Understanding why Japan’s recent quake defied expectations
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.
Historic events have suggested that this strain is generally released along relatively narrow segments of a fault. During a large quake, one or two of these segments would typically shift, releasing much of the strain and transferring the rest to the flanking segments. This process made Japan the site of frequent earthquakes, many of them quite large, but few reaching anywhere close to the magnitude seen during Tohoku-Oki. Most of these occurred deep in the fault, closer to Japan than the trench.
That’s one of the reasons that the March event was so unexpected. The other is the fact that, historically, this segment of the fault appeared to be relatively inactive. There were two ways to think about that. Either it was completely stuck, making quakes rare but building up excessive pressure, or it was moving with relatively little resistance, releasing the remaining tension through lots of small earthquakes.
The LOZ are now prepared to unleash a large devastating earthquake in the U.S.