China quake rare and unexpected, says new MIT studyJuly 01, 2008Could be a long wait before next big temblor according to first scientific look at quake 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. Massachusetts Institute of Technology |
|||||||||||||||||||||
| Related Earthquake Current Events and Earthquake News Articles Global challenges and opportunities in fighting HIV/AIDS and neglected diseases Responding to the HIV/AIDS pandemic and tackling so-called neglected tropical diseases are the focus of the November/December 2009 edition of Health Affairs. Fortuitous research provides first detailed documentation of tsunami erosion Tsunamis are among the most-devastating natural calamities. These earthquake-generated waves can quickly engulf low-lying land and bring widespread destruction and death. They can deposit sand and debris far inland from where they came ashore. West Antarctic ice sheet may not be losing ice as fast as once thought New ground measurements made by the West Antarctic GPS Network (WAGN) project, composed of researchers from The University of Texas at Austin, The Ohio State University, and The University of Memphis, suggest the rate of ice loss of the West Antarctic ice sheet has been slightly overestimated. Tsunami evacuation buildings: another way to save lives in the Pacific Northwest Some time soon, a powerful earthquake will trigger a massive tsunami that will flood the Pacific Northwest, destroying homes and threatening the lives of tens of thousands of people, says Yumei Wang, a geotechnical engineer at the Oregon Department of Geology and Mineral Industries in Portland. Satellite data look behind the scenes of deadly earthquake Using satellite radar data and GPS measurements, Chinese researchers have explained the exceptional geological events leading to the 2008 Wenchuan Earthquake that killed nearly 90 000 people in China's Sichuan Province. Scientists obtain rocks moving into seismogenic zone An international group of scientists aboard the Deep-Sea Drilling Vessel CHIKYU, operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for the Integrated Ocean Drilling Program (IODP), return from a 40-day scientific expedition off the shore of the Kii Peninsula, Japan on Oct. 10, 2009. San Andreas affected by 2004 Sumatran quake U.S. seismologists have found evidence that the massive 2004 earthquake that triggered killer tsunamis throughout the Indian Ocean weakened at least a portion of California's famed San Andreas Fault. Plastic surgeons should be part of disaster relief planning, response When a terrorist bomb explodes, a tornado rips through a town, a hurricane devastates a region, or wildfires ravage homes and businesses, plastic surgeons are not typically atop the list of emergency responders. Scientists return from first ever riser drilling operations in seismogenic zone he Deep-sea Drilling Vessel CHIKYU successfully completed riser drilling operations on Aug. 31, for IODP Expedition 319, Stage 2 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). Study reveals seismic shift in methods used to track earthquakes The team, led by scientists from the University of Edinburgh, says that the new method, which uses data collected from earthquakes, potentially allows the Earth's seismic activity to be mapped more comprehensively. More Earthquake Current Events and Earthquake News Articles |
|||||||||||||||||||||
|
|||||||||||||||||||||
|
|||||||||||||||||||||