Saturday 3 November 2012

Earthquake off Mindanao Island in the Philippines.

On Saturday 3 November 2012, slightly after 2.15 am local time (slightly after 6.15 pm on Friday 2 November, GMT) the The Philippine Institute of Volcanology and Seismology recorded a Magnitude 6.5 Earthquake at a depth of 78 km, off the northeast coast of Mindanao Island. No damage or casualties have been reported, but the United States Geological Survey, which measured the quake as a magnitude 6.1 event at a depth of 38.1 km, estimate that a quake on this scale in this area would have a 35% chance of leading to fatalities.

Map showing the area where the quake could be felt  (black circle) and where damage to buildings was likely (red circle). Geoscience Australia/Google Maps.

The geology of the central Philippines is Complex. The west of Mindanao Island is located on the Banda (or Sunda) Microplate, and the east on the Philippine Sea Plate, which is being subducted beneath the Sunda (or Banda) Microplate along the central part of the island. Immediately to the east of the Island the Pacific Plate is being subducted along the Philippine Trench, and passes beneath eastern Mindanao as it sinks into the Earth; this is the margin along which the 3 November quake took place.

Diagram showing the relationship between the Banda (or Sunda), Philippine Sea, and Pacific Plates beneath Mindanoa. Pubellier & Meresse (2012).

As the plates pass under one another they constantly stick together until the pressure from the westward movement of the Pacific Plate builds up sufficiently to force them to break apart and shift relative to one-another, causing Earthquakes. In addition, as the plates sink beneath each-other, they are heated by the friction and the heat of the planet's interior, causing them to partially melt. Some of the melted material then rises through the overlying plates, fueling volcanism.

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