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.”

自然災害通常被人們描繪成“天災”。去年五月，中國四川省發(fā)生了毀滅性的大地震。在此之后，中國內外的地震學家們都在思索：人類是不是在這場大地震中起了推動作用。如今，第一位研究者公開表示：紫平鋪大壩后蓄滿的水所產生的壓力，可能觸發(fā)了臨近斷裂帶上的脆弱點破裂，它的破裂導致了近300公里斷裂帶的斷裂，使8萬多人喪生。

然而，還沒有人能證明汶川地震是一例由水庫引起的地震。“毫無疑問，地震都是有誘因的”，紐約拉蒙多特利地球觀測中心的地質學家萊昂賽博說。事實和新的證據都表示，地震和大壩的聯系值得進一步去深究，但要去證明這個誘因“可不容易”。而且中國ZF緊緊地掌握著關鍵數據。

地震學家已經收集了40年能夠誘發(fā)地震的樣本。“令我吃奇的是，只需要一點點結構性的擾動，就能誘發(fā)一場地震”，賽博說。“在地殼上移動流體或巖石，例如開采石油或煤炭挖掘，都有可能導致地震。同樣可能的誘因還有向地底填注液體廢料，或者隔離二氧化碳，或者，在大壩的后面蓄上一百米高的水。”

無論這種擾動的本質是什么，它一定會讓它附近的脆弱點斷裂，引發(fā)地震。在水庫誘發(fā)的地震中，水的重量通過集中壓力，有可能將斷裂帶兩邊擠到一起，緊緊地固定住，從而消除脆弱點。相反，新增加的水也可能會增加壓力，讓斷裂帶兩邊分離，使斷裂帶斷裂。1967年，印度科亞納大壩后的蓄水引發(fā)了已知最大的一起由水庫引起的地震。一場里氏6.3級的大地震，200人喪生。地震學家們一共確認了十幾起由水庫引發(fā)的地震，從3級到6級不等。

因此，面對7.9級的汶川大地震，很多科學家們都在研究是否應該批評水庫。三峽大壩受到了很多詆毀和誹謗，但它太遠，專家們將它排除考慮。他們轉而考慮離脆弱點只有500米，離震中只有 5.5公里的紫平鋪大壩。“時間吻合。紫平鋪水庫在2004年12月開始蓄水，兩年的時間里，水位迅速上升了120米”。四川省地質區(qū)調隊總工程師范曉（音）說。

“紫平鋪大壩后幾千萬噸重的水，在相鄰的北川斷裂帶上施加了錯誤的壓力”。哥倫比亞大學從事地質風險研究的克里斯蒂克洛斯在上個月舊金山召開的美國地理聯盟大會的閉幕會議上說。在發(fā)言中，克洛斯含混地解釋了，增加的水量如何改變斷裂帶上的壓力。但沒有指明哪一處大壩。按照他的計算，增加的重量使斷裂帶上的擠壓變得松動，減弱。并且增加引起斷裂帶破裂的巨大應力�？寺逅拐f，這相當于每年構造運動所產生的自然壓力的25倍。他還指出，當斷裂帶最后真的破裂時，它就會朝著水庫負重推動它的方向移動。

克洛斯的演講撩起了聽眾們的興趣，但還遠遠不能說服他們。他們想聽到更多關于水位變化、當地底層地震活動性的細節(jié)。沒有出席會議的方先生提供了一些相關細節(jié)，所有的這些細節(jié)都讓人對紫平鋪水庫和地震的聯系產生興趣。他表示，根據歷史上已知的由水庫引起的地震來看，紫平鋪水庫過快的蓄水速度、以及相當可觀的蓄水深度都有助于誘發(fā)地震。水庫蓄水和大地震發(fā)生之間的間隔，給蓄水滲入地殼深層、削弱斷裂帶提供了時間。誘因最威險的時候不是當水庫蓄滿水，而是當水位下降的時候。“如我們所知，在5.12地震前的一個星期，水庫水位急劇下降，前所未有。”范說。

發(fā)表在上個月中國《地震地質》月刊上的一篇文章得出了相似的結論。紫平鋪水庫的蓄水“明顯地影響了地震活動性”。文章的主要執(zhí)筆者雷新林（音）這樣說，他是中國地震協會（北京）和日本國家工業(yè)科技研究所（筑波）的地質學家。雷強調，現在要得出穩(wěn)妥的結論還為時過早。但他認為水庫的蓄水滲透進斷裂帶、以及水量在2007年12月和 2008年5月的下降，是“與四川大地震的核心相關的主要因素”。

范曉同樣不認為紫平鋪和汶川地震之間的聯系已經被證明，但他認為現在已被發(fā)現的足夠向人們提出警告。“我們應該重新調整現有的規(guī)劃，對項目規(guī)劃采取更審慎的態(tài)度。但我對很多的大項目是否能被取消持悲觀態(tài)度，因為它們能給水電開發(fā)商和當地政府提供巨大的利潤。”

中外的研究者們表示：要成為更有說服力和警示作用的實例，就需要得到更多的詳細數據。四川地震局的地質學家聞雪澤（音）說：“水庫附近和周邊的小地震，是實時變化的地震活動性的證據。我們還需要更多的數據，像水庫的水位、負荷等等。”范曉認為，中科院的研究者們已經有此類研究的初步成果，“但他們懶得和我們分享。”