Did the Zipingpu Dam Trigger China ' s 2008 Earthquake ? The Scientific Case

After the May 12, 2008 earthquake in Wenchuan, Sichuan, suspicion grew among Earth scientists that the Zipingpu Dam reservoir had triggered the devastating earthquake, which killed more than 80,000 Chinese citizens. Since then, some 50 to 60 articles and studies have investigated this massive seismic event and its relationship to the Zipingpu reservoir. This paper reviews the literature and concludes that the mounting body of evidence and analysis indicates that the magnitude 8 earthquake was triggered by the mass loading and increased pore pressure caused by the Zipingpu reservoir. It also concludes that the initial seismogenic rupture of the Wenchuan earthquake did not occur along the Yingxiu Fault Belt at a depth of 14 to 19 kilometres, as previously thought, but at a depth of 6 to 9 km along the ShuimoMiaoziping Fault Belt, which passes underneath the Zipingpu reservoir. This initial seismogenic rupture subsequently expanded and spread in a series of rupture events that were closely linked to each other for 90 seconds along the Longmenshan Central Fault, moving 200-300 km from southwest to northeast. The near absence of a typical precursor before the Wenchuan earthquake, in addition to seismic recordings of abnormal, small earthquakes in the reservoir area as early as April 5, 2008, suggest that this was not a conventional case of reservoir-induced seismicity (RIS) in which the accumulation of stress in a fault zone is nearing the critical point, and the impounding activities of a reservoir merely trigger the inevitable seismic event. Rather, the new findings suggest that the filling and drawdown of the Zipingpu reservoir triggered clusters of small earthquakes which caused new ruptures in the rock that, in turn, altered the stress field in the Longmenshan region and led to an accelerating release of energy. This series of events culminated in the giant rupture that became the MS8.0 Wenchuan earthquake. In light of these findings, Earth scientists and decision-makers alike must now address a dangerous new reality: if reservoir-induced seismicity can be considered humaninduced foreshocks to a major earthquake, then the science of reservoir-induced seismicity must consider the possibility that reservoirs can trigger unanticipated tectonic activity. Most urgently, the findings presented in this paper about the Wenchuan earthquake make a review of current plans to build dozens of large dams with accompanying large reservoirs, in and near areas of high regional tectonic stress in western China, a high priority. The mounting body of evidence and analysis indicates that the magnitude 8 earthquake was triggered by the mass loading and increased pore pressure caused by the Zipingpu reservoir. INTRODUCTION: World attention on the Zipingpu Dam reservoir as the trigger of China's May 2008 earthquake grows The initial rupture depth of the Wenchuan earthquake was only 6 to 9 kilometres Figure 1: Seismic frequency and seismic depth in the Zipingpu reservoir area from August 16, 2004 – May 10, 2008 The true focal point of the Wenchuan earthquake is located on the Shuimo-Miaoziping Fault Belt which passes through the bottom of the Zipingpu reservoir catchment area Figure 2: Diagram of the process by which the Zipingpu reservoir induced the M8 Wenchuan earthquake Figure 3: Surface rupture zone of the Wenchuan earthquake The rapid rate and dramatic scale of filling and drawdown of Zipingpu's reservoir was unprecedented and dangerous Table 1: The impoundment phases of the Zipingpu reservoir from September 2004 to May 2008 Figure 4: Major phases of loading and unloading of the Zipingpu reservoir Figure 5: Major phases of loading and unloading of the Zipingpu reservoir Conclusions drawn from calculations of "Coulomb stress" Seismic activity significantly increased after filling the Zipingpu reservoir Figure 6: Three seismic clusters after inundation of the Zipingpu Dam reservoir and before the May 12, 2008 Wenchuan earthquake Figure 7: Comparison of seismic activity before and after filling the Zipingpu Dam reservoir Were the reservoir-induced small earthquakes in the Shuimo seismic cluster foreshocks of the Wenchuan earthquake? Other evidence of the effect of small earthquakes on seismic activity in the Zipingpu reservoir area Is the absence of an obvious precursor before the Wenchuan earthquake consistent with the features of RIS? Whether other unusual events before the Wenchuan earthquake were related to the impoundment of the reservoir Can a reservoir induce an earthquake with a magnitude as large as Ms8.0? Figure 8: The phases of the rupture process of the Wenchuan earthquake CONCLUSION: As the complete picture of what happened on May 12, 2008 becomes clearer, scientists and decision-makers face an urgent new reality