M 7.5 - Mindanao, Philippines

Tectonic Summary

The March 5, 2002, M 7.5 earthquake south of Mindanao, Philippines, occurred as the result of shallow oblique reverse faulting within the Sunda plate. Focal mechanism solutions indicate that rupture occurred on either a moderately dipping, southeast-striking fault, or on a thrust fault dipping shallowly towards the northeast. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the northwest-striking thrust fault. At the location of the earthquake, the Philippine Sea plate moves west-northwest with respect to the Sunda plate at a velocity of approximately 100 mm/yr. Locally, lithosphere of the Celebes Sea Basin is subducting to the east-northeast beneath the Philippines at the Cotabato Trench, and the March 5th earthquake is likely associated with thrust faulting along that microplate boundary fault.

Northeastern Indonesia and southern Philippines are characterized by complex tectonics in which motions of numerous small microplates accommodate the large-scale convergence among the Philippine Sea, Sunda, Pacific, and Australia plates. The tectonic setting of the Philippines is unusual in that it is characterized by opposite-facing subduction systems on its east and west sides. North of the Cotabato system that likely generated this March 5 earthquake, Sunda lithosphere is also subducting at the Sulu Trench in the south, at the Negros Trench in central Philippines, and at the Manila Trench in the north.

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 March 5, 2002, earthquake are typically about 85x30 km (length x width); modeling of this earthquake implies similar dimensions, predominantly surrounding the hypocenter.

The most recent large earthquake of at least M 7 that occurred in the 200-km vicinity of the March 5, 2002, event was a M 7.5 event in November 1984, approximately 140 km to the southeast. Additionally, a M 7.9 earthquake occurred less than 50 km to the northwest of the March 5th event in August 1976. The 1976 earthquake and its associated tsunami caused upwards of 7,000 fatalities and 10,000 injuries and widespread damage along local coastlines, making the event one of the most devastating earthquakes in the history of the Philippines.

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)