A major earthquake with a magnitude of 7.6 (M7.6) occurred off the Philippines on 31 August 2012, sparking fears of a tsunami. Preliminary information from the United States Geological Survey indicates that the tremor occurred around 60 miles offshore to the north and east of the island of Mindanao, at a depth of just over 20 miles. The tremor was followed by several significant aftershocks, greater than M5.
Earthquake Leading to Tsunami?
Offshore earthquakes of high magnitude can trigger tsunamis, if the movement which causes the tremor is vertical, and so displaces vast amounts of water. The August 31 tremor clearly satisfied the first two requirements and although detailed seismological information is not available, it appears that the third has also been fulfilled.
Although tsunami warnings for large parts of the Pacific have been issued, and cancelled, the Pacific Tsunami Warning Center issued a bulletin stating that “sea level readings confirm that a tsunami was generated. This tsunami may have been destructive along coastlines of the region near the earthquake epicenter.” At the time of writing this warning was still in force.
Tectonic Setting of the Philippines
The Philippine archipelago is itself a product of large-scale earth movements. At the largest and most simple scale, the convergence of the Pacific and Eurasian plates is the main feature shown on a tectonic map of the Pacific Ocean. As in many other areas of the western Pacific, however, the actual situation is more complex, with several smaller plates (microplates) caught between the two larger ones. The different directions and types of movements of these plates, relative to one another, set up complex inter-plate stresses, generating friction, and thus earthquakes.
It appears that the August 31 earthquake was caused along the leading edge of the largest of these microplates, the Philippine plate, as it is forced beneath the microplates underlying the archipelago. Such a situation, known as a subduction zone, is characterized by a deep ocean trench (in this case the Philippines Trench).
Seismic History of the Philippines
Typically, subduction zones are subject to regular earthquakes, including some of the largest in the planet’s recorded history. The earthquake history of the Philippines, which forms part of the Pacific Ring of Fire (a circum-Pacific zone of earthquakes and volcanoes) reflects this, although it does not display either the frequency or magnitude of earthquake activity that occurs elsewhere – for example, the islands of Java and Sumatra.
The USGS listing of major earthquakes in the Philippines shows 10 events of ≥M6.0 in the Philippines since 1976, of which seven were ≥M7.0, three of them occurring on a single day. The largest tremor is an M7.9 which occurred in 1976, with the loss of some 8,000 lives. Typically, most of these larger earthquakes have occurred to the western side of the Philippine archipelago, whereas the most recent event occurred to the east.
Pacific Tsunami Warning Center/NOAA. Tsunami Bulletin 003. Accessed 31 August 2012
USGS. M7.6 – 96km E of Sulangan, Philippines. Accessed 31 August 2012
USGS. Tectonics and Volcanoes of the Philippines. Accessed 31 August 2012