M 7.6 - 80 km NE of Tilichiki, Russia

Tectonic Summary

The April 20, 2006, M 7.6 earthquake near Koryakia, Russia, occurred as the result of shallow reverse faulting in northeastern Russia several hundred kilometers north of the intersection between the Aleutian and Kuril-Kamchatka Trenches. Focal mechanism solutions indicate that rupture occurred on either a northeast- or southwest-striking, moderately dipping reverse fault. Of these two possible fault orientations, finite-fault modeling of globally recorded seismic data is more consistent with slip on the southwest-striking fault. The Koryakia earthquake occurred in sparsely populated northeastern Russia, directly north-northeast of the Kamchatka Peninsula. The earthquake occurred in a complicated geological setting where the configuration and interaction of the tectonic plates between northeastern Asia and northwestern North America are poorly understood. Recent models of the tectonic plates in eastern Russia show that the earthquake occurred near the junction of two plates. These are (1) the Okhotsk plate, which includes the Sea of Okhotsk, the Kamchatka Peninsula, and northern Japan, and (2) the extreme northwestern margin of the North America plate. Some evidence indicates that a third plate, the Bering plate, may be present and may play a role in the regional tectonics, but this plate is poorly defined with uncertain boundaries.

Geologically, northeastern Russia in the vicinity of the Koryakia earthquake is composed of a sequence of volcanic island arcs that have been attached or accreted to the margin of a continent. In addition, about 65–160 million years ago, sedimentary basins formed in the region, and magma intruded the crust and erupted onto it as volcanoes. The boundaries between accreted island arcs are commonly large faults that, under suitable conditions, might be reactivated, resulting in earthquakes. Because many of these old faults formed during the accretion process, it is difficult to determine which, if any of them, ruptured to cause the Koryakia earthquake.

Current models of plate interactions in northeastern Russia indicate that the region is being squeezed in a southeast-northwest direction, which causes shortening at a rate that might be only a few millimeters per year. This southeast-northwest direction of long-term shortening is consistent with the focal mechanism solution of the Koryakia earthquake, which shows that shortening produced thrust movement on faults that are oriented in a northeast-southwest direction.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the April 20, 2006, earthquake are typically about 90x45 km (length x width); modeling of this earthquake implies dimensions of about 120x30 km, predominantly up-dip to the north of the hypocenter.

This M 7.6 event was followed approximately 3 minutes later by a M 6.1 aftershock. The only other event of M 6+ within 400 km of the April 20th event over the preceding century was a M 6.6 earthquake in March 1991, just 6 km to the south.

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)