China quake rare and unexpected, new study says

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 longâ€"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.

Source: Massachusetts Institute of Technology

Related stories:

Long Wait Before Next China Quake?
A new analysis of the setting for May's devastating earthquake in China shows that the quake resulted from faults with little seismic activity--and that similar events in that area occur, on average, only once every 2,000 to 10,000 years. However, geologists caution that because earthquakes can sometimes occur in clusters, people should still be wary of another possible large-scale earthquake.
May 2008 earthquake in China could be followed by another significant rupture
Researchers analyzing the May 2008 Wenchuan earthquake in China's Sichuan province have found that geological stress has significantly increased on three major fault systems in the region. The magnitude 7.9 quake on May 12 has brought several nearby faults closer to failure and could trigger another major earthquake in the region.
Meteorites 'behind volcanic eruptions' say scientists
(PhysOrg.com) -- Gases that cause volcanoes to erupt may have spewed from meteorites that smashed into the earth billions of years ago, according to research presented at The BA Festival of Science in Liverpool today.
Chinese earthquake provides lessons for future
The May 12 Sichuan earthquake in China was unexpectedly large. Analysis of the area, however, now shows that topographic characteristics of the highly mountainous area identified the mountain range as active and could have pointed to the earthquake hazard. Topographic analysis can help evaluate other, similar fault areas for seismic risk, according to geologists from Penn State and Arizona State University.
Geologists study China earthquake for glimpse into future
The May 12 earthquake that rocked Sichuan Province in China was the first there in recorded history and unexpected in its magnitude. Now a team of geoscientists is looking at the potential for future earthquakes due to earthquake-induced changes in stress.

Black holes have simple feeding habits
The biggest black holes may feed just like the smallest ones, according to data from NASA's Chandra X-ray Observatory and ground-based telescopes. This discovery supports the implication of Einstein's relativity theory that black holes of all sizes have similar properties, and will be useful for predicting the properties of a conjectured new class of black holes.
Fossils found in Tibet revise history of elevation, climate
About 15,000 feet up on Tibet's desolate Himalayan-Tibetan Plateau, an international research team led by Florida State University geologist Yang Wang was surprised to find thick layers of ancient lake sediment filled with plant, fish and animal fossils typical of far lower elevations and warmer, wetter climates.
Fatal mine collapse covered 50 acres
New calculations show that the deadly Crandall Canyon mine collapse – which registered as a magnitude-3.9 earthquake – began near where miners were excavating coal and quickly grew to a 50-acre cave-in, University of Utah seismologists say in a report on the tragedy.

News discussion:

Space & Earth science news