M 7.0 - 211 km SE of Pondaguitan, Philippines

Tectonic Summary

The January 21, 2021, M 7.0 earthquake off the southeast coast of the Philippines occurred as a result of reverse faulting likely within the subducted oceanic lithosphere of the Philippine Sea plate. Focal mechanism solutions indicate that rupture occurred on either a north-northwest-striking, shallow-dipping reverse fault or a south-striking, steeply-dipping reverse 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.

Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed "intermediate-depth" earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters.

While commonly plotted as points on maps, earthquakes the size of the January 21, 2021 event are more appropriately described as slip over a larger fault area. Shallow reverse-faulting earthquakes of this size are typically about 40x20 km (length x width). The typical fault dimensions for intermediate depth earthquakes are less well known.

This region of the Philippines experiences moderate to large earthquakes frequently. Eighty-six events of M 6+ have occurred over the past 50 years within 250 km of the January 21, 2021, earthquake. While magnitude 7.0 earthquakes are capable of causing severe damage to infrastructure, this earthquake was fortunately located far from built structures. Reports of shaking on Mindanao Island indicate minor damage is possible but significant damage is not expected.