Tectonic Summary
The May 30, 2015 M7.8 earthquake WNW of Chichi-shima, Japan occurred as the result of oblique-normal faulting at a depth of over 660 km beneath the Pacific Ocean. The earthquake is located within the interior of the Pacific plate that subducts beneath Japan starting at the Izu trench, several hundred kilometers to the east of the event (and at the surface of the Earth). At the location of the earthquake, the Pacific plate moves approximately westwards with respect to the Philippine Sea plate at a rate of 39 mm/yr. This earthquake occurred in response to stresses generated by the slow distortion of the Pacific plate at depth, rather than occurring on the interface between the Pacific plate and the overriding Philippine Sea plate.
Earthquakes that have focal depths greater than 300 km are commonly termed "deep-focus". Deep-focus earthquakes cause less damage on the earth’s surface above their foci than is the case with similar magnitude shallow-focus earthquakes. Large deep-focus earthquakes may be felt at great distance from their epicenters. The largest recorded deep-focus earthquake was a 2013 M 8.3 earthquake that occurred at a depth of 600 km within the subducted Pacific plate beneath the Sea of Okhotsk, offshore northeastern Russia. The M 8.3 Okhotsk earthquake was felt all over Asia, as far away as Moscow, and across the Pacific along the western seaboard of the United States. Over the past century, 66 earthquakes with a magnitude of M7 or more have occurred at depths greater than 500 km; three of these were located in the same region as today's event. The largest nearby event at these depths was a 1968 M 7.3 earthquake, several hundred kilometers to the south of this earthquake.
Dave