Study On The Maximum Credible Earthquake At The Dam Site Of Important Hydro-electric Structures

On May 12,2008, a strong earthquake of magnitude 8.0 occurred in Wenchuan County, Sichuan Province of China. During the earthquake, many hydropower projects located in high intensity area, such as the Zipingpu concrete faced rockfill dam and Shapai arch dam are respectively 17 km and 30km away from the epicenter, are actually subjected to much higher intensity than the seismic fortification level. After the 2008 Wenchuan earthquake the National Development and Reform Commission proposed, in order to responding to serious earthquake disaster, it is not enough for major projects to meet the requirements of seismic design levels; a checking of the major projects to the effects of the maximum credible earthquake is also needed, which means the strongest earthquake may occur under the seismic and geological conditions in the dam site. The maximum credible earthquake ground motions are most probably near field ones from large magnitude earthquakes. Aseismatic safety is commonly regarded as the controlling condition for important hydropower projects design when it refers to large-magnitude earthquakes to occur in near field zones, in which case the validity of seismic design ground motion parameters determined by conventional ways is questioned. Consequently, it becomes a key scientific subject urgently to be solved to determine input seismic ground motion parameters. This subject is intended to come up with more rational seismic ground motion parameters for seismic design of China's important hydropower projects based on bedrock and near field seismic ground motions research under circumstance of earthquakes with large magnitude. The main research contents and results include:1. On account of the stochastic method of ground motions simulation based on seismology theory and the finite fault source model which can describe the complexity of the earthquake source accurately, the stochastic finite fault method is applied directly to generating the artificial accelerograms of the maximum credible earthquake in this study. The advantages and problems of the artificial accelerograms simulation by using the stochastic finite fault method are comparatively analyzed. Besides, the values of some key parameters such as the source spectrum, the source scale, the stress drop, the kappa parameter and the Q value are discussed particularly. Furthermore, the paper focuses on how to establish the source model and obtain the relevant parameters based on the inversion of actual records and empirical statistics when applying the stochastic finite fault method to the artificial accelerograms simulation of the maximum credible earthquake.2. Taken parameters relevant such as earthquake source, path and site etc. provided by different researchers with insight into Wenchuan and Lushan earthquakes, in accordance with the observed strong motions at 51PXZ and 51MXT station from the Wenchuan earthquake and at Shapai arch dam abutments from the Lushan earthquake, Stochastic point-source and stochastic finite-fault methods were adopted to predict the strong motions at Shapai arch dam abutments during the Wenchuan earthquake by parameters correction through repeated tentative computing. The results show that the average peak ground acceleration of simulated accelerograms of the Shapai dam site reaches 262cm/s2, while the value of the Zipingpu dam site is 516cm/s2, the average simulated acceleration amplification coefficient spectrum of the Shapai dam site is approximate to the revised design response spectrum with site class I0 based on "Specification for seismic design of hydraulic structures".3. Taking the Shapai Arch Dam, a major hydropower projects in western China, as an example, both finite fault model and stochastic method of seismology are combined, and the seismic source is treated as a three-dimensional spatial fault rupture to study the maximum credible earthquake of dam site under the condition of large earthquakes in the near-source region. The uncertainty analysis of different combinations of the seismic sources and parameters is carried out to obtain the reasonable range of the source models and parameters, serving as the basis of models and parameters for the study in this paper. The Longmenshan fault zone is determined as the seismogenic structure which has the greatest influence on the area of Shapai dam site. First, suppose there will be another Ms8.0 earthquake recurring in the central fault of the Longmenshan fault zone, and set the models of earthquake sources of different high risks for comparison, whose results show that Wenchuan Earthquake is not the maximum potential earthquake as for the Shapai dam site, the form of earthquake source in which the distance between the energy center of the greatest asperity and the dam site is the shortest is the most dangerous, and the peak acceleration of the simulated acceleration time histories at the dam site is generally below 380cm/s2. Secondly, if there will be a Mw7.5 earthquake occurring in the back range faults of the Longmenshan fault zone where the seismogenic potential is a little weaker, the uncertainty of earthquake source is considered by the following two ways, that is to set up the most dangerous earthquake source and randomly set a large number of earthquake source models, both reckoning in uncertainty of the key parameters. The peak accelerations of the Shapai dam site obtained from those two ways are very close. However, according to the engineering importance, the peak acceleration of the Shapai dam site is determined 735cm/s2 by choosing the bigger ones.