M 7.1 - 106 km WSW of Sangay, Philippines

Tectonic Summary

The July 11, 2024, M 7.1 earthquake in the Moro Gulf, south of Mindanao, Philippines, resulted from deep normal faulting within the inclined seismic zone defining the deep limit of the Molucca Sea microplate beneath the Celebes Sea Basin. The earthquake 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. 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.

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 distances from their epicenters. The largest recorded deep-focus earthquake was an 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 distances from the epicenter.

Since 1900, 105 earthquakes with a magnitude of M 7+ have occurred globally at depths greater than 300 km. This July 11, 2024, earthquake occurred roughly 50 km south of a sequence of 3 deep M7+ earthquakes on July 23, 2010, none of which resulted in fatalities.