Tectonic Summary

The August 31, 2012, M 7.6 earthquake off the east coast of the Philippines occurred as a result of shallow reverse faulting within the oceanic lithosphere of the Philippine Sea plate. Focal mechanism solutions indicate that rupture occurred on either a north-northwest- or south-southwest-striking, moderately dipping reverse fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the northwest-striking fault. At the location of the earthquake, the Philippine Sea plate moves west-northwest at a velocity of about 100 mm/yr with respect to the Sunda plate. The preliminary location of the earthquake indicates that this is an intraplate event, 50 km or more to the east of this subduction zone plate boundary between these two plates.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the August 31, 2012, earthquake are typically about 90x45 km (length x width); modeling of this earthquake implies dimensions of about 60x60 km, predominantly surrounding the hypocenter.

While this region of the Philippines experiences moderate to large earthquakes fairly frequently—approximately 40 events of M 6+ have occurred over the past 40 years within 250 km of the August 31, 2012, earthquake—large events outboard of the subduction zone are rare. While several moderate-sized events have occurred in this intraplate region, most have been the result of normal faulting within the shallower (depth less than 15 km) oceanic lithosphere, rather than deeper (greater than 30 km) reverse faulting similar to the August 31st event. The largest nearby event regardless of mechanism was the October 1975 M 7.6 earthquake, approximately 200 km to the north of the August 31st event, which resulted in at least one fatality.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)

Event Details

Event Map

Tectonic Summary

Historical Magnitude 4.5+ (100km)

Tectonic Summary

Historical Magnitude 4.5+ (100km)