Tsunami was 'unexpectantly large'

Series of unexpected geological events caused size of the tsunami

PALO ALTO, CA (UPI) | May 27, 2011

The tsunami that followed the devastating March 11 earthquake in Japan was given extra strength by a sequence of unexpected geologic events, seismologists say.

Researchers at Stanford University said an earthquake of magnitude 9 was not expected, given the local seismic configuration.

The earthquake occurred in a subduction zone, where one great tectonic plate is being forced down under another tectonic plate and into Earth's interior along an active fault, a Stanford release said Tuesday.

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

The fault on which the Tohoku-Oki earthquake took place, which slopes down from the ocean floor toward the west, first ruptured mainly westward from its epicenter -- about 20 miles below the seafloor -- toward Japan, shaking the island of Honshu violently for 40 seconds. Unexpectedly, the fault then ruptured eastward from the epicenter, researchers said, up toward the ocean floor along the sloping fault plane for about 30 or 35 seconds.

As the rupture neared the seafloor, the movement of the fault violently deformed the seafloor above the fault plane, pushing the overlying water upward and triggering the tsunami.

"When the rupture approached the seafloor, it exploded into a tremendously large slip," Beroza said. "It displaced the seafloor dramatically."

The sort of "two-faced" rupture seen in the Tohoku-Oki earthquake has not been seen in other subduction zones, the Stanford researchers said.

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