May 23, 2011
Scientists knew something was wrong with their understanding of the offshore fault that was the source of the March 11 earthquake in Japan almost immediately after the shaking began. That part of the ocean floor, where the ocean plate subducts beneath Japan, was supposed to be simple and uniform, sometimes sticking and building up stress that should have released in segments, creating large (magnitude 7 or 8 ) earthquakes every few decades or centuries or so. A magnitude 9 quake was not possible, so they thought.
In the months that have followed, geophysicists have been looking into exactly what happened—aided by what might be the best earthquake sensor network on the planet, with hundreds of GPS data recorders documenting movements on land and the sea floor, along with other sensors that measured wave heights from the tsunami. And now some of these researchers have published early results (which are freely available online from the journal Science) from what one scientist called “the best recorded earthquake ever.” Here are a few of the highlights:
1 ) The geologic fault where the Japanese earthquake originated is far more complex than scientists once thought. Geophysicists suspect that a bit of the plate that is sinking beneath Japan, perhaps a seamount, had stuck—for reasons still unknown—causing strain to slowly build up over hundreds of years. Some researchers had previously assumed that the area had been slowly slipping without causing any quakes, but that was not correct.
2 ) About 250 kilometers of fault experienced significant slip during the event, about half the length of what would be expected in an earthquake of this magnitude. And the the most slip—30 meters or more—occurred in an even smaller area, only 50 to 100 kilometers long. Nothing like that had ever been recorded before. These realizations call into question previous conclusions that the fault nearer Tokyo could not create a similarly sized earthquake. “It is important to note that we are not predicting an earthquake [for the fault closer to Tokyo],” says Caltech geophysicist Mark Simons. “However, we do not have data on the area, and therefore should focus attention there, given its proximity to Tokyo.”
3 ) Different parts of the fault produced high- and low-frequency waves. High-frequency waves, which are generated by areas under the highest levels of stress, came from the edges of the area of slip, not where the fault began its break as seismologists had previously assumed. If the fault were compared with a piece of paper being torn in half, “the highest amounts of stress aren’t found where the paper has just ripped, but rather right where the paper has not yet been torn,” Simons says.
It’s somewhat heartening to realize that out of the death and destruction of the quake and tsunami is coming even better earthquake knowledge that will help us better prepare for—and perhaps one day even predict—earthquakes. For almost all of humankind’s existence, all we’ve been able to do in the face of earthquakes, tsunamis and other natural events is to clean up whatever is left after the disaster has hit. But science has changed much of that, and now we can plan and prepare for the inevitable and often stave off the worst possible outcomes.
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