M7.7 - SEA OF OKHOTSK

Tectonic Summary

The Sea of Okhotsk earthquake of 5 July 2008 occurred in the inclined seismic zone that dips to the west-northwest beneath Kamchatka and the Kurile Islands. In the region of Kamchatka, the Pacific plate moves to the west-northwest with respect to the Okhotsk plate with a velocity of about 80 mm/y. The Pacific plate thrusts under the Okhotsk plate at the Kuril-Kamchatka trench and is subducted into the mantle. The earthquake occurred within the Pacific plate, in response to stresses generated by the plate’s slow distortion, rather on the thrust fault that constitutes the interface the between the Okhotsk and Pacific plates and which is seismically active near the earth’s surface. The Pacific plate is active to depths of about 650 km in the region of the earthquake. 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.