On July 29, 2016, the 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.
Slipping 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 at the 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 from 70 to 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 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.