Poster of the Izu Islands, Japan Earthquake of 09 August 2009 - Magnitude 7.1

Tectonic Summary

The magnitude 7.1 earthquake of August 9, 2009, occurred off the south coast of Honshu within the Izu-Bonin subduction system. This system extends 1,200 km from southern Honshu to the Mariana Islands and constitutes part of the boundary between the Pacific plate and the Philippine Sea plate. The Pacific plate moves westwards relative to the Philippine Sea plate with a velocity of about 50 mm/y and subducts beneath it at the Izu-Bonin trench. The earthquake occurred at depth within the westward dipping Pacific plate, well beneath the overriding Philippine Sea plate. Subduction of the Pacific plate along the northern and western margins of the Pacific Ocean generates a large amount of seismicity. Earthquakes typically occur as thrust faulting on interfaces that separate the subducting Pacific plate from the overriding plates, and earthquakes occur within the subducting or overriding plates as the result of stresses associated with intraplate deformation. The August 9, 2009, earthquake was an intraplate event. Its focal depth of 303 km classifies it is a "deep" earthquake, an earthquake with depth greater than or equal to 300 km. Earthquakes have been accurately located to depths of 450 km in the section of the Izu-Bonin subduction system within which the August 9 earthquake occurred. Due to its great depth, the August 9 earthquake did not cause any damage and no damaging tsunami was generated.