Anna Cobb, Stanford News Service
Fault motion sequence: 1. Rupture of the fault plane begins at the epicenter. 2. Rupture travels westward. 3. The upward sloping east side of the fault plane begins to rupture, forcing up the seafloor. 4. The water above the seafloor is pushed into an unstable dome that then flows out in all directions as a tsunami.
OurAmazingPlanet
updated 5/24/2011 5:40:54 PM ET 2011-05-24T21:40:54

The devastating tsunami that swept ashore in Japan after a massive 9.0-magnitude earthquake in March had such extraordinary power because of the unusual way in which the fault that generated the quake ruptured, new research suggests.

"It was not appreciated before that this size of earthquake was possible on this plate boundary," Stanford University geophysicist Greg Beroza said in a statement. "It was thought that typical earthquakes were much smaller."

The earthquake occurred in a subduction zone, where one of Earth's tectonic plates is forced down under another plate and into the Earth's interior.

The fault on which the Tohoku earthquake took place slopes down from the ocean floor toward the west. It first ruptured westward from its epicenter — about 20 miles below the seafloor — toward Japan, violently shaking the main island of Honshu for 40 seconds.

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Surprisingly, the fault then ruptured eastward from the epicenter, up toward the ocean floor along the sloping fault plane for about 30 or 35 seconds, which displaced the seafloor dramatically, Beroza said.

"The depth of the water column there is also greater than elsewhere," Beroza said. "That, together with the slip being greatest where the fault meets the ocean floor, led to the tsunami being outlandishly big."

Beroza said this sort of "two-faced" rupture seen in the Tohoku earthquake has not been seen in other subduction zones, but that could be because of the limited amount of data available for analyzing other earthquakes.

Japan possesses the most intricate network of seismometers anywhere in the world, and the sensors provide detailed data on the region's earthquakes.

The damage from the March 11 earthquake was so extensive in part simply because the earthquake was so large. But the way it ruptured on the fault plane, in two stages, made the devastation greater than it might have been otherwise, Beroza said.

The research was published last week in the journal Science's online prepublication service Science Express.

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