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Earthquake risk in SA

More information about what to do before, during and after an earthquake (external site, opens in new window)

History

There have been 3 earthquakes since 1837 that have caused some damage to Adelaide. These were in 1897, 1902, and 1954. The last of these was the smallest, but also the closest and most damaging.

Eyewitness and newspaper reports of these have been collected and compiled by local historian Tom Dyster. This collection is available as a downloadable report entitled The strong shock of earthquake: the story of the four greatest earthquakes in the history of South Australia (.pdf 312.7kb, opens in new window).

Faults

There have been very few earthquakes occurring near Adelaide over the last decade; only about 1 per year within 30km of the city. The low accuracy with which the epicentres can be calculated makes it difficult to tell if the events that have occurred are on the major fault lines near Adelaide.

From experience in the Flinders Ranges it would appear that the earthquakes are scattered widely and do not cluster along the faults. It is therefore considered that although the next major event may occur on a known fault line, there is also a very good probability that it will not. (A 3D model of Flinders Ranges Earthquake Hypocentres is available for download.)

Soft, deep sediments amplify earthquake vibrations. Fortunately most of the Adelaide metropolitan area is underlain by fairly stiff sediments, however some amplification will still occur. Deep sediments under the city will also cause some amplification for tall buildings.  The intensity map of the 1954 earthquake did not however show any areas of very strong amplification. There was some damage from the 1954 event to buildings on soils in steeply sloping areas.

Building damage

The most common damage from earthquakes includes:
  • Poor quality mortar, or no mortar at all, leading to collapsed walls
  • Tall brick chimneys which break easily
  • Corroded ties between walls, or no ties at all, leading to wall collapse
  • Gable ends which have no support at the top
  • Parapets that are cracked or have not lateral support
  • Suspended awnings where the supporting structure is not strong enough

Most of these things can be easily avoided, especially in a new building. It is old buildings that are more likely to be at risk. It is also clear from the Modified Mercalli (MM) Intensity Scale, a scale of seismic intensity used to measure the effects of an earthquake, that a wooden structure performs better than brick veneer, which performs better than double brick. The basic principle is that flexible structures survive in better condition.

Probability

The question of risk is related more directly to intensity than magnitude. (The worst damage to Adelaide was caused by the smallest of the 3 large events, but it was the closest.) The average return period for damage (MM VI) in Adelaide is thought to be about 180 years, but it could be considerably more or less.

Higher intensities may occur, but with much lower probabilities.

More information

For more information on earthquakes, download Earth Science Information Sheet M13 Faults, earthquakes and house damage (.pdf 945.2kb, opens in new window)