M 8.3 - Sea of Okhotsk

Tectonic Summary

The May 24, 2013, M 8.3 earthquake beneath the Sea of Okhotsk, Russia, occurred as a result of deep normal faulting at a depth of approximately 600 km. Focal mechanism solutions indicate that rupture occurred on either a near-vertical, north-striking normal fault, or on a shallow, south-striking normal fault. At the location of this earthquake, the Pacific and North America plates are converging at a velocity of about 78 mm/yr in a west-northwest–east-southeast direction, resulting in the subduction of the Pacific plate beneath Eurasia at the Kuril-Kamchatka Trench. Note that some authors divide this region into several microplates that together define the relative motions among the larger Pacific, North America and Eurasia plates; these include the Okhotsk and Amur microplates that are part of North America and Eurasia, respectively. The depth and faulting mechanism of the May 24th earthquake indicate that it ruptured a fault deep within the subducting Pacific lithosphere rather than on the shallow 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.

This deep section of the Pacific slab beneath the Sea of Okhotsk has hosted several large earthquakes in the past—four M 6+ events within 200 km of the May 24th event since 1988. These included a M 7.7 earthquake in July 2008, 115 km to the southwest at a depth of 630 km, and a M 7.3 event in November 2008, 95 km to the southeast at a depth of 490 km. Neither of these events resulted in documented casualties or damage. The Pacific slab in the region of the May 24th earthquake is seismically active to depths of greater than 650 km.

Earthquakes that have focal depths greater than 300 km are commonly termed “deep-focus” earthquakes. Deep-focus earthquakes cause less damage on the ground surface above their foci than similar-magnitude shallow-focus earthquakes, but large deep-focus earthquakes may be felt at great distance from their epicenters. Prior to this event, the largest recorded deep-focus earthquake was a M 8.2 event that occurred at a depth of 630 km within the subducted Nazca plate beneath South America near the northern Bolivian border in 1994. The May 2013 Okhotsk earthquake is approximately 1.5 times larger than the Bolivian event. The Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific Ocean along the western seaboard of the United States (though at distant locations, individuals reporting having felt the event were likely very favorably situated for the perception of small ground motions). The M 8.2 Bolivian deep-focus earthquake in 1994 had similarly been reported by individuals in North America at great distance from the epicenter.

Over the past century, 85 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 8 of these were located in the same region as the May 24, 2013, event. The largest nearby event at these depths was a M 7.7 earthquake in July 2008, just 100 km to the south and 40 km deeper than the May 24, 2013, event.

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)