Unique Quake Explained

Scientific analysis of the effects of Wednesday’s earthquake in Dusky Sound is starting to be published, providing insight into what is turning out to be a rather unique event.

Saturday 18th July 2009

Since the magnitude 7.6 earthquake that struck Fiordland on Wednesday night, scientists have been puzzled by the apparent lack of damage from such a large and shallow event. GNS Science has had several teams working in the area, looking for landslides, deploying seismic instruments to study the aftershocks, and analysing Global Positioning System (GPS) data to determine land movement.

The mainshock occurred when the Australian Plate thrust forward and under the Pacific Plate in the vicinity of Dusky Sound. The rupture began at a depth of about 30 km, rupturing upwards and to the south, focusing energy offshore. The motion was more like a lurch than a sudden snap allowing the energy to be released more slowly ”“ typical of a subduction thrust earthquake. This resulted in lower frequency shaking, which explains the steady rolling sensation that made many people feel queasy over the next minute or so.

The lack of high frequency waves from the earthquake is directly related to the low level of building damage and landslides. By comparison, the magnitude 7.2 Secretary Island quake of August 2003, is thought to have triggered four times as many landslides.

Data from permanent GPS stations in the area shows that the Puysegur Point GPS station has moved south-west, registering the largest movement of 30 cm westward. Displacements of 2 cm were recorded at Alexandra, 3 cm at Bluff and 5 cm at Mavora Lakes.

Subduction thrust earthquakes are not common in New Zealand, though they are known to have occurred prior to recorded history near Wellington. The Dusky Sound quake has provided a unique opportunity for scientists to study such an event at the same time that nearby communities can be thankful that such a large quake has caused lighter damage than expected.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST.]

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