From: Andy Soos, ENN
Published May 23, 2011 08:10 AM

Japanese Quake News



Among the study’s findings:

- The length of fault that experienced significant slip during the quake was about 155 miles about half of what would be conventionally expected for an event of this magnitude. The area of greatest slip -- 98 feet (30 meters) or more -- happened within a 31- to 62-mile-long segment.

-  High- and low-frequency seismic waves can come from different areas of a fault. The quake’s high-frequency seismic waves were generated much closer to the coast, away from the area of the fault slip, where low-frequency waves were observed.

- The amount of strain associated with the quake’s displacement was five to 10 times larger than normally seen in large megathrust earthquakes. Before now, it was generally believed that the relatively soft material of the seafloor near the Japan Trench could not support such a large amount of stress. Because of this local strengthening of the seafloor, the researchers believe the Pacific and Okhotsk tectonic plates had been pinned together for a long time, perhaps 500 to 1,000 years.

- The area just south of where the fault slipped in March, which is close to Tokyo, should be a focus area for researchers because they do not have data on the area and don’t know yet what it might do in the future.

The Okhotsk Plate is a tectonic plate covering the Sea of Okhotsk, the Kamchatka Peninsula, and Eastern Japan. It was formerly considered a part of the North American Plate, but recent studies indicate that it is an independent plate, bounded on the north by the North American Plate while to the east is the Pacific.

"This event is the best recorded great earthquake ever," says Mark Simons, professor of geophysics at Caltech’s Seismological Laboratory and lead author of the study.

For Jean Paul Ampuero, assistant professor of seismology at Caltech’s Seismological Laboratory who studies earthquake dynamics, the most significant finding was that high- and low-frequency seismic waves can come from different areas of a fault. "The high-frequency seismic waves in the Tohoku earthquake were generated much closer to the coast, away from the area of the slip where we saw low-frequency waves," he says.

Simons says there are two factors controlling this behavior; one is because the largest amount of stress (which is what generates the highest-frequency waves) was found at the edges of the slip, not near the center of where the fault began to break. He compares the finding to what happens when you rip a piece of paper 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," he explains.

"We had previously thought high-frequency energy was an indicator of fault slippage, but it didn’t correlate in our models of this event." Equally important is how the fault reacts to these stress concentrations; it appears that only the deeper segments of the fault respond to these stresses by producing high-frequency energy.

For further information: http://www.jpl.nasa.gov/news/news.cfm?release=2011-152&rn=news.xml&rst=3006 or http://media.caltech.edu/press_releases/13417
Photo: Credit: Mark Simons/Caltech Seismological Laboratory ]

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