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When a devastating earthquake struck off the coast of Japan in March, it surprised many experts. Geologists say they didn't expect that big a quake in that spot, though they do fear more big quakes may be in the offing in or near Japan.

Scientists can't tell when, but as NPR's Christopher Joyce reports, they're trying to narrow down where.

CHRISTOPHER JOYCE: The earth's crust is made up of huge plates, kind of like floating ice sheets on a lake. Where plates meet edge to edge, sometimes one shoves itself under the other. The plate may just creep smoothly underneath or get stuck. In that case, stress builds up until the edges snap free like an overstretched rubber band.

Mr. ROSS STEIN (Geologist, U.S. Geological Survey): We have the junction of three different tectonic plates and some of them near Tokyo are highly distorted.

JOYCE: That's Ross Stein at the U.S. Geological Survey. Distortions create stress. But Stein says geologists didn't view the site of the March quake, along the so-called Japan Trench in the Pacific, as a hot spot. They thought that section was a quiet creeper.

Mr. STEIN: I mean, in retrospect, we all should have seen that this kind of earthquake was a possibility, and we should have read the tea leaves better.

JOYCE: Now, there are new tea leaves to read - lots of them, since Japan's seismic monitoring system is second to none. One discovery, during the quake parts of the Japan Trench slipped an astonishing 180 feet or more.

A quake that big effects neighboring faults. It's like a game of pick-up sticks. You move one stick and it relieves or adds stress to the other sticks.

Mark Simons at the California Institute of Technology has been looking nervously to a plate boundary south of where the March quake hit.

Professor MARK SIMONS (California Institute of Technology): It's potentially creeping and not producing large earthquakes. And it's potentially stuck and has just been loaded by a magnitude nine next to it, and could produce another eight or larger earthquake.

JOYCE: Simons describes his research in the journal Science.

Stein notes that the magnitude nine-plus quake in the Indian Ocean in 2004 was followed months later by another huge shock. He says that pattern could repeat in Japan.

Mr. SIMONS: So far, the Tohoku earthquake is producing aftershocks at a rate that is typical for earthquakes of this size, which means we could have many large earthquakes to come.

JOYCE: When? No one knows. But scientists think likely spots are offshore again, also a site due east of Tokyo, and another right under Tokyo. And there's Mount Fuji - the region near the volcano experienced a sizable aftershock after the March quake.

Christopher Joyce, NPR News.

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