Last week, Science released three papers and a perspective, all focused on understanding what happened during the March earthquake that struck Japan. Now officially termed the Tohoku-Oki quake, the event is estimated as a magnitude 9 quake—one of the biggest in recorded history—and it has triggered significant aftershocks. But it's not the size alone that has people worried; it's the fact that something this size occurred on a segment of fault that we didn't think was capable of producing a quake of this magnitude (an estimate that has had disastrous consequences at the Fukushima nuclear reactors). Understanding what happened and why can potentially tell us a lot more about risks elsewhere along this fault.
The quake occurred along a segment of fault that creates the Japan Trench, where the Pacific plate slides underneath the one that plays host to Japan. This subduction zone gives rise to Japan's volcanoes, and the pressure helps push Japan upwards, creating more of its topography. As with many faults, the two plates sporadically stick as they slide past each other, triggering large earthquakes when the strain is released. All told, the earthquakes have to release a strain that results from a relative motion of the plates that's estimated at about 8.5cm every year.