August 11, 2021 Earthquake: Mindanao, Philippines

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Tectonic Summary

The August 11, 2021, M 7.1 earthquake off the southeast coast of the island of Mindanao, Philippines occurred as a result of oblique reverse and strike-slip faulting at moderate depth in the Earth. Preliminary estimates of the depth and mechanism of the earthquake are consistent with the earthquake having occurred in the continental lithosphere above the ~90 km deep subducted Philippine Sea plate. Focal mechanism solutions indicate that rupture occurred on either a west-southwest or east-southeast dipping reverse fault with a component of either right-lateral strike-slip motion to the northeast or left-lateral strike-slip motion to the southeast. 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.

While commonly plotted as points on maps, earthquakes the size of the August 11, 2021 event are more appropriately described as slip over a larger fault area. Earthquakes of this size are typically about 50x20 km (length x width).

As at many subduction zone plate boundaries, the Philippines frequently experiences moderate to large earthquakes. 142 earthquakes of M6 or larger have occurred over the past 100 years within 250 km of the August 11, 2021 earthquake, including a M7.0 earthquake on January 21, 2021, approximately 180 km SSE. Earthquakes in the Philippines region exhibit diverse styles of faulting that are indicative of the locally complex tectonic motions. Moreover, this region experiences earthquakes across a range of depths, from shallow surface rupturing strike-slip earthquakes, to deep earthquakes located within subducting oceanic slabs. While magnitude 7.0 earthquakes are capable of causing severe damage to infrastructure, this earthquake was fortunately located far from built structures.

Reference #13005:

A 20-year-old man in Tandag City, Surigao del Sur reportedly died after being hit in the head by falling coconuts following the magnitude 7.1 earthquake that shook parts of Mindanao on Thursday dawn.