Why New Zealand earthquakes can impact Australia
31 July 2009
On 15 July 2009 the first tsunami warning from Australia's now fully operational Tsunami Warning System was issued for the east Australian coast, following a large undersea earthquake. The threat didn't come from the great Sumatra subduction zone that caused the Indian Ocean tsunami in 2004, but from the relatively unknown Puyseger subduction zone off the coast of New Zealand.
A magnitude 7.9 earthquake occurred in the Fjordland region along the southwest coast of the South Island of New Zealand at approximately 7:30pm AEST and was widely felt throughout the region though no major damage was reported.
The earthquake occurred along the boundary where the Australian and Pacific tectonic plates meet. At this plate boundary, the Australian plate is being forced under the Pacific plate ('subducted') at a rate of about 40mm a year, resulting in the uplift of the Southern Alps of New Zealand and causing the deep Puyseger trench in the sea floor to the south.
"The subduction of the Australian plate causes stresses to build up where the plates slide past each other. Periodically, the rocks give way under this stress causing earthquakes, and we observe the effects on land such as ground shaking and displacement of the ground surface", said Clive Collins, a senior seismologist at Geoscience Australia.
Earthquakes which occur along the Puyseger subduction zone, which runs southwards from southwest New Zealand, can potentially generate tsunamis which will reach the Australian coastline. They are typically thrust earthquakes, in which the upper Pacific plate 'thrusts' up over the downgoing Australian plate when the boundary between the plates slips during an earthquake. This has the effect of suddenly pushing upwards all the water above and spawning a tsunami wave.
"When these type of large, shallow earthquakes occur in this area off the South Island, such as this one, they do have the potential to create tsunami which propagates towards the eastern seaboard of Australia", said Mr Collins.
A small tsunami was generated during the earthquake and was recorded on tide gauges. In New Zealand, a tide gauge at Jackson Bay measured the height of the tsunami wave at 50cm. A tsunami warning was issued for the marine areas of New South Wales, Victoria, Norfolk Island and Tasmania, and a land inundation warning for Lord Howe Island. No visible effects were reported at any of these locations, but tide gauges measured a 15cm wave at Port Kembla in New South Wales and a 12cm wave at Spring Bay in Tasmania.
"What's also interesting is according to GPS measurements following the earthquake, the southwest tip of New Zealand shifted westwards towards Australia by about 30cm during the earthquake, and south by about 7cm. So now our neighbours are a little closer", he said.
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