M 7.4 - 118 km ESE of Shing?, Japan

Tectonic Summary

The September 05, 2004, M 7.4 earthquake southeast of Osaka, Japan, occurred as the result of shallow, oblique reverse faulting on or near the plate boundary between the Philippine Sea and Eurasia plates, where the Philippine Sea plate subducts northwestward beneath southern Japan at the Nankai Trough. This M 7.4 event occurred less than 5 hours after a nearby M 7.2 earthquake with the same faulting mechanism. Focal mechanism solutions indicate that rupture occurred on either an east- or west-striking, moderately dipping reverse fault. At the location of the earthquake, the Philippine Sea plate moves towards the northwest relative to the Eurasia plate at a velocity of about 52 mm/yr. Note that some authors divide this region into several microplates that together define the relative motions among the larger Pacific, North America and Eurasia plates; these include the Yangtze and Amur microplates local to this earthquake, both part of the eastern edge of Eurasia. Given the obliquity of both nodal planes in the focal mechanism solution to the trend of the local subduction zone and the depth of the event relative to the interplate thrust, the event likely represents intraplate faulting within the subducting Philippine Sea plate, rather than thrusting on the overlying plate boundary. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

The plate boundary region surrounding the September 5, 2004 earthquake has hosted large and damaging earthquakes in the past. This M 7.4 (14:57 UTC) event was preceded by a M 7.2 earthquake that occurred 45 km west just 5 hours earlier. The epicenters of both September 5th earthquakes are also just northeast of the 1946 Nankai earthquake, a M 8.3 event that caused 1,300 fatalities and severe damage in southern Japan, as well as an extensive tsunami. Similar megathrust earthquakes are thought to recur on this section of the Nankai subduction zone every 90–150 years.

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)