送交者: 老中乙号 于 2009-01-17, 06:12:22:

Science 16 January 2009:
Vol. 323. no. 5912, p. 322
DOI: 10.1126/science.323.5912.322
News of the Week
A Human Trigger for the Great Quake of Sichuan?
Richard A. Kerr and Richard Stone

Natural disasters are often described as "acts of God," but within days of last May's devastating earthquake in China's Sichuan Province, seismologists in and out of China were quietly wondering whether humans might have had a hand in it. Now, the first researchers have gone public with evidence that stresses from water piled behind the new Zipingpu Dam may have triggered the failure of the nearby fault, a failure that went on to rupture almost 300 kilometers of fault and kill some 80,000 people.

Still, no one is near to proving that the Wenchuan quake was a case of reservoir-triggered seismicity. "There's no question triggered earthquakes happen," says seismologist Leonardo Seeber of the Lamont-Doherty Earth Observatory in Palisades, New York. That fact and the new evidence argue that the quake-dam connection "is worth pursuing further," he says, but proving triggering "is not easy." And the Chinese government is tightly holding key data.

Seismologists have been collecting examples of triggered seismicity for 40 years. "The surprising thing to me is that you need very little mechanical disturbance to trigger an earthquake," says Seeber. Removing fluid or rock from the crust, as in oil production or coal mining, could do it. So might injecting fluid to store wastes or sequester carbon dioxide, or adding the weight of 100 meters or so of water behind a dam.

Whatever the nature of the disturbance, it must bring a nearby fault to the point of failure to trigger a quake. In the case of reservoir-triggered seismicity, the water's weight can weaken a fault by counteracting the stresses that are squeezing the two sides of the fault together and tightly locking it. Or, the added weight can increase the stress already tending to push opposing sides past each other and break the fault. In 1967, impoundment behind the Koyna Dam in India triggered the largest known reservoir-triggered quake, a magnitude-6.3 temblor that killed 200 people. Seismologists recognize dozens of other reservoir-triggered quakes in the range of magnitude 3 to 6.

So when the magnitude-7.9 Wenchuan earthquake struck, many scientists wondered if a reservoir was to blame. Ruling out the much-maligned Three Gorges Dam as too distant, experts considered the Zipingpu Dam, just 500 meters from the fault that failed and 5.5 kilometers from the quake's epicenter. The timing was right. The Zipingpu reservoir began filling in December 2004, and within 2 years the water level had rapidly risen by 120 meters, says Fan Xiao, a chief engineer of the Sichuan Geology and Mineral Bureau in Chengdu.

Figure 1 Heavy. The several hundred million tons of water behind Sichuan's Zipingpu Dam may have triggered a devastating earthquake.


The several hundred million tons of water piled behind the Zipingpu Dam put just the wrong stresses on the adjacent Beichuan fault, geophysical hazards researcher Christian Klose of Columbia University said at a session last month at the Fall Meeting of the American Geophysical Union in San Francisco, California. In his talk, Klose coyly explained--without ever mentioning a dam--how the added water changed the stresses on the fault. According to his calculations, the added weight both eased the squeeze on the fault, weakening it, and increased the stress tending to rupture the fault. The effect was 25 times that of a year's worth of natural stress loading from tectonic motions, Klose said. When the fault did finally rupture, it moved just the way the reservoir loading had encouraged it to, he noted.

Klose's listeners were intrigued but far from convinced. They wanted to hear more details about changing water levels and local, lower-level seismicity. Fan, who was not at the meeting, provides some of those details, all of which favor a link between the Zipingpu Reservoir and the earthquake. Judging by the history of known reservoir-triggered quakes, the rapid filling of Zipingpu as well as its considerable depth would have favored triggering, he says. The delay between filling and the great quake would have given time for reservoir water to penetrate deep into the crust, where it can weaken a fault. And the greatest danger of triggering comes not at the time of maximum filling, he argues, but when the water level is falling. "As we now know, a week before the May 12 earthquake, the water level fell more rapidly than ever before," says Fan.

A paper in last month's issue of the Chinese journal Geology and Seismology arrives at a similar conclusion. Zipingpu's impoundment "clearly affected local seismicity," says lead author Lei Xinglin, a geophysicist at the China Earthquake Administration in Beijing and the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan. Lei emphasizes that a firm conclusion is premature, but he sees penetration of reservoir water into the fault and the reservoir decline between December 2007 and May 2008 as "major factors associated with the nucleation of the great Sichuan earthquake."

Fan also does not see the Zipingpu-Wenchuan connection as proven yet, but he's seen enough to urge caution. "We should readjust our existing plans and take a more cautious attitude when planning projects," he says. "But I am pessimistic that many of these large-scale constructions will be canceled, because of the strong economic interests that benefit hydropower developers and local governments."

Building a stronger case for restraint, researchers in and out of China say, will require access to even more detailed data. "Time-variation evidence for seismicity of small earthquakes near and surrounding the reservoir, as well as for the water levels and loading of the reservoir, are needed," says geophysicist Wen Xue-ze of the Sichuan Seismological Bureau in Chengdu. Fan believes that researchers in the Chinese Academy of Sciences have preliminary results from such studies, "but they are reluctant to share them."



笔名: 密码: 注册笔名请按这里


内容: (BBCode使用说明