M 7.7 - Pagan region, Northern Mariana Islands

Tectonic Summary

The July 29, 2016 M 7.7 earthquake in the Northern Mariana Islands region occurred as the result of oblique reverse faulting at an intermediate depth, approximately 210 km beneath the Pacific Ocean and 200 km west of the Mariana Trench, which marks where the Pacific plate begins its subduction beneath the overriding Philippine Sea plate. Focal mechanism solutions indicate oblique rupture occurred on either a south-southwest or northwest striking reverse fault. Slip on a fault of either orientation is consistent with the intraplate compressional tectonics implied by the faulting mechanism and earthquake depth. At the location of the earthquake, the Pacific plate moves to the west relative to the Philippine Sea plate with a velocity of about 40 mm/yr, and at about 60 mm/yr relative to the Mariana microplate. The earthquake likely represents the release of stress resulting from the distortion of the Pacific plate at depth.

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. "Deep-focus" earthquakes, those with focal depths greater than 300 km, also occur in the subducted Pacific plate beneath the Mariana island arc. Earthquakes have been reliably located to depths of about 630 km beneath the Mariana arc.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse faulting events of the size of the July 29, 2016 earthquake are typically about 110x40 km in size (length x width).

The Mariana Arc hosts frequent moderate-to-large earthquakes, and four other events over M 7 have occurred within 250 km of the July 29th earthquake in the preceding century. These include a deep (595 km) M 7.1 event in August 1995, approximately 70 km to the northwest, and a M 7.2 in October 2007, also at an intermediate depth (207 km), and 60 km to the north-northwest of the July 29th, 2016 earthquake. The remote location of all of these earthquakes makes them minimally disruptive.