News

The big earthquake still building SE Australia

Media Release, Thursday 19 December 2002

Two separate geological studies have concluded that an area from Adelaide to south-east Victoria is seismically active and the next 'big one' could endanger lives and infrastructure.

Contrary to the popular notion that Australia is an ancient continent that has for millions of years been geologically comatose, University of Melbourne geologists have uncovered evidence that parts of South-eastern Australia recently stirred from their geological slumber and are in an active mountain building phase. These mountains are being shaped by earthquakes, some reaching greater than 6 on the Richter scale.

"When these big quakes reoccur, they have the potential to cause catastrophic damage to cities such as Melbourne, Adelaide, and the La Trobe Valley area, which straddle some of these major faults lines," says Professor Mike Sandiford, who conducted one of the studies.

Possibly, the most dramatic indication of this geological stirring, which the studies estimate began suddenly about ten million years ago, can be found in the landscape of the Mount Lofty Ranges near Adelaide.

"Some faults around Adelaide have moved slabs of the continent up to 30 metres in the last one million years," says Sandiford.

"A typical earthquake of magnitude 6.0 might produce a displacement of about one metre. Thirty metres is equivalent to 30-50 big earthquakes in the last million years," he says.

Other areas of intense mountain building have been around Victoria's Otway Ranges, Mornington Peninsula and Strzeleckis. In some of these areas, similar uplift and erosion over the last 10 million years have thrust chunks of Australia upwards in the order of one kilometre.

Tectonic movements have pushed the Otways 250 metres higher in the last three million years, and The Selwyn fault, which runs from Mt Martha, on Victoria's Mornington Peninsula, east to the Dandenong Ranges has possibly produced six metres of uplift in the last 100,000 years.

"This is potentially six big earthquakes," says Sandiford.

"We are still trying to determine the slip rates along these fault lines, but our evidence so far suggests that we should expect, on any one of the major faults, a large earthquake every 10-20,000 years. The estimated return period of a quake greater than 6.0 in south-east Australia is about 30 years, but none have been recorded in the last 100 years," he says.

"Most earthquakes experienced by this region are less than three on the Richter scale and occur several times a year. It is unusually quiet at the moment with nothing over 1.5 for the last few months."

Sandiford's evidence for the mountain building comes from extensive airborne geophysical data that measure radioactivity and magnetic field of the soil and rock. Rocks of different ages and types display different levels of radioactivity and magnetic properties. Faults and uplift which bring older rocks to the surface or bury younger strata can be detected through such measurement.

A second study led by the University's Dr Malcolm Wallace investigated sediments and seismic data from petroleum surveys to determine the long-term history of earthquakes and seismic activity in South East Australia.

Evidence of faulting, buckling and uplift can be clearly seen in the young sediment record from this region. The team obtained an age for the various faults and folds by using a combination of fossils and radioactive isotope dating methods.

The findings confirm that the young mountain building and earthquake activity began around 10 million years ago and continues to the present day.

"This young faulting and folding has had very important economic effects for Australia. The giant oil and gas fields of the Gippsland Basin are largely trapped in young geological deformations produced by the seismic activity. Faulting, however, can also rupture the reservoirs and cause leakage.

"In the La Trobe Valley it is this tectonic activity that has made the thick sequences of brown coal that Victoria relies on for its power generation economically accessible," says Wallace.

In the Murray Basin, the same activity was largely responsible for the heavy mineral deposits such as titanium and rare earths. It also caused the damming of the Murray River only 60,000 years ago forming the Barmah Swamp near Echuca, Victoria.

"While this is still nothing compared to the activity along the plate margins of, for example, New Zealand and California, it defies the notion that Australia is an inactive continent," says

Dr Wallace's research team is Julie Dickinson (PhD student) and Dr Guy Holdgate, all from the University's Department of Earth Sciences.