Earthquake attack in the Sydney Basin: what is the risk?

D. Denham

Abstract

Earthquakes pose a major threat to life and property, particularly in major conurbations where there is the potential for considerable damage to buildings. In the Sydney Basin region, which is home to approximately four million people and contains the largest urban development in Australia, earthquakes have been experienced since European settlement in 1788. However, it is only in the last thirty years that significant damage has been caused. In this context, the 1989 Newcastle earthquake dominates. It killed 13 people and caused damage estimated at $1.5 billion. Before that event, the 1961 Robertson and the 1973 Picton earthquakes, which were of similar size to the Newcastle earthquake (M ~5.5), also caused significant, but much less damage ? $3 million and $2 million respectively in 1990 dollars. Current estimates of earthquake risk in the Sydney Basin indicate that on average, there is a 10 per cent chance of ground accelerations exceeding 0.11 g in 100 years, or in terms of velocity, a 1000 year return period of approximately 90 mm/s. The earthquakes tend to occur within basement rocks beneath the basin and are caused by NE-SW compressive stress in the earth's crust. This information is derived from in-situ stress measurements in basin rocks and focal mechanism determinations from earthquakes. It is evident that while the earthquake risk is much lower than in cities such as Tokyo and Los Angeles, which are on active plate margins, the risk in the Sydney region must be addressed, particularly for special buildings like schools, hospitals, dams and high rise office blocks.

Exploration Geophysics 23(4) 579 - 587  doi:10.1071/EG992579