Friday, 31 August 2012

Massive Earthquake to the east of the Philippines.

On Friday 31 August 2012, slightly after 8.45 pm local time (slightly after 12.45 pm, GMT), the United States Geological Survey recorded an Earthquake roughly 100 km east of Samar Island in the Philippines as measuring 7.6 on the Richter Scale, at a depth of 34.9 km. This is an extremely large quake and is likely to cause considerable problems throughout the Philippines, and potentially neighbouring countries. No reports of any damage or casualties have emerged yet, but the USGS estimate such a quake in this location will have a 35% chance of causing at least one fatality. The NOAA Tsunami Centre issued a tsunami alert for the Philippines, Japan, Indonesia, Taiwan, Papua New Guinea, Hawaii,Guam, Yap and the Northern Mariana Islands, though this has now been lifted for some of these areas.

Map showing the areas that suffered the worst shaking. Areas within the greenish circle are likely to have suffered damage to buildings. USGS.

The geology of the Philippines is complex, with the majority of the islands located on the east of the Sunda Plate. To the east of this lies the Philippine Sea plate, which is being subducted beneath the Sunda Plate; further east, in the Mariana Islands, the Pacific  Plate is being subducted beneath the Philippine Sea Plate. 

The 31 August 2012 quake appears to have happened on the Philippine Trench, where the Philippine Sea Plate is being subducted beneath the Sunda Plate. Quakes on such margins are not uncommon, as tectonic plates do not sink smoothly past one-another, but rather stick then break apart. This presents a high risk of tsunami generation on subductive margins, as the overlying plate can stick to the underlying plate and be drawn back like a bow-string, until the pressure causes the rocks to rift sharply, and the plate margin snaps back suddenly.

Cartoon animation showing how a tsunami can be genrated on a subduction zone. EarthScope.

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