Seismotectonics of Japan and Vicinity
The North America plate, Pacific plate, Philippine Sea plate, and Eurasia plate all influence the tectonic setting of Japan, Taiwan, and the surrounding area. Some authors divide the edges of these plates into several microplates that together take up the overall relative motions between the larger tectonic blocks, including the Okhotsk microplate in northern Japan, the Okinawa microplate in southern Japan, the Yangzee microplate in the area of the East China Sea, and the Amur microplate in the area of the Sea of Japan.
The seafloor expression of the boundary between the Pacific and North America plates lies 300 km off the east coasts of Hokkaido and Honshu at the Kuril-Kamchatka and Japan trenches. The subduction of the Pacific plate beneath the North America plate, at rates of 83-90 mm/yr, generates abundant seismicity, predominantly as a result of interplate slip along the interface between the plates. The 1958 M 8.4 Etorofu, 1963 M 8.6 Kuril, 2003 M 8.3 Tokachi-Oki, and the 2011 M 9.0 Tohoku earthquakes all exemplify such megathrust seismicity. The 1933 M 8.4 Sanriku-Oki earthquake and the 1994 M 8.3 Shikotan earthquake are examples of intraplate seismicity, caused by deformation within the lithosphere of the subducting Pacific plate (Sanriku-Oki) and of the overriding North America plate (Shikotan), respectively.
At the southern terminus of the Japan Trench the intersection of the Pacific, North America, and Philippine Sea plates forms the Boso Triple Junction, the only example of a trench-trench-trench intersection in the world. South of the triple junction the Pacific plate subducts beneath the Philippine Sea plate at the Izu-Ogasawara trench, at rates of 45-56 mm/yr. This margin is noteworthy because of the steep dip of the subducting Pacific plate (70° or greater below depths of 50 km depth), and because of its heterogeneous seismicity; few earthquakes above M 7 occur at shallow depths, yet many occur below 400 km. The lack of large shallow megathrust earthquakes may be a result of weak coupling at the plate interface, or simply a reflection of an incomplete earthquake catalog with respect to the length of typical seismic cycles.