[Editors] MIT/China Quake: First scientific analysis

[Jen Hirsch]

For Immediate Release
MONDAY, JUN. 30, 2008

Contact: Jen Hirsch, MIT News Office
T. 617-253-1682   E.: jfhirsch at mit.edu

PHOTO AVAILABLE

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China quake rare and unexpected, says new MIT study
--Could be a long wait before next big tremblor as per first  
scientific look at quake
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CAMBRIDGE, Mass.—A new analysis of the setting for last month’s  
devastating earthquake in China by a team of geoscientists at MIT  
shows that the quake resulted from  faults with little seismic  
activity, and that similar events in that area occur only once in  
every 2,000 to 10,000 years, on average.

However, the researchers caution that because earthquakes can  
sometimes occur in clusters, people should still be wary of another  
possible large-scale earthquake.

The magnitude 7.9 quake struck Sichuan province on May 12 at around  
noontime, which may have increased the human death toll because many  
people were at school, and the school buildings turned out to be  
especially vulnerable to collapse because of poor construction. More  
than 69,000 people have been confirmed dead so far, and more than  
374,000 injured, with fears of further casualties because several  
lakes created by rockfall dams may give way and cause sudden flooding.

Clark Burchfiel, Schlumberger Professor of Geology, and Leigh Royden,  
professor of geology and geophysics in the Department of Earth,  
Atmospheric and Planetary Sciences at MIT, have been doing extensive  
research in that region of China and the Tibetan plateau for more  
than two decades, but had found no hints that suggested such a large  
earthquake might strike the area. They and several colleagues,  
including MIT's Robert D. van der Hilst and Bradford H. Hager, who  
are both Cecil and Ida Green Professors of Earth Sciences, have  
published a paper analyzing the causes of the quake that appears in  
the July issue of GSA Today, a publication of the Geological Society  
of America.

The team operated an array of 25 broadband seismograph stations in  
this region of western Sichuan for more than a year. “Nobody was  
thinking there would be a major seismological event” in that area,  
Royden says. “This earthquake was quite unusual,” and may have  
involved a simultaneous rupture of two separate but contiguous  
faults, she continued.

The region is extremely unusual geologically, Royden says, because of  
the very steep slopes at the boundary between the Sichuan Basin to  
the east and the Tibetan plateau to the west. The elevation rises  
sharply by about 3,500 meters (more than two miles) over a span of  
only about 50 kilometers (about 30 miles).

The area where the quake occurred is part of the boundary between two  
of the Earth’s tectonic plates, where the Indian and Asian plates  
converge in an ongoing collision that has created the Himalayan  
mountains and the Tibetan plateau. But in central and eastern Tibet,  
unlike most other areas of continental collision, much of the  
movement of crust is hidden from view. Instead of thickening the  
entire crust by folding and faulting, the surface of the eastern  
Tibetan plateau is undeformed and is being lifted upward by  
thickening of a weak crustal layer more than 15 km below the surface.

The crust in this deep weak layer is flowing eastward away from  
central Tibet to escape from the area directly north of the Indian  
plate. But, in the area where the earthquake occurred, this rapidly  
flowing material is obstructed by a major obstacle, the Sichuan  
Basin. “The crust and mantle beneath the basin appears to form a  
hard, cold knot” that extends to 250 km depth, Royden says, that  
forces the flow to “wrap around the knot.” The huge elevation  
difference between the surface of the plateau and the Sichuan Basin  
provides the underlying stress that led to the quake, she says.

As the surface of the eastern plateau has risen, it has become  
increasingly incised by rivers. Four of the world’s 10 largest  
rivers, including the Yangtze, flow through the region, Royden says.  
“There are gorges two and a half to three kilometers deep, and  
hundreds of kilometers long—they dwarf the Grand Canyon,” she says.

The steep slopes within the river gorges make the region especially  
vulnerable when earthquakes occur, she says. “When you shake those  
valleys, everything just slides down into the river gorges and  
eventually washes out to sea,” she says.

Because of the extreme geological environment of this region, Royden  
says, it may be possible to learn about mechanisms taking place there  
that may also occur, at a smaller scale, in other places. In this  
way, it may reveal processes that are also relevant in other parts of  
the world but that would be much harder to discover in these other  
locations because they would be more subtly expressed.

The research was funded by the National Science Foundation. The MIT  
scientists are currently collaborating with geophysicists of the  
China Seismological Bureau (in Beijing and Chengdu, Sichuan) on a  
study of the structure and seismic hazard of the region.

By David Chandler, MIT News Office

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