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.

Obviously, the March experience argues for the former. Understanding what actually happened could go a long way towards helping us understand why we had misread recent history near Japan, and the three papers use various pieces of seismic data, location data, and tsunami readings to put that picture together.

When the plates at the Japan Trench stick, the edge of the North American plate (which contains Japan) is pushed downward, while the pressure pushes the nearby terrain slightly upwards. The rupture that occurred during the earthquake changed all of these. The edge of the plate was released and bounced upwards; the release allowed it to extend outwards, eliminating the corresponding buckling. In Japan itself, this registered as a horizontal displacement that exceeded four meters in spots, while some areas dropped over half a meter in elevation.

But what happened at the plate itself? That's what the new papers address, at the same time they trace the timeline of the event. The initial period of rupture occurred relatively deep under the plate, and lasted for up to 40 seconds. That was followed by an extremely brief but violent horizontal rupture, as the North American plate released the built-up pressure by extending back out over the Pacific plate. In fact, one group indicates it extended so far that it experienced what's termed "dynamic overshoot," and several aftershocks occurred as this corrected itself. This violent horizontal motion was followed by up to a hundred seconds of further deep rupture.

As a result of all these changes, the differences that occurred in the ocean are much more dramatic. One group estimates that the peak vertical drop occurred 50km from shore, and lowered the North American plate by 2m. Near the trench, the plate edge rose by as much as 9m. But those are dwarfed by the evidence of horizontal motion. In order of increasing spatial resolution, the papers estimate the horizontal motion near the trench at 24m, over 30m, and as much as 60m. This motion was so violent that it effectively dragged neighboring segments of the plate along, causing it to be one of the most extensive ruptures in the area that we're aware of. All together, the events released 9 x 1018J—a back-of-the-envelope calculation suggests that this is somewhere in the neighborhood of 2,400 megatons of TNT.

What's this mean? A few things. For one, the horizontal motion was the primary cause of the tsunami, but only provided a portion of the energy released in the quake. Thus, if a quake can generate that sort of motion on its own, then we could see much smaller quakes generating tsunamis that are far larger than we would have predicted.

It also has implications for Japan itself. For one, we shouldn't be confident that a lack of historic earthquakes means that the plates in the area are sliding past each other—one paper suggests that having a sea mount dragged down into the trench could have caused the long-term quiet that the Tohoku-Oki section had apparently experienced. Thus, we probably need to go back to examine other areas that have seemingly enjoyed periods of quiet near the trench border. Ominously, the authors note that one such area is a bit further to the south—and much closer to Tokyo.

Globally, the quake has also told us that it's possible for a relatively small fault segment to spawn a giant earthquake. A perspective quotes the US Geological Survey's David Wald as saying, "If you can get a 9 that is this compact, it increases the number of places you can [fit in] a 9 where you may not have expected one." So, coastal nations around the globe might want to start re-evaluating the risks they face from nearby subduction zones.

Science, 2011. DOI: 10.1126/science.1206731. 10.1126/science.1207020. 10.1126/science.1207401 (About DOIs).