The northeastern half of Japan lies on the North American Plate, which dips down from the Aleutian Chain, crosses the middle of Honshu (Japan's large central island) and then angles NNW across Siberia, west of the Kamchatka Peninsula. The southwestern half of Japan lies on the Eurasian Plate.
Today's magnitude 7.3 earthquake, off the northeastern coast of Japan, was another subduction quake, like the massive earthquake in March, 2011, that produced the destructive tsunami; fortunately, today's quake resulted in a relatively small tsunami. Both earthquakes occurred along a subduction zone where the Pacific Plate, moving northwest, is forced beneath the North American Plate; friction pulls the edge of the latter plate downward and, when it slips and rebounds upward, an earthquake results and displacement of the overlying sea water may cause a tsunami. Off the southern coast of southwestern Japan, the Philippine Plate is dipping beneath the Eurasian Plate and similar subduction quakes and secondary tsunamis have and will continue to occur along that tectonic boundary as well.
It is interesting to note that the residents of northeastern Japan live on the North American Plate while those of southern California and the Baja Peninsula do not (they, in fact, live on the Pacific Plate). Contrary to a popular assumption, the tectonic plates do not correspond to the contours of the Continents for which they are named and the Continents may be composed of land segments from multiple tectonic plates (Asia, for example, includes segments from the Eurasian, Arabian, Indian, Philippine and North American Plates).
