plenty of seismic activity,鈥� says Mark Leonard of Geoscience Australia, the government鈥檚 department of geology.
Sandiford is also trying to uncover why the quakes hit where they do. Most earthquake-prone areas straddle a plate boundary zone, like the San Andreas fault in California, for example. But Australia is near the middle of a plate 鈥� the nearest boundary is thousands of kilometres away in New Zealand (see Graphic). The Indo-Australian plate is under huge stresses as it smashes into the Eurasian and Pacific plates, so the crust has to give somewhere, but why in south-eastern Australia? Sandiford has now found that the active regions on his tectonic map all contain relatively high levels of heat-producing radioactive elements. The excess heat may weaken the crust enough to make quakes more likely, he speculates.
