Poster of the Mariana Islands Region Earthquake of 28 September 2007 - Magnitude 7.5

Tectonic Summary

The Mariana Islands earthquake of September 28, 2007, occurred in the inclined seismic zone that marks the position of the subducted Pacific plate beneath the overriding Philippine Sea plate. The Pacific plate moves to the west with respect to the Philippine Sea plate with a velocity of about 40 mm/yr and thrusts under the Philippine Sea plate at the Mariana Trench. The earthquake represents the release of stress resulting from the distortion of the Pacific plate.

Earthquakes, such as this one, that have focal depths between 70 and 300 km are commonly termed intermediate-depth earthquakes. Deep-focus earthquakes, 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 Marianas arc.