M 7.7 - 156 km ENE of Poronaysk, Russia

Tectonic Summary

The August 14, 2012, M 7.7 earthquake near Poronaysk, Russia, occurred as a result of oblique reverse faulting within the subducting Pacific plate, approximately 580 km beneath the Sea of Okhotsk offshore of northeast Russia. Focal mechanism solutions indicate that reverse slip occurred on either a steep, southwest-striking fault or on a moderately dipping, northeast-striking fault. At the location of the earthquake, the Pacific plate moves towards the west-northwest relative to the North America plate at a velocity of about 81 mm/yr. 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 descent 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 that 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 greater than 650 km in this region. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

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. The largest recorded deep-focus earthquake prior to this August 2012 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. A larger event has since occurred—namely the M 8.3 earthquake that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk offshore of northeastern Russia in 2013. The M 8.3 Sea of 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, 84 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 7 of these were located in the same region as the August 14, 2012, event. The largest nearby event at these depths was another M 7.7 earthquake in July 2008, several hundred kilometers to the northeast and 50 km deeper than the August 14th event, with no recorded damage or casualties.

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)