Slightly before 8.30 am on Wednesday 30 November 2011 an Earthquake with a magnitude of 6 on the Richer Scale occurred at a depth of about 2 km on the Manila Trench 200 km to the northwest of the city of Manila. While this was a fairly severe quake, and shallow enough to be extremely dangerous, it was far enough off shore to cause serious damage, though it was felt across much of the west of Luzon island, including in the Philippine capital, Manila. No tsunami warning has been issued.
The location of the 30 November quake.
The Manila Trench runs along the western edge of Luzon and the northern Philippines, at the boundary of the Eurasian Plate and the Philippine Mobile Zone, an area of small fragmented tectonic plates underlying the Philippines. To the east these are in turn being subducted beneath the Philippine Sea Plate, and to the south beneath the Sunda Plate. All of these small plates form part of a boundary between the Eurasian Plate and the Pacific Plate, which creates a complex tectonic setting running from Japan to Papua New Guinea.
Along the Manila Trench the Eurasian Plate is being forced beneath the Philippine Mobile Belt creating friction. As the subducting plate sinks it catches on the overlying plate, so that it moves in stops and starts, causing earthquakes. As the plate is forced further down it partially melts, and the melted material rises up through the overlying Mobile Belt, erupting at the surface to form volcanoes; these volcanoes are ultimately responsible for all the land that makes up the Philippines.
It is sometimes reported that the Philippine Volcanic Arc is caused be the subduction of the Pacific beneath the Philippine Plate. This is incorrect; the Pacific is being subducted beneath the Philippine Sea Plate, which lies to the east of the Philippine Islands. Where this subduction occurs it does create a volcanic island arc; the Marianas, far to the east of the Philippines.
All this makes the Philippines a very tectonically active place. The Philippine Institute of Volcanology and Seismology recorded eighteen on 20 November 2011, though most were quite small. The Philippines have suffered seven earthquakes measuring greater than 6 on the Richer Scale since 2000. Only one of these, in 2002, caused recorded fatalities. The islands also boast 37 volcanoes, of which 18 are active.
Mount Mayon in the Philippines erupting, with a dramatic pyroclastic flow to the left.