M 7.6 - 61 km W of Bantogon, Philippines

Tectonic Summary

The July 23, 2010, (22:51:12 UTC) M 7.6 earthquake in the Moro Gulf, south of Mindanao, Philippines, occurred as the second of three M 7+ earthquakes within just over an hour (following an earthquake of M 7.3 at 22:08 UTC, and preceding a M 7.5 event at 23:15 UTC). Each was the result of deep normal faulting within the inclined seismic zone defining the deep limit of the Molucca Sea microplate 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 the July 23rd earthquakes, 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/yr range. Locally, arc-arc collision is occurring between the Sangihe microplate and the Philippine Sea plate, wedging between them the Molucca Sea microplate, which subducts beneath both (to the east and west) and forms an inverted-U-shaped seismic zone. At the location of the July 23rd 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 microplate 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 microplate at depth, rather than on one of the shallower interfaces with the overriding Sangihe and Philippine Sea plates. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

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 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 prior to these July 2010 earthquakes was a M 8.2 event that occurred at a depth of 630 km within the subducted Nazca plate beneath South America near the northern Bolivian border in 1994. A larger event has since occurred—namely the M 8.3 earthquake that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk offshore of northeastern Russia in 2013. The M 8.3 Sea of Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific Ocean along the western seaboard of the United States (though at distant locations, individuals reporting having felt the event were likely very favorably situated for the perception of small ground motions). The M 8.2 Bolivian deep-focus earthquake in 1994 had similarly been reported by individuals in North America at great distance from the epicenter.

Over the past century, 80 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; four of these were located in the same region as the July 23, 2010, events. The largest nearby event at these depths was a M 7.3 earthquake in March 1983, less than 200 km to the north and 10–60 km deeper than the July 23rd events, with no associated damage or casualties.

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)