Poster of the Japan Earthquakes of 16 July 2007 - Magnitude 6.6 and 6.8

Tectonic Summary

This magnitude 6.6 earthquake occurred near the west coast of Honshu, Japan, in a zone of compressional deformation that is associated with the boundary between the Amur plate and the Okhotsk plate. At this latitude, the Okhotsk plate is converging to the west-northwest towards the Amur plate with a velocity of about 9 mm/yr. The Amur and Okhotsk plate are themselves relatively small plates that lie between the Eurasia plate and the Pacific plate. The Pacific plate converges west-northwest towards the Eurasia plate at over 90 mm/yr. Most of the relative motion between the Pacific plate and the Eurasia plate is accommodated approximately 400 km to the east-southeast of the epicenter of the earthquake, where the Pacific plate subducts beneath the Okhotsk plate.

This shallow crustal earthquake was followed 13 hours later by a deep focus magnitude 6.8 quake roughly 330 km to the west, 350 km below the Sea of Japan. The two earthquakes were generated by different mechanisms. The first earthquake was caused by deformation within the crust of the Okhotsk plate and the second quake was likely caused by faulting resulting from internal deformation of the subducted Pacific plate. Given their different mechanisms and physical separation of at least 10 rupture lengths, the second earthquake is not considered an aftershock of the first.

Shallow earthquakes cause more damage than intermediate- and deep-focus ones since the energy generated by the shallow events is released closer to the surface and therefore produces stronger shaking than is produced by quakes that are deeper within the Earth.

Other deadly earthquakes have occurred in this region: In 2004, a magnitude 6.6 earthquake occurred in the Niigata Prefecture, 50 km to the southeast of the recent quake, killing 40 people and injuring about 3,000. In June, 1964, a M7.5 earthquake occurred about 115 km to the north-northeast of the recent quake, killing 37 people; and in April, 1995 a M5.4 earthquake 80 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.