Tectonic Summary
According to the US Geological Survey, the earthquake was caused by thrust faulting on the boundary between the Australian Plate and the Sunda Plate. At the location of the earthquake, offshore of the west coast of Sumatra, the Australia plate moves northeast with respect to the Sunda Plate at a velocity of about 69 mm/year, oblique to the orientation of the plate boundary. The component of plate motion perpendicular to the boundary produces thrust faulting on the offshore plate-boundary.
The magnitude 7.9 earthquake was also a result of thrust faulting on the same plate boundary. It occurred about 225 km northwest of the magnitude 8.4 earthquake, at the northern end of the aftershock zone.
The magnitude 8.5 earthquake was the most powerful earthquake of 2007 to date, more powerful than the two March 2007 Sumatra earthquakes nearby and the 2007 Peru earthquake. It was the second most powerful earthquake since the magnitude 9.3 2004 Indian Ocean earthquake, exceeded only by the magnitude 8.6 2005 Sumatra earthquake. The two largest earthquakes were the fourth and fifth earthquakes with magnitude 7.9 or greater to have occurred on or near the plate boundaries offshore of western Sumatra in 7 years, the others being the magnitude 7.9 earthquake of 4 June 2000; the magnitude 9.3 2004 Indian Ocean earthquake on 26 December 2004; and the magnitude 8.6 2005 Sumatra earthquake on 28 March 2005.
According to data from the Japanese PALSAR microwave sensor aboard the Daichi(ALOS) satellite, South Pagai Island was uplifted, creating 6 new islands, and exposing previously submerged coral reefs as well as enlarging existing islands.
Read more about this topic: September 2007 Sumatra Earthquakes
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