M 7.1 - 161 km S of ?yama, Japan

Tectonic Summary

The August 9, 2009, M 7.1 earthquake off the south coast of Honshu, Japan, occurred as the result of deep oblique-reverse faulting, approximately 300 km beneath the Pacific Ocean off the southern coast of Japan, within the Izu-Bonin subduction system. Focal mechanism solutions indicate that rupture occurred on either a near-vertical reverse fault or a shallowly dipping thrust fault. Slip on a fault of either orientation would accommodate the down-dip compression of the Pacific slab that is implied by the reverse component of the faulting solution. At the location of the earthquake, the Pacific plate moves westward relative to the Philippine Sea plate at a velocity of about 50 mm/yr, subducting at the Izu-Bonin Trench several hundred kilometers to the east of this earthquake. The Izu-Bonin subduction zone as a whole 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 August 9, 2009, 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, as well as within the subducting or overriding plates as the result of stresses associated with intraplate deformation. The August 9, 2009, earthquake was an intraplate event. Earthquakes have been accurately located to depths of 450 km in the section of the Izu-Bonin subduction zone within which the August 9th earthquake occurred.

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 2009 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, 77 earthquakes with a magnitude of M 7+ have occurred at depths greater than 300 km globally; 12 of these were located in the same region as the August 9, 2009, event. The largest nearby event at these depths was a M 7.3 earthquake in October 1968, several hundred kilometers to the southeast and 200 km deeper than the August 9th event, which did not result in any recorded damage.

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)