Are Oz rocks leading stressful lives?

30 April 2002

An average of one earthquake happens every ten years which is powerful enough to rupture the ground's surface on the Australian landmass. Compared to the other states, Western Australia has experienced more than its fair share of large earthquakes over the last forty years.

But no consistent pattern of earthquake locations has emerged in Australia, even though records began about one hundred years ago. So scientists are thinking laterally to find ways of gauging the earthquake hazard for various communities.

For three weeks in May, Geoscience Australia will send researchers into the field — they will use state-of-the-art equipment to track fifty ground locations in south-west Western Australia, to see if rocks in the region are being distorted by stress.

The researchers are looking for horizontal and vertical motions within the Australian landmass. The tell-tale sign will be the movement of the ground locations relative to each other. The Earth could literally be moving in this neck of the woods.

This work will mean better estimates of earthquake hazard in south-west Western Australia, leading to safer communities and infrastructure. However, scientists point out that there is still no scientifically-proven method for predicting earthquakes.

The project is a collaborative effort involving the Department of Land Administration in Western Australian, the Institute of Geological and Nuclear Sciences (New Zealand), the Curtin University of Technology, and the University of Western Australia.

Stress-induced earthquakes

The south-west of Western Australia is one of the most active regions in Australia for earthquakes.

An area which is centred about 150 kilometres east of Perth is known as the South-West Seismic Zone and measures roughly 300 kilometres by 500 kilometres.

According to Geoscience Australia, four magnitude 5 earthquakes and thousands of smaller ones have occurred in a small area of the Zone in the last year alone, centred near the town of Burakin. Since 28 September 2001 more than 12,000 earthquakes have been recorded in the area, with most of these being aftershocks of the large events. Many buildings within 25 kilometres of the activity have suffered minor damage, but none has suffered major structural damage.

This is the highest level of seismic activity experienced in Australia since the 1988 Tennant Creek earthquake, which measured 6.8 on the Richter scale. The sequence of earthquakes near Burakin is thought to be related to the 1979 Cadoux earthquake.

Earthquakes occur when rocks suddenly release stored stress that has built up over long periods of time. Stress builds up in rocks that are being distorted by pushing, pulling or shearing forces. These forces largely originate from Australia's continuous journey northwards. Australia is part of a larger unit, called a tectonic plate, which is made of continental and oceanic crust. This plate is slowly moving north at a rate of roughly seven centimetres per year.

There was movement at the station...

As the first step in this project, the researchers will install a Global Positioning System (GPS) station at each of the fifty ground locations in the South-West Seismic Zone.

GPS is a way of obtaining the position of geographical points on the Earth's surface and allows the position of the each station to be measured very precisely — to within a few millimetres, in fact. The team will measure each station's initial position as part of fieldwork in May. Then station positions will be measured again in two to five years, to allow movements large enough to be accurately measured by the GPS equipment.

The repeat measurements will allow Geoscience Australia to map the direction of any rock movement, and may show areas that are distorting the fastest — these areas may be prone to earthquake activity in the future. The South-West Seismic Zone is likely to be distorting by as much as two to five millimetres per year, a rate which equates with 20 to 50 centimetres in 100 years.

The information gathered will be used to better understand why earthquakes have occurred in this region and where they might occur down the track.

The results will be especially vital for evaluating the seismic hazard of south-west Western Australia, particularly the hazard from large earthquakes for the Perth area. The project also contributes to the international scientific community's effort to monitor the Earth's changing shape and motions.

Surface rupturing earthquakes in Western Australia in the last 40 years
Event Magnitude Felt radius (approx.) Damage description
1968 Meckering 6.8 650 km Township and lifelines destroyed. Extensive damage over approx. 15,000 square kilometres.
1970 Calingiri ~5.8 250km Minor structural damage in township.
1979 Cadoux 6.0 500km Township destroyed. Extensive damage over 4000 square kilometres.

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