Two sixth magnitude earthquakes and related aftershocks have struck off-shore near Milford Sound this morning.
The sequence of earthquakes began with a magnitude 6.8 earthquake located 60 km west of Milford Sound, 90 km north-west of Te Anau at 1:29 this morning, Tuesday 16th October 2007. The 24 km-deep quake woke sleepers throughout the South Island, and has been followed by more than 20 aftershocks.
Main aftershocks of magnitude 4.6 struck at 2:50 a.m., 4.9 at 6:34 a.m. and magnitude 4.9 at 8:22 a.m. These three aftershocks were at the shallower depth of 5 km.
A second main event struck at 10:28 this morning. Also 5 km deep, this magnitude 6.2 quake was located 50 km west of Milford Sound, 80 km north-west of Te Anau. Quakes of magnitude 4.1 and 4.5 have been reported since.
This morning’s quakes have occurred on or near the interface between the Australian and Pacific tectonic plates. The South Island lies along a transition zone in the boundary of the two plates where, off the east coast of the North Island, the Pacific Plate slides under (subducts) the Australian Plate and, to the south of the South Island, the Australian Plate slides under the Pacific Plate. In the middle, along much of the Southern Alps, they grind past each other.
Today’s earthquakes have struck in an area off the coast of Fiordland where the Alpine Fault heads out to sea through Milford Sound, and the Australian Plate begins sliding under the Pacific Plate. In a media release this morning, the US Geological Survey stated that the magnitude 6.8 quake indicated either thrusting of the Australian Plate towards the east or thrusting on a westward oriented structure on the interface between the two tectonic plates. However, the release added, “Slip vectors for this earthquake [the magnitude 6.8 event], which indicate the direction of motion along the ruptured fault, do not align with plate motions and thus indicate a complicated tectonic setting for this event. Whether this event, and other similar ones over the past decades, represents plate motion between Australia and Pacific or some other tectonic processes is not clearly understood, and is presently being debated by the scientific community.”
This morning’s quakes are occurring about 50 km northeast of the magnitude 7.0 earthquake and 6.2 aftershock of the 22nd of August 2003. This quake, close to Secretary Island, was 12 km deep and triggered more than 200 landslides in Fiordland National Park. Geologists who visited the area after that quake saw large numbers of trees and vegetation floating in Doubtful Sound, and recorded at least seven major summit-to-valley floor landslides covering vertical distances of 1500m.
A media release on the Geonet website from GNS Science this morning reports landsliding in the Milford Sound area, and minor damage to house contents which have been shaken from shelves. Two staff will be sent to the area to deploy portable seismographs to study the aftershocks. Other quakes in the area since 1990 have been a magnitude 6.2 event on the 1st of November 2000 and a magnitude 6.7 earthquake on the 10th of August 1993.
Two magnitude 6.1 quakes struck the area in 1988 and 1989. Writing in “Earthquakes,” George Eiby reports Fiordland earthquakes of magnitude 7.0 in May 1960 and magnitude 6.5 in May 1976. Further south in the Puysegur Bank, a quake of magnitude 6.5 was reported in October 1979, magnitude 7.0 in September 1945, and 6.8 in November of 1918. Two magnitude 7 quakes were reported by Eiby in February and August of 1950.
In recent weeks, earthquakes of magnitude 6.6 and 7.4 have been reported near the plate boundaries close to the Auckland Islands, and magnitude 5.5 near Macquarie Island.
[Compiled from data provided by the Geonet project and its sponsors EQC, GNS Science and FRST; and the US Geological Survey and its contributing agencies. Notes from “Earthquakes,” G.A. Eiby, Heinemann Reed, 1989.]