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【转帖】最新一期Science上的文章 An Unpredictably Violent Fault
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说预测其实有时赌博的成分很大，赌对了，就是功臣，赌错了，那后果不堪设想。汶川地震已经过去快两个月了，我们渐渐发现过去的许多认识都是错误的，把大地震发生地点这个赌注压在了离龙门山并不远的鲜水河断裂带上。而后来的事情发展表明我们的科研管理体制还有some有待改进的地方。 有句名言，潮退了，看谁在裸泳。放在这里，有两点意思，其一是有问题，迟早会暴露的，踏踏实实做自己的事情，若不认真做事，迟早会为之付出代价的。其二，我们社会的进步，一般而言是要付出一些代价的，只是有时候代价过于高昂了，比如这次地震。 我们的国家和民族能从中吸取点什么教训呢？我们能否从国家和民族的最大利益出发，真正做到资源共享，包容彼此，而不是死抱着本属于国家财产的东西。 Science 20 June 2008: Vol. 320. no. 5883, pp. 1578 - 1580 DOI: 10.1126/science.320.5883.1578 NEWS FOCUS GEOPHYSICS: An Unpredictably Violent Fault Richard Stone* Chinese researchers placed a dense array of seismometers around a dangerous-looking seam in the rocks of Sichuan--only to be blindsided by the true killer Taken by surprise. Locals evacuate from devastated Beichuan County. CREDIT: CHEN XIE/XINHUA PRESS/CORBIS BEIJING--Geophysicists knew that the rugged mountains of Sichuan Province were primed for a "big one." But they didn't know when or which fault would give way first. Two years ago, Liu Qiyuan, a geophysicist at the China Earthquake Administration's (CEA's) Institute of Geology, bet on the Anninghe fault, which has been shifting to the east about 10 millimeters a year as the Indian subcontinent shoves the Tibetan Plateau against the Sichuan Basin. Liu deployed 300 broadband seismometers--the $6 million array is one of the densest in the world--around Anninghe and two faults to the north. Thanks to a prodigious 1.8 terabytes of seismic data per year, he began compiling a high-resolution threedimensional map of the underlying crust. Liu and his colleagues guessed wrong. Mountain building. During the Wenchuan earthquake, land west of Longmenshan lurched eastward. At Shiyan village in Beichuan County, the rupture lifted a road more than 4 meters, destroying houses along the scarp. CREDITS: JIE CHEN; SOURCE: USGS Northeast of Anninghe, on 12 May, a complex fault system ruptured under the Longmenshan, or Dragon's Gate Mountains, releasing energy equivalent to about 2000 Hiroshima-size atomic bombs. Nearly 70,000 people are known to have died, thousands are missing, and more than 1.5 million people lost their homes in the magnitude-7.9 Wenchuan earthquake. Land west of the Longmenshan fault system had been edging eastward toward the Sichuan Basin at a rate of only a couple of millimeters per year, according to Global Positioning System (GPS) measurements. Liu says the GPS readings blinded researchers to the real threat: "We did not imagine such a big event happening in Longmenshan." Scientists will need to redraw seismicintensity maps that guide planners on safe areas for construction and how much shaking buildings must be designed to withstand. The U.S. Geological Survey (USGS) estimates that the Wenchuan earthquake subjected 1.3 million people to violent or extreme shaking. The revisions must be made posthaste in Sichuan, which is keen to begin reconstruction. But with many of China's 1.3 billion people living in seismic zones, every province will have to check its intensity maps. Municipalities will have to upgrade building codes and strengthen enforcement. "They can begin to prepare for the inevitable," says geophysicist Walter Mooney, USGS's top expert on China. But in the long run, a deeper understanding of the titanic forces at play in Earth's crust is necessary to refine predictions of where future big quakes might strike. Other revelations are sure to follow. For the past month, a few dozen Chinese scientists have been braving strong aftershocks to survey the rupture on the central Longmenshan fault and a shorter gash on a parallel fault. Their observations, coupled with readings from the northeastern corner of Liu's Sichuan array, which covers much of the Longmenshan system, should give an unprecedented look at how a powerful temblor warps geological structure. "The Sichuan earthquake is very important because it's rare to see this happen on a thrust fault inside a continent," says seismologist David Simpson, president of Incorporated Research Institutions for Seismology in Washington, D.C., a university consortium that has wired up the world with digital seismometers. Not surprisingly, Liu has been barraged with requests for data from the CEA array; he expects to release preliminary analyses in the next several weeks. In the shadow of Dragon's Gate For someone who has spent nearly three solid weeks surveying Longmenshan and ducking aftershock-induced landslides, geologist Wen Xue-ze looks surprisingly refreshed. Chengdu is sweltering on 5 June, but Wen, after a couple of arduous days in the field around Wenchuan, has just showered and changed and can now review his team's copious findings at the Sichuan Seismological Bureau (SSB) in Chengdu, CEA's biggest provincial bureau. At 2:28 p.m. on 12 May, when the earthquake shook the bureau violently, "we realized it was a big one, but we didn't know where it struck," Wen says. In a few minutes, they learned that the epicenter was Wenchuan, just 70 kilometers northwest of Chengdu. That brought a fresh worry: the possibility that the 156-meter-high Zipingpu Dam, several kilometers east of the epicenter, would collapse. Engineers determined that Zipingpu suffered cracking but was structurally sound. Some Chinese and Western geophysicists privately say it's necessary to investigate whether the dam, completed less than 2 years ago, triggered the earthquake. A couple of hours after the quake struck, Wen and other SSB staff reached the historic town of Dujiangyan, situated on the edge of the Sichuan Basin in the shadow of the Longmenshan. Much of the town lay in ruins, the road wending through the mountains to Wenchuan blocked by landslides. To aid relief efforts, Sichuan officials in Dujiangyan asked SSB to produce a map of the hardest hit areas. Based on aftershocks in the first several hours after main shock, Wen and SSB Director Wu Yao-qiang circled an area encompassing a mind-numbing 20,000 square kilometers. That night, SSB delivered the sobering map to authorities. "They realized the destroyed area was enormous," Wen says. Wen led a team into the field on 17 May to look for surface ruptures. Their first stop was Beichuan, which straddled the main fault and had been reduced to rubble. Wen's group found that land on the northwestern side of the fault had been thrust up as much as 5 meters. "This is mountain building," says Liu. (The third-largest temblor ever recorded--the magnitude-9.3 earthquake off Sumatra that spawned the tsunami in 2004--ruptured 1600 kilometers, shifting the seabed fault a staggering 20 meters in places.) Based on the dramatic scar at Beichuan, CEA chiefs in Dujiangyan asked Wen to rev up his survey work and assigned him a team of 30 scientists. They fanned out in eight groups and over 2 weeks mapped a rupture running more than 200 kilometers along the main fault. In addition to lifting 3 to 5 meters, the fault had shifted areas to the west 1 to 4 meters relative to those in the east. "The earthquake lifted up the mountains and pushed them to the side," says Mooney. The section near Beichuan showed a strike-slip movement, a grinding twist as two slabs of crust moved in opposite directions. Aftershocks have rattled 300 kilometers of the main fault, including a roughly 100-kilometer section to the northeast that Wen says did not rupture. His team also discovered a rupture more than 50 kilometers long on a secondary fault 10 to 20 kilometers to the southeast. A third fault in the Longmenshan system northwest of the main one appears not to have ruptured. Before the Wenchuan earthquake, Wen and his colleagues, like Liu, perceived two immediate threats. One was the Anninghe fault--which has a 90-kilometer seismic gap, or eerily quiet stretch with few tremors. The other was the Xianshuihe fault, which runs southwest to northeast, forming a "V" with the Longmenshan fault, and which, like Anninghe, has been moving about 10 millimeters per year. Longmengshan's giving way before the others, says Wen, "is a challenge to the traditional idea of active fault segmentation." http://www.sciencemag.org/cgi/content/full/sci;320/5883/1578?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=China&searchid=1&FIRSTINDEX=0&issue=5883&resourcetype=HWCIT [ Last edited by 极度灵感 on 2008-7-8 at 23:33 ] |
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Out of the blue? The Wenchuan earthquake certainly wrong-footed CEA headquarters. After the quake struck, it took less than 6 minutes for the initial seismic waves to leave Chinese soil. By the time the waves had reached the far corners of the globe 20 minutes later, USGS's National Earthquake Information Center in Colorado had pinpointed the epicenter and assigned a preliminary magnitude of 7.5. Meanwhile, CEA's automated software determined, erroneously, that a magnitude-3.9 earthquake had struck the Beijing suburb of Tongzhou and posted the information to CEA's Web site. Minutes later, CEA identified the correct epicenter and updated the Web site. By evening, a 230-person-strong CEA team had ar rived in Dujiangyan (Science, 23 May, p. 996). But an incorrect calculation of the moment tensor--a mathematical description of a fault's movement during a quake--lingered on the Web site for 4 days. The errors left CEA's cadre of scientists red-faced. The massive agency employs some 10,000 people, but only about 100 are Ph.D. scientists. "The problem is at the top," says Chen Yuntai, honorary director of CEA's Institute of Geophysics. "The root cause of the mistakes is not placing importance on the science." Perhaps as a result, says Peking (Beijing) University geophysicist Huang Qinghua, "CEA is isolated from the scientific community." One long-standing gripe is a lack of data sharing. The main entities involved in earthquake research--CEA, China's Geological Survey, the Chinese Academy of Sciences, and universities under the Ministry of Education--each collect similar data using their own instruments. "We need a policy to force scientists to upload data to a common server," says Zhou Shiyong, a geophysicist at Peking University. Collaboration across disciplines must also improve. "Before, seismologists and geohazards researchers were working completely separately. Now we realize we have to cooperate," says Cui Peng, a geomorphologist at the Institute of Mountain Hazards and Environment in Chengdu. Peking University and CEA last year established a joint seismology research center. "Theoretically, we can get data from CEA," says Huang. But it hasn't worked very well. U.S. experts, meanwhile, complain that China's policy of delaying the release of seismic data by 30 minutes impedes emergency response. Chinese researchers say their hands are tied by the military, but some say the Wenchuan quake may give momentum to arguments for real-time data release. A more fundamental issue is CEA's mission: not only to monitor and respond to earthquakes but also to predict them. "CEA issues many earthquake warnings, but they are just guessing. It's not science," says Zhou. One recognized precursor of some major quakes is foreshocks that increase in frequency and intensity, but "very few earthquakes have identifiable foreshocks," says Simpson--and Wenchuan had none. Reading portents Nonseismic warning signs are even more problematic. In the hours before the Wenchuan earthquake, a Taiwanese meteorology satellite reportedly detected a decrease in density of charged particles in the ionosphere above Wenchuan. Although some researchers speculate that it may have been due to radon seeping into the air, Huang notes that a link between earthquakes and ionosphere anomalies is controversial. A few days before that, the streets of a Sichuan village near the fault were filled with toads migrating from the mountains. "Everyone hopes that animals can tell us something us humans don't know," says Mooney. "But animal behavior is way too unreliable." Even in hindsight, he and other s say, they've seen no geophysical connection between these anomalies and the earthquake. Chinese scientists devoted considerable energy to research on potential precursors after the late Premier Zhou Enlai in 1966 tasked CEA with earthquake prediction. But a decade later, a disaster laid bare the limitations of this effort. On 28 July 1976, a magnitude-7.8 earthquake leveled Tangshan, 160 kilometers east of Beijing. Officially, 240,000 people died, the highest earthquake death toll in the 20th century. Before the quake, the geology beneath Tangshan was restless: In early July, for example, locals reported fluctuations in the water table, and on the eve of the disaster, there were reports of odd lights emanating from the ground. Some experts were convinced an earthquake was imminent, but others "were waiting for the foreshocks," says Chen Xiaofei, a geophysicist at Peking University. None came. "Tangshan is why I believe that precursors exist, but we don't understand them yet," says Huang. The primitive state of the field, says a senior CEA geophysicist, is similar to that of weather forecasting a century ago, when people relied on sky observations and animal behavior. "Meteorologists have made the transition from empirical to physical prediction," he says. "We haven't." The danger below. CEA's seismometer array near Beijing has revealed a tumultuous--and still dangerous--geology beneath the city of Tangshan, which was leveled by an earthquake in 1976. CREDIT: COURTESY OF LIU QIYUAN In the past decade, Chinese research has largely followed the lead of Japanese and U.S. efforts that focus on deep geophysical processes. For instance, Liu notes, CEA's other broadband array--107 seismometers in the "Capital Circle" region around Beijing--has revealed a convoluted geology under Tangshan caused by a localized upwelling of magma into the crust (see diagram, above). Such mapping can flag hot spots for future megaquakes where GPS reveals little deformation. "The findings suggest that Tangshan remains perilous," warns Liu. Other Chinese scientists argue that their country should chart its own course, with an emphasis on characterizing nonseismic anomalies preceding major temblors. "We pay too much attention to deep structure," argues Zhou, who would like to see China's seismically restive Xinjiang Province turned into an earthquake prediction laboratory. Huang says that "rigorous and reliable" research could allow scientists to design a precursor monitoring network. "We should study geophysical and geochemical signals," says Liu. But he cautions that precursors are likely to be more complicated than the earthquakes they presumably foretell. "Some of my colleagues are in too big a hurry to succeed in earthquake prediction," he says. In the short term, all eyes will be on Sichuan. Liu's team is processing data from the Longmenshan broadband stations; only three of the pricey seismometers were damaged. Wen's group will mount field surveys and comb records of past earthquakes in Longmenshan to better estimate the intervals between major quakes. "We want to understand the relationship between this large earthquake and historical seismicity," he says. The SSB researchers will also investigate whether the Wenchuan earthquake transferred stress to surrounding faults such as southwestern Longmenshan and Anninghe. Geophysicists will help guide reconstruction, which the Sichuan government aims to complete in 3 years. "We want people to have a better life than they had before the earthquake," says Cui, whose mountain hazards institute is one of dozens of organizations participating in reconstruction planning. The most urgent task, Wen says, is to remap the faults. Within the next few weeks, he says, a CEA team will produce an active fault map of the region, which will be revised as new information comes in. Although geophysicists do not expect another huge earthquake in the 12 May rupture zone for another century or two, Sichuan authorities have already chosen another location for a new Beichuan; there are no plans to rebuild other villages on the Longmenshan fault. "People feel the area is no good," says Cui. Adding to the agony is the observation that many buildings that collapsed were poorly constructed. That was the biggest lesson of the Wenchuan earthquake, says the senior CEA geophysicist. "We got this knowledge at the expense of many lives. We should never let it happen again." ________________________________________ With reporting by Hao Xin. |
2楼2008-07-08 21:15:58