Friday 22 June 2012

Earthquake shakes Macquarie Island.

Slightly after 3.30 pm local time (slightly after 4.30 am GMT) on Friday 22 June 2012, Macquarie Island in the southwest Pacific Ocean was shaken by an Earthquake measured by the United States Geological Survey as 6.0 on the Richter Scale, at a depth of 9.9 km, 24 km northwest of the Island. This is a fairly large quake at quite a shallow depth; in an inhabited region it could be devastating, but on Macquarie Island, uninhabited except for a few scientists at the Australian Antarctic Division's  Macquarie Island Station (and a lot of penguins), it is unlikely to have caused any serious harm.

The location of the Macquarie Island Quake. The large landmass at the top right is South Island, New Zealand. The red lines are the boundaries between tectonic plates. USGS.

Macquarie Island is located on the boundary between the Pacific and Australian Plates. This is a convergent margin, where the Pacific Plate is being subducted beneath the Australian. This causes uplift in the overlying Australian Plate, which has lifted Macquarie Island above the waves. This is usually accompanied by some degree of vulcanism, but this does not seem to be the case on Macquarie Island, to date at least. This makes the geology of the island rather unique, it is formed from basaltic seafloor, but has been exposed above the sea surface for between 80 000 and 700 000 years, during which time it has been exposed to constant erosion from the wind and storms of the Southern Ocean, but never actually glaciated. Due to its unique geology Macquarie Island was declared a World Heritage Site by UNESCO in 1997.

Macquarie Island; Macquarie Island Station in the foreground. IPS - Radio and Space Services.

The process of subduction is not a smooth one, the tectonic plates frequently stick together only to break apart again as the pressure builds up. This causes frequent Earthquakes in subduction zones, including Macquarie Island. In December 2004 the Island was subject to an Earthquake measuring 8.1 on the Richter Scale, one of the largest ever recorded instrumentally.

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