The Puysegur Trench Quakes of 30-9-2007 – Part 1

Part 1 – What Happened
Two large earthquakes struck the Puysegur Trench in the Southern Ocean south of New Zealand on the evening of 30th September 2007.

At 6:24 on the evening of Sunday September 30th, a magnitude 7.3 earthquake struck the Puysegur Trench at a location 200 km north-west of the Auckland Islands. The quake occurred near the interface between the Australian and Pacific tectonic plates, in an area where the Australian Plate is sliding under the Pacific Plate at a rate of approximately 32 mm per year. The motion is not smooth and, while averaged out at 32 mm per year, actually occurs through spasmodic events such as earthquakes which account for months or years of accumulated strain.

The quake was located 475 km south-west of Invercargill and the shockwave of the earthquake registered on seismographs located in Otago and Southland at about 6:25 p.m. Residents of Southland, Otago, Fiordland and South Canterbury felt the shaking, and 273 of them reported feeling the event to the GeoNet website.

The earthquake was calculated to be 12 km deep, and its under-sea location and size meant that a tsunami wave might have been generated. The Pacific Tsunami Warning Centre analysed the quake’s characteristics and compared them with data for other earthquake events that have been recorded in the area. In a bulletin issued at 6:38 p.m. the warning centre advised that a widespread tsunami event was unlikely, but that a local wave might have been generated.

The next stage was to wait for data to come in from wave buoys in the area or any reports of waves coming ashore in inhabited areas. At 6:57 p.m. a 4 cm variation in sea level was noticed at Dart, Australia and, at 8:25 p.m. a 10 cm variation in sea level was recorded at Spring Bay in Australia. This second wave was recorded as an 11 cm variation at Port Kembla six minutes later. The warning centre issued its report of these small waves at 10:14 p.m. New Zealand Daylight Time, adding that no further bulletins would be issued on the event unless further information came to hand.

At 10:48 p.m. another large earthquake struck the Puysegur Trench area. This quake was measured at Magnitude 6.8 and located at a depth of 10-12 km about 35 km away from the first event, being at 235 km west-north-west of Auckland Island.

The shaking from this event 505 km south-west of Invercargill reached the seismograph at Wanaka less than a minute later and the tremor was felt in Southland, Otago, Fiordland, South Canterbury and Christchurch city. By this time many people were aware of the earlier earthquake, and hence more alert to any sign of another event. 744 of them filed a report on the GeoNet website describing their experience.

Once again the Pacific Tsunami Warning Centre assessed the likelihood of a tsunami being generated by another strong and shallow undersea earthquake, using information from seismic instruments, historical data and, of course, their knowledge of the quake more than 4 hours earlier. It was determined that a destructive widespread tsunami threat did not exist, but considered that a locally destructive tsunami wave could have been generated. A bulletin to this effect was issued by the warning centre at 11:01 p.m. New Zealand Daylight Time.

Since last night, the area has been relatively quiet, with seismographs in the southern part of the South Island registering at least 6 much smaller earthquake events which may or may not be in the same area as the two large events. The US Geological Survey has a policy of publishing data for earthquakes of magnitude 4.0 or greater outside United States territory, but this policy is not always applied in the case of New Zealand territory where earthquakes as large as magnitude 5.0 often do not appear on their website.

The Puysegur Trench subduction zone is part of the collision zone between the Pacific and Australian tectonic plates. On-shore in the South Island, the boundary between the two plates is known as the Alpine Fault, a section of the boundary where the two plates collide and grind past each other. As the boundary area heads out to sea toward Antarctica, the boundary goes through a number of transitions where one plate slides under the other or they slide past each other. Distinct segments of the plate boundary are known as the Puysegur Trench, Macquarie Ridge and Hjort Trench.

Various parts of the boundary have hosted very large quakes in recent historic times. On December 24th 2004 a magnitude 8.1 earthquake struck about 200 km west of last night’s quakes. At the time, it was the largest quake worldwide since an event off the coast of Peru early in 2001 but it was quickly supplanted by the infamous Boxing Day earthquake and tsunami in the Indian Ocean two days later. A magnitude 8.0 earthquake in May 1989 was located along the Macquarie Ridge about 450 km southwest of last night’s quakes.

Magnitude 7 quakes along the undersea plate boundary are more common, with a magnitude 7.2 quake 70 km off the Fiordland coast on the 23rd of November 2004 prompting more than 800 people to register reports with GeoNet.

[Compiled from data supplied by the US Geological Survey and its contributing agencies, the U.S. National Oceanic & Atmospheric Administration’s Pacific Tsunami Warning Centre, the GeoNet project and its sponsors EQC, GNS Science and FRST, and Geoscience Australia.]

Next – Part 2: How They Were Managed
A timelined record of how the various government agencies managed their responsibilities for analysing, reporting and managing the risk presented by the Puysegur Trench earthquakes.

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