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Natural disasters are often described as “acts of God,” but within days of last May’s devastating earthquake in China’s Sichuan Province, seis-mologists 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 kilo-meters 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 seis-mologist 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 seis-micity 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 Heavy.The 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. 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 Seismologyarrives 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 read-just 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 hydro- power 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.”
自然灾害通常被人们描绘成“天灾”。去年五月，中国四川省发生了毁灭性的大地震。在此之后，中国内外的地震学家们都在思索：人类是不是在这场大地震中起了推动作用。如今，第一位研究者公开表示：紫平铺大坝后蓄满的水所产生的压力，可能触发了临近断裂带上的脆弱点破裂，它的破裂导致了近300公里断裂带的断裂，使8万多人丧生。
然而，还没有人能证明汶川地震是一例由水库引起的地震。“毫无疑问，地震都是有诱因的”，纽约拉蒙多特利地球观测中心的地质学家莱昂赛博说。事实和新的证据都表示，地震和大坝的联系值得进一步去深究，但要去证明这个诱因“可不容易”。而且中国ZF紧紧地掌握着关键数据。
地震学家已经收集了40年能够诱发地震的样本。“令我吃奇的是，只需要一点点结构性的扰动，就能诱发一场地震”，赛博说。“在地壳上移动流体或岩石，例如开采石油或煤炭挖掘，都有可能导致地震。同样可能的诱因还有向地底填注液体废料，或者隔离二氧化碳，或者，在大坝的后面蓄上一百米高的水。”
无论这种扰动的本质是什么，它一定会让它附近的脆弱点断裂，引发地震。在水库诱发的地震中，水的重量通过集中压力，有可能将断裂带两边挤到一起，紧紧地固定住，从而消除脆弱点。相反，新增加的水也可能会增加压力，让断裂带两边分离，使断裂带断裂。1967年，印度科亚纳大坝后的蓄水引发了已知最大的一起由水库引起的地震。一场里氏6.3级的大地震，200人丧生。地震学家们一共确认了十几起由水库引发的地震，从3级到6级不等。
因此，面对7.9级的汶川大地震，很多科学家们都在研究是否应该批评水库。三峡大坝受到了很多诋毁和诽谤，但它太远，专家们将它排除考虑。他们转而考虑离脆弱点只有500米，离震中只有 5.5公里的紫平铺大坝。“时间吻合。紫平铺水库在2004年12月开始蓄水，两年的时间里，水位迅速上升了120米”。四川省地质区调队总工程师范晓（音）说。
“紫平铺大坝后几千万吨重的水，在相邻的北川断裂带上施加了错误的压力”。哥伦比亚大学从事地质风险研究的克里斯蒂克洛斯在上个月旧金山召开的美国地理联盟大会的闭幕会议上说。在发言中，克洛斯含混地解释了，增加的水量如何改变断裂带上的压力。但没有指明哪一处大坝。按照他的计算，增加的重量使断裂带上的挤压变得松动，减弱。并且增加引起断裂带破裂的巨大应力�？寺逅顾�，这相当于每年构造运动所产生的自然压力的25倍。他还指出，当断裂带最后真的破裂时，它就会朝着水库负重推动它的方向移动。
克洛斯的演讲撩起了听众们的兴趣，但还远远不能说服他们。他们想听到更多关于水位变化、当地底层地震活动性的细节。没有出席会议的方先生提供了一些相关细节，所有的这些细节都让人对紫平铺水库和地震的联系产生兴趣。他表示，根据历史上已知的由水库引起的地震来看，紫平铺水库过快的蓄水速度、以及相当可观的蓄水深度都有助于诱发地震。水库蓄水和大地震发生之间的间隔，给蓄水渗入地壳深层、削弱断裂带提供了时间。诱因最威险的时候不是当水库蓄满水，而是当水位下降的时候。“如我们所知，在5.12地震前的一个星期，水库水位急剧下降，前所未有。”范说。
发表在上个月中国《地震地质》月刊上的一篇文章得出了相似的结论。紫平铺水库的蓄水“明显地影响了地震活动性”。文章的主要执笔者雷新林（音）这样说，他是中国地震协会（北京）和日本国家工业科技研究所（筑波）的地质学家。雷强调，现在要得出稳妥的结论还为时过早。但他认为水库的蓄水渗透进断裂带、以及水量在2007年12月和 2008年5月的下降，是“与四川大地震的核心相关的主要因素”。
范晓同样不认为紫平铺和汶川地震之间的联系已经被证明，但他认为现在已被发现的足够向人们提出警告。“我们应该重新调整现有的规划，对项目规划采取更审慎的态度。但我对很多的大项目是否能被取消持悲观态度，因为它们能给水电开发商和当地政府提供巨大的利润。”
中外的研究者们表示：要成为更有说服力和警示作用的实例，就需要得到更多的详细数据。四川地震局的地质学家闻雪泽（音）说：“水库附近和周边的小地震，是实时变化的地震活动性的证据。我们还需要更多的数据，像水库的水位、负荷等等。”范晓认为，中科院的研究者们已经有此类研究的初步成果，“但他们懒得和我们分享。”
