M 7.1 - 20 km SSW of Merizo Village, Guam

Tectonic Summary

The April 26, 2002, M 7.1 earthquake near Guam occurred as the result of reverse faulting at intermediate depth, approximately 85 km beneath the Pacific Ocean less than 50 km off the southern coast of Guam. Focal mechanism solutions indicate that rupture occurred on either a near-vertical reverse fault or a very shallowly dipping oblique thrust fault. At the location of the earthquake, the Pacific plate subducts to the northwest beneath the Philippine Sea plate at a velocity of about 18 mm/yr, beginning its descent into the mantle at the Mariana Trench to the south of the April 26th earthquake. The depth, location, and focal mechanism solutions of this earthquake suggest reverse faulting associated with internal deformation of the subducted Pacific slab, rather than occurring on the shallower thrust interface between the two plates. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

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 Island Arc.

In the past decade, this location has been the site of two other M 7+ events. The larger of the two was a M 7.8 earthquake in August 1993 about 20 km to the southeast of the April 26th event, which caused injury and extensive damage on the island of Guam.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)