Dusky Sound Earthquake

The magnitude of Wednesday’s earthquake has been revised downward slightly and attention has turned to why greater damage has not occurred. Aftershocks continue to unnerve Fiordland, Southland and Otago residents, while damage claims begin to flow in to EQC.

Friday 17th July 2009

Following further study, the U.S. Geological Survey has adjusted the magnitude of Wednesday’s earthquake in Dusky Sound downward slightly. Originally estimated at between 7.8 and 8.2, the 9:22 p.m. quake is now listed as magnitude 7.6 at a depth of 12 km.

An earthquake felt widely in the southern part of the North Island at 9:38 p.m. the same evening was an entirely separate event, and not one of the Dusky Sound aftershocks. This magnitude 5.4 quake was located off-shore in the South Taranaki Bight (160 km north-west of Wellington) at a depth of 160 km and was felt from Auckland to Westport. Some reports of this quake were filed from Banks Peninsula, Christchurch and Invercargill, but they may relate to the magnitude 6.1 aftershock in Fiordland, which struck 3 minutes later.

Residents of Fiordland, Southland and Otago continue to experience aftershocks, with hundreds having been recorded by GeoNet. Most are too small to be felt, but thirteen have been magnitude 5 or greater, with two of magnitude 6. A magnitude 6.0 earthquake (reported by GeoNet as magnitude 5.5) struck at 10:18 this morning, followed by a magnitude 5.2 event at 11:01.

GeoNet has acknowledged difficulty in assessing the magnitude of large on-shore earthquakes as the mathematical techniques are more suited to data from distant instruments. For earthquakes above magnitude 7, the rapid estimation method used to provide information for the website can be inaccurate and gave a false reading of magnitude 6.6 on Wednesday night.

Wednesday’s mainshock occurred in Fiordland’s Dusky Sound, on the interface between the Pacific and Australian tectonic plates. The quake was located in a transition zone where the sliding-past movement of the Alpine Fault changes to the Puysegur subduction zone where the Australian Plate is sliding beneath the Pacific Plate. It is thought that the earthquake resulted from the Australian Plate thrusting forward beneath Fiordland, perhaps by as much as 5 metres.

Most of Fiordland’s earthquake activity occurs about 100 km north-east of Wednesday’s event where the Alpine Fault heads out to sea into the transition zone. A magnitude 7.2 earthquake in the area (beneath Secretary Island) on the 22nd of August 2003 caused significant landslides and damaged property in Southland and Otago. The quake generated a 30 cm tsunami wave locally and caused a seiche of 15 cm at Port Kembla, Australia.

On Christmas Eve in 2004 a massive magnitude 8.1 quake struck further south (north of Macquarie Island) and was widely felt in the South Island. Minor damage was reported, and the shaking was felt in Tasmania and Australia. The size of the quake attracted a lot of initial attention but it was soon forgotten when the infamous magnitude 9.0 quake struck off northern Sumatra resulting in the Boxing Day Tsunami which killed more than 280,000 people.

Earthquake activity following Wednesday’s earthquake is occurring at two distinct locations in the transition zone off Fiordland, with one centre to the north-west of Invercargill clustered around Dusky Sound, and another centred around 190 km west of Invercargill. Six quakes of magnitude 5 have been reported near Dusky Sound, and five of magnitude 5 and two of magnitude 6 at the other centre of activity.

Insurance claims have started flowing in to disaster insurer EQC. By midday today, 401 claims totalling $520,000 had been lodged. 151 relate to damage in Invercargill, 30 to Dunedin, with the Invercargill claims estimated at $346,000.

Attention is now turning to why the damage from such a large earthquake has been so light. Land slippage has been lighter than expected based on past experience, but it has been suggested that recent rainfall has been near normal and insufficient to make slopes prone to landslides. One comment made by a geologist on National Radio yesterday suggested that the energy was directed westward when the initial earthquake occurred.

Today, it was reported that Dr Euan Smith of Victoria University of Wellington suggested that the rupture generated less high frequency tremor thereby reducing the damage from the large earthquake.

Whatever the cause, there is now concern that the minor damage generated by a magnitude 7.6 earthquake may encourage complacency about the hazard presented by a rupture of the nearby Alpine Fault. It is widely thought that this fault is due to move, possibly generating a series of earthquakes with a combined magnitude of 8. Whether the date of an Alpine Fault earthquake has been brought forward by stress transferred from Wednesday’s earthquake remains to be seen.

[Compiled from data provided by the U.S. Geological Survey and its contributing agencies; the GeoNet project and its sponsors EQC, GNS Science and FRST and EQC media release.]

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