M 7.4 - Bonin Islands, Japan region

Tectonic Summary

The August 6, 2000, M 7.4 earthquake in the Bonin Islands, Japan region occurred as the result of deep normal faulting within the subducted Pacific plate, approximately 390 km beneath the Philippine Sea south of Japan. Focal mechanism solutions for the earthquake indicate that rupture occurred on either a steeply dipping normal fault or on a shallowly dipping oblique normal fault within the interior of the subducted Pacific slab. Slip on a fault of either orientation would accommodate the down-dip extension of the Pacific slab that is implied by the normal-component of the faulting solution. At the location of the earthquake, the Pacific plate moves westward with respect to the Philippine Sea plate at a velocity of about 40 mm/yr and subducts at the Izu Trench, about 300 km to the east of the August 6th earthquake. The subducted Pacific plate is seismically active to a depth of about 560 km.

Earthquakes that have focal depths greater than 300 km such as this 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 2000 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 preceding century, 67 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 6, 2000, event. The largest nearby event at these depths was a M 7.3 earthquake in October 1968, approximately 200 km to the south and 110 km deeper than the August 6th event, with no recorded damage or casualties.

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)