Earthquake Depths in Australia
Earthquakes occur in the outer crust where rocks are cold enough to be brittle. At some depth the temperature is high enough for the rocks to react to stress by plastic flow.
In eastern Australia, earthquakes occur to depths of about 20 km. Less than 5 km are regarded as shallow, while those at depths greater than 15 km are deep.
In subduction zone areas, like Indonesia or Fiji, earthquakes occur to 700 km deep. Those that are within 70 km of the surface are considered shallow, while those greater than 300 km are deep.
Shallow earthquakes cause much damage, but deep earthquakes rarely cause damage.
Mechanisms of Australian Earthquakes
Almost all Australian earthquakes have mechanisms with horizontal compression producing reverse or thrust faults
Under compression these faults produce uplift, so Australian earthquakes are more likely to be found under areas of rising topography (Eastern Highlands, Flinders Ranges).
They are less frequent in sedimentary basins (Murray Basin, Great Artesian Basin)
Maximum Credible Magnitude in Australia
Because Australian earthquakes occur only in the top 20 km, one dimension of the rupture area is constrained.
This means that the fault rupture for a large earthquake must be quite long.
There is a practical limit for fault length of a little over 100 km in Australia.
This corresponds to the rupture area of an earthquake of magnitude about 7.5
Australia has large earthquakes, but they occur infrequently
An earthquake exceeding magnitude 7 occurs somewhere in Australia every 100 years or so.
A typical site in Australia will be within 50 km of a magnitude 7 event every 100,000 years or so.
In active areas like Japan, Philippines or California, earthquakes of magnitude 7 occur every few years. The activity in these places is restricted to a much smaller area than that of Australia, so a typical site may be within 50 km of a magnitude 7 event every 100 years or so.
Any location in Australia will eventually experience very strong earthquake motion.
Low seismicity does not mean weak ground motion. It means that strong earthquake motion happens less often.
Earthquakes of magnitude 8 and larger are termed great earthquakes, and normally only occur at plate boundaries. These are unlikely to ever occur within Australia. Earthquakes of magnitude 9 and larger will rupture faults for hundreds of kilometres, so usually only occur on subduction zones such as along the west coast of South America, or the south coast of Alaska.
Low attenuation in Western and Central Australia
As seismic waves radiate away from an earthquakes, their amplitude decreases due to geometric spreading. In addition, some energy may be absorbed within the rocks, especially in soft or hot rocks.
The rocks in Western and Central Australia are old, relatively cold, and hard, so seismic waves are not greatly attenuated by absorption of energy.
The rocks in Eastern Australia are younger and softer, and absorb energy at a rate that is about world average or greater. Earthquakes in Western and Central Australia will be felt over greater distances than those in Eastern Australia.
Summary of Characteristics of Australian Earthquakes
Intraplate, so they are infrequent
- A person feels an event only every 5 -10 years.
- One of the most active intraplate areas.
Distributed over many small faults
- Low maximum magnitude, perhaps Mw 7.5.
- Hazard is widely distributed.
Shallow depth, from surface to 20 km
- Small events often felt and heard.
- Moderate magnitudes can cause damage.
- Above Mw 6 usually gives surface rupture.
Usually horizontal compression
- Reverse faults predominate.
- High stress drop, giving high amplitude, high frequency, short duration motion.
Attenuation varies across Australia
- The old, cold, hard rocks of Western and Central Australia do not absorb seismic energy greatly, so earthquakes in these areas are felt over longer distances than in Eastern Australia.