M7.7 - SEA OF OKHOTSK

Tectonic Summary

The August 13, 2012 M 7.7 earthquake near Poronaysk, Russia occurred as a result of oblique-reverse faulting deep within the subducting Pacific plate beneath the Sea of Okhotsk, offshore of northeast Russia. The earthquake ruptured a fault in the interior of the inclined subduction zone that dips to the west-northwest beneath the Kuril Islands and Kamchatka, having begun its decent into the mantle at the Kuril-Kamchatka trench. The event resulted from stresses generated by the slow distortion of the subducting plate as it descends through the mantle, rather than on the thrust interface the constitutes the boundary between the Pacific and overlying North America plates; the latter is active only near the Earth's surface, while the subducting Pacific plate is active to depths of over 650 km in this region. At the latitude of the August 13 earthquake, the Pacific plate moves approximately 81 mm/yr towards the west-northwest with respect to the North America plate. The plate boundary here is sometimes divided into several microplates that together define the relative motions between the larger Pacific, North America and Eurasia plates, including the Okhotsk and Amur microplates that are respectively part of North America and Eurasia. Deep earthquakes in this region of the Pacific plate are not uncommon; there have been 10 similar events deeper than 450 km over the past 450 years, within 300 km of the August 13 2012 earthquake. The largest was a M 7.3 event in November of 2003, approximately 230 km to the south-southeast. 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 is the case with similar magnitude shallow-focus earthquakes, but large deep-focus earthquakes may be felt at great distance from their epicenters. The largest recorded deep-focus earthquake had a magnitude of 8.2, and occurred deep beneath Bolivia in 1994.