The islands of Japan sit at the convergence of four major tectonic plates: the Eurasian, North American, Pacific and Philippine Plates. Throughout most of their history, the Pacific Plate was moving northwestward, bringing in terranes and producing volcanic island arcs as it subducted beneath the North American and Eurasian Plates.
About 45 million years ago, the movement of the Pacific Plate shifted to a westward direction (as evidenced by the angle change of the Hawaiian Ridge), creating the Philippine Plate as the Pacific Plate began to subduct along the Izu Bonin Trench. Surrounded by subduction trenches, the Philippine Plate now subducts beneath the southern half of Japan (which lies on the Eurasian Plate), including Kyushu Island, fueling volcanic activity and triggering earthquakes.
This week's earthquake on Kyushu Island, which killed at least 9 and injured hundreds, was a magnitude 6.2 quake, centered beneath the Island just west of Kumamoto. While subduction forces initiated the earthquake, it occurred due to slippage along a fault that bisects the Island, separating older and younger terranes. Shifting pressure along this and associated faults led to numerous aftershocks that may continue for weeks. Since the earthquake developed on land (not at the subduction trench itself), there was no risk of a tsunami; on the other hand, the quake could have triggered a volcanic eruption, a complication that, to date, has fortunately not occurred.
Addendum: A second, far more powerful (magnitude 7.0) earthquake has struck near Kumamoto. Initial reports indicate widespread damage, including landslides, and at least 26 deaths (in addition to those killed by the quake earlier this week).
