Massive Earthquake shakes Cebu & Negros Islands in the Philippines.

Slightly before 11.50 am, local time (slightly before 3.50 am, GMT), on Monday 6 February 2012, the Philippine Islands of Cebu and Negros were shaken by an Earthquake measured by the United States Geological Survey as a 6.7 on the Richter Scale at a depth of 20 km, midway between the islands. Local press have widely reported this as a magnitude 6.9 quake, which may come from the Philippine Institute of Volcanology and Seismology, though their website is down at the time of writing, making this hard to check. The quake has been followed by a number of severe aftershocks, one at 6.10 pm, local time (10.10 am, GMT), measuring 6.0 on the Richter Scale according to the United States Geological Survey.

Map showing the location of the quakes, cluster of blue squares. The first quake is the yellow square behind the blue squares, other yellow squares represent other quakes within the last week. The red line is the Philippine Trench, where the Philippine Sea Plate to the east is being subducted beneath the Philippine Mobile Belt, on which most of the Philippine Islands sit. From the United States Geological Survey.

At the time of writing the official death toll from the quake is 15, with unofficial sources giving numbers as high as 52. Official figures also give 71 missing, 52 hospitalized and many hundreds more injured. The majority of the deaths are likely to have been caused by landslides, though there is apparently also widespread damage to buildings and bridges. The full extent of the destruction will probably not be known for several days. At the time of the initial quake there was widespread fear that it would be followed by a dangerous tsunami, but this did not occur.

Sudden inrush of water in Cebu, following the quake; technically this is a tsunami, but not life-threatening. Irene Nipaya, Balamban, Cebu.

The geology of the Philippines is complex, with the Philippine Sea Plate to the east and the Sunda Plate to the west both being subducted beneath the Philippine Mobile Belt, on which most of the islands sit. The Mobile Belt is slowly being pulled in several different directions by this, and geologists have mapped hundreds of faults in the islands - often by measuring quakes. On this occasion the quake may have revealed a previously unknown fault, according to Ishmael Narag of the Philippine Institute of Volcanology and Seismology's Seismological Observation and Earthquake Prediction Division.

Map of the Philippines showing Subductive Margins (Trenches) and known faults. From the Philippine Institute of Volcanology and Seismology.

See also Earthquake shakes Yamanashi Prefecture, Explosion on Suwanosejima, Earthquake in the Babuyan Islands of the Northern Philippines, Eruptions from the Tompaluan Crater and Earthquakes on Sciency Thoughts YouTube.

Manila shaken by mild earthquake. 30 November 2011.

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.

See also Hokkaido Earthquake, Earthquake in Vanuatu, Fukashima shaken by large earthquake, Fresh eruption on Mount Karangetang and Earthquakes on Sciency Thoughts YouTube.