Looking at a map of the North Pacific, one finds that the Aleutian Chain of Alaska is continuous with other islands off the east coast of Asia; the latter include the islands of Japan. This curving line of islands marks the southern edge of the North American Plate, which extends northward to include eastern Siberia; south of Tokyo, the Japanese islands lie on the eastern edge of the Eurasian Plate. Indeed, Japan lies at the intersection of four major tectonic plates: the North American, Eurasian, Philippine and Pacific Plates.
The North American and Eurasian Plates are colliding in the center of Honshu, Japan's largest island; though the exact position of this collision zone remains controversial, most geologists place it just southwest of Tokyo. The Philippine Plate is subducting beneath the Eurasian Plate along the southern margin of Japan while the Pacific Plate is subducting beneath the North American Plate along the eastern edge of this island nation. Finally, to the southeast, the Philippine and Pacific Plates are scraping against one another.
All of this tectonic activity makes Japan very susceptible to earthquakes; today's quake was off the east coast of Hokkaido (Japan's large, northern island), reflecting a release of pressure along the Pacific-North American margin. Volcanism along both of the subduction zones produced the islands of Japan and, in concert with this tectonic activity, poses an ongoing threat to the country's residents.
