M6.8 - SEA OF JAPAN

Tectonic Summary

The recent magnitude 6.8 earthquake occurred 140 km off the west coast of Honshu, Japan at a depth of 350 km below the Sea of Japan. This oblique thrust event was likely caused by the release of stress built by the internal deformation of the Pacific plate as it subducts beneath the Okhotsk plate. The Okhotsk plate is a thin plate that lies between the Pacific Ocean and the Eurasian landmass. The north-south trending Japan Trench lies 700 km to the east of this earthquake, and is the surface expression of the boundary between the plates.

This deep-focus earthquake was preceded 13 hours earlier by a shallow crustal magnitude 6.6 quake roughly 330 km to the east. The two earthquakes were generated by different mechanisms. The shallow-focus earthquake was caused by deformation within the crust of the Okhotsk plate and the deep-focus quake was likely caused by the internal deformation of the Pacific plate. Given their different mechanisms and physical separation of at about 10 rupture lengths, the deep-focus earthquake is not considered an aftershock of the first.

Intermediate- and deep-focus earthquakes generally cause less damage than shallow-focus earthquakes since the energy generated by the deep events is released further from the surface and therefore produces less shaking than by quakes that are closer to the surface.

Deadly earthquakes have occurred in this region: In 2004, a magnitude 6.6 earthquake occurred in the Niigata Prefecture, 360 km to the northeast of the recent quake, killing 40 people and injuring about 3,000. In June, 1964, a M7.5 earthquake occurred about 425 km to the northeast of the recent quake killing 37 people; and in April, 1995 a M5.4 earthquake 380 km to the northeast, injured at least 39 people.

During the 20th century Japan has suffered nine devastating earthquakes that killed more that 1000 people each. Among these are the 1923 M7.9 earthquake that triggered the great Tokyo fire that killed 143,000 people and the more recent 1995 M6.9 Kobe earthquake that killed about 5,500 people.