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[交流] 【讨论】汶川还有大震么？转篇文章

May '08 China Quake Could Hasten Other Big Shocks
Scientists Say Nearby Faults Now Twice as Likely to Produce Strong Quakes

WASHINGTON—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 12 May has brought several nearby faults closer to failure and could trigger another major earthquake in the region.
Geophysicists used computer models to calculate the changes in stress along the Xianshuihe, Kunlun, and Min Jiang faults, which lie about 150 to 450 kilometers (90 to 280 miles) from the Longmen Shan rupture that caused the devastating quake. The research team also examined seismic activity in the region over the past decade.
The scientists found that the 12 May event has doubled the probabilities of future earthquakes on these fault lines. Specifically, they estimated the probability of another earthquake of magnitude 6 or greater in the region is 57 to 71 percent over the next decade. There is an 8 to 12 percent chance of a quake larger than magnitude 7 in the next decade and 23-31 percent in the next 30 years.
The research team reported its findings on 9 September in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).
"One great earthquake seems to make the next one more likely, not less," says Ross S. Stein of the U.S. Geological Survey (USGS) in Menlo Park, California, a member of the team led by Shinji Toda of the Geological Survey of Japan, in Tsukuba. "We tend to think of earthquakes as relieving stress on a fault. That may be true for the one that ruptured, but not for the adjacent faults," Ross adds.
In 1999, a magnitude 7.4 (M7.4) earthquake in Izmit, Turkey, was followed four months later by an M7.1 event in nearby Duzce. The devastating December 2004 Sumatra earthquake (M 9.2) and tsunami were followed by an M8.7 quake three months later.
"Because the Tibetan Plateau is one of the most seismically active regions in the world, we believe there is credible evidence for a new major quake in this region," says Jian Lin of the Woods Hole Oceanographic Institution (WHOI), who is also on the team. "The research community cannot forecast the timing of earthquakes, and there are still significant uncertainties in our models. But the Turkey and Sumatra events indicate that one major earthquake can indeed promote another."
Researchers see it as a domino-like effect, where the movement of one piece of Earth's crust means that another piece must move up, down, or away. While the stress in the crust gets reduced in some locations, it is transferred to other faults nearby.
Large aftershocks that occurred on 1 August and 5 August in the Sichuan region of China may fit with this predicted pattern.
"Earthquake prediction is a bit like the thundercloud and lightning," Toda explains. "We can forecast that lightning will come from a thundercloud, but we cannot predict the exact time and place where the lightning will hit. With earthquakes, we can roughly forecast the probability of activity over broad ranges of time, magnitude, and location, but we cannot determine the exact value for any of these."
On 12 May 2008, about 300 kilometers (190 miles) of the Longmen Shan fault zone ruptured in an earthquake that killed at least 69,000 people and left another 5 million homeless. It was the deadliest and strongest earthquake to hit China since the 1976 Tangshan earthquake, which killed at least 240,000.
As pieces of the Longman Shan fault slipped by as much as nine meters (28 feet) in the May quake, stress increased along the neighboring Xianshuihe, Kunlun, and Min Jiang faults, according to Toda and colleagues. All three faults have a history of large quakes, though portions of each have been quiet for most of the past century. All three faults were considered to be primed for an earthquake even before the recent events.
In addition to the broad prediction of earthquake triggering, the researchers have also forecasted the rate and distribution of seismic shocks greater than magnitude 6, a prediction that they plan to test from seismic stations over the next decade.
"Our paper predicts the change in the rate of small earthquakes for the faults in the region, and now we can test that prediction," says Stein. "If the rate of shocks increases on the adjacent faults, then we can confirm at least part of our hypothesis that large shocks are also more likely. It may take time, but it is a testable hypothesis."
In western China, the intrusion of the Indian sub-continent pushes the Tibetan Plateau up and over the older Sichuan Basin and other parts of the Eurasian continent. An estimated 33 percent of world's continental earthquakes occur in China, even though it only occupies 7 percent of the planet's land mass. Nearly 55 percent of all human loss to earthquakes occurs in China.
"Earthquakes do not kill people, buildings do," says Lin, who was a high school student in China when the devastating Tangshan earthquake struck. "There needs to be widespread education in earthquake preparedness, as well as systematic inspection of buildings in these regions of heightened risk. Every new building inspection and evacuation plan could potentially save lives."
"We hope the long-term forecasting allows the Chinese government to make it a priority to mitigate future damage," Toda adds. "We recommend that Chinese scientists carefully observe changes in seismicity by installing new seismometers in the region."
Mustapha Meghraoui of the Institute of Geophysics in Strasbourg, France also collaborated on the research. Lin, Toda, and Stein were preparing to teach an earthquake modeling course to Meghraoui's students and colleagues in France when the 12 May earthquake occurred. The researchers immediately went into action, working with an international group of scientists to analyze the new stresses on the system.
An early version of the manuscript by Toda et al. was circulated to several dozen Chinese scientists and government officials as they sought to assess the risk of aftershocks in the weeks after the earthquake. Chinese government organizations and scientists are now examining the paper in detail, the researchers say.

Source: http://www.agu.org/sci_soc/prrl/2008-30.html
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计算结果显示2008年5月12日中国汶川7.9级地震可能导致
三条主要断层带的应力及地震发生率增加

Shinji Toda(远田晋次)1, Jian Lin(林间)2, Mustapha Meghraoui3, Ross S. Stein4
1日本地质调查局活动断层研究中心
Active Fault Research Center, Geological Survey of Japan, AIST,
Tsukuba 305-8567, Japan; (e-mail) s-toda@aist.go.jp
2美国伍兹霍尔海洋研究所
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA;
(e-mail) jlin@whoi.edu
3法国斯特拉斯堡地球物理研究所
EOST-Institut de Physique du Globe de Strasbourg, 67084 Strasbourg
cedex, France; (e-mail) Mustapha.Meghraoui@eost.u-strasbg.fr
4美国地质调查局
U.S. Geological Survey, Menlo Park, CA 94025, USA;
(e-mail) rstein@usgs.gov

2008 年5月12日的中国汶川7.9级地震夺去了六万九千多人的生命并造成巨大损失。计算结果显示,该地震可能使部分鲜水河断层、东昆仑断层大部以及部分岷江断层更加接近断裂状态（见附左图）。这三条位于青藏高原东部的活动断层带均在离汶川发震带150-400公里的库仑静应力影响范围内。由于部分受力断层上的应变累积至少已有一个世纪之久，最近的汶川地震有可能触发或加速这些断层带上新的7级以上强震的发生，从而可能造成从四川省康定至道孚，青海省玛沁至甘肃省洛大以及邻近区域的强烈震动。我们使用由计算得到的汶川地震引起的应力变化以及过去十年里观测到的背景地震发生率，对该地区在未来十年里发生破坏性地震的概率以及可能的空间分布进行预测。结果表明，该分析区域内在未来十年里发生M≥6级地震的概率为57-71%,而发生M≥7概率为8-12%。
来源 http://quake.usgs.gov/research/d ... odaetalchinese.html
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