Earthquakes Earthquakes

Instrumentally recorded seismicity in Tasmania is low, but are large earthquakes credible in the Tasmanian setting? Tasmania is part of the Australian continental plate and since European settlement a small number of large intra-plate earthquake events have been observed (i.e. within the continental plate instead of at the margins). The most damaging of these earthquakes was the 1989 magnitude 5.6 ML Newcastle Earthquake (13 lives lost and $4.5 billion in costs).

In Tasmania, a significant earthquake swarm (ranging up approx 6.9 Mw) was recorded in the late 1800s with the epicentre located east of Flinders Island near the edge of the continental shelf. These earthquakes caused damage to buildings at Launceston. From this evidence alone a large future earthquake must be regarded as possible, although the low seismicity and short duration baseline of instrumental monitoring makes it very difficult to determine likelihood.

There is a further line of evidence from the geological record that indicates large earthquakes have occurred in the past and are likely to occur again. The evidence lies principally in the Lake Edgar Fault, an active fault known to have moved multiple times during the Quaternary. Investigations have shown that the fault has moved at least three times between 60 000 and 17 000 years BP (before present) and with calculated magnitudes of 6.8-7.0 MW. The effect of an earthquake from this source on a population centre such as Hobart, 80 km away, has yet to be determined and is an open question. Ground rupture from this fault would impact on the Edgar Dam built over it and have other effects on the water storage lakes. Landslides will almost certainly occur. There are also other features in Tasmania that may be active faults and there could be unknown active faults offshore, all of which could possibly be reactivated.

The undesirable effects of a large earthquake include strong ground shaking that may include amplification, liquefaction and surface rupture along fault lines locally. Strong ground motion can damage buildings, particularly the many un-reinforced brick-masonry or stone buildings that were built before building standards were improved. In many cases modern construction techniques that incorporate wind-loading may effectively earthquake-proof the structure, although this will not address unsecured objects in homes from falling over and causing damage. Structures built on low strength material, such as modern flood plains at Launceston where the thickness of material exceeds about ten metres, are susceptible to amplification at certain resonant frequencies. Multi-storey buildings whose resonant frequency is similar to that of the underlying ground frequency could be significantly affected. Areas containing young uncompacted fine sands may also subside or liquefy, causing foundation damage.