Poster of the Moro Gulf, Mindanao, Philippines Earthquakes of 23 July 2010 - Magnitudes 7.3, 7.6, 7.4

Tectonic Summary

The July 23, 2010 earthquakes in the Moro Gulf, south of Mindanao, Philippines, occurred within the inclined seismic zone defining the deep limit of the Molucca Sea micro plate beneath the Celebes Sea basin. Northeastern Indonesia and southern Philippines are characterized by complex tectonics in which motions of numerous small plates accommodate the large-scale convergence between the Philippine Sea and Sunda plates. In the region of today's earthquake, the Philippine Sea plate moves west-northwest with respect to the Sunda plate at a velocity that various models would place in the 60-110 mm/year range. Locally, arc-arc collision is occurring between the Sangihe microplate and the Philippine Sea plate, wedging between them the Molucca Sea micro plate, which subducts beneath both (i.e. to the east and west) and forms an inverted-U-shaped seismic zone. At the latitude of the July 23 earthquakes, the top of the Molucca Sea microplate is at a depth of about 150 km beneath the earth's surface. Seismicity within the Molucca Sea micro plate is active to depths of approximately 260 km to the east and 650 km to the west. The tectonic setting of this region is unique in that it is the only global example of an active arc-arc collision consuming an oceanic basin via subduction in two directions.

The July 23rd events occurred in response to stresses generated by the slow distortion of the Molucca Sea micro plate at depth, rather than on the shallower interfaces with the overriding Sanglehe and Philippine Sea plates, active boundaries in this region near the earth's surface.

Earthquakes that have focal depths greater than 300 km are commonly termed "deep-focus" earthquakes. Deep-focus earthquakes cause less damage on the ground surface above their foci than is the case with similar magnitude shallow-focus earthquakes, but large deep-focus earthquakes may be felt at great distance from their epicenters. The largest recorded deep-focus earthquake had a magnitude of 8.2, and occurred deep beneath Bolivia in June 1994. Over the past 50 years, approximately 50 earthquakes with magnitudes of M7 or more have occurred at depths greater than 500 km; just two of these were located in the same region as today's events.

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