沙牌坝址的最大可信地震研究

Study on the maximum credible earthquake of Shapai dam site

如何给出能够反映近场大震特点的坝址的最大可信地震是在我国西部修建高坝时需要考虑的重要问题。以我国西部重大水电工程沙牌拱坝为例,结合有限断层模型和地震学随机方法,将震源视为三维的空间面源,进行坝址在近场大震条件下的最大可信地震研究。选择茂县地办台站(051MXT)记录,对汶川地震的震源模型和参数进行校核,作为本文研究的模型和参数依据。确定对沙牌坝址区域影响最大的发震构造为龙门山断裂带。(1)假定龙门山中央断裂未来再次发生Ms8.0级地震,设置不同高危险的震源模型,比较结果显示,对沙牌坝址而言汶川地震不是最大潜在地震,最大凹凸体的能量中心距坝址最近的震源形式最危险,坝址的峰值加速度一般不超过380 cm/s2。(2)若发震能力稍弱的龙门山后山断裂未来发生Mw7.5级地震,遵循两种思路考虑震源的不确定性,即设置最危险震源和随机设定大量震源模型,且均计入关键参数的不确定性。两种思路合成的沙牌坝址的峰值加速度非常接近,根据工程重要性取较大值,得到沙牌坝址的峰值加速度为735 cm/s2。

It is an important issue to consider in the construction of high dam in western China how to provide the maximum credible earthquake of dam site which is able to reflect the characteristics of large earthquakes in near field. 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 recorded accelerograms at 051MXT rock station from the Wenchuan earthquake is chosen to validate the finite earthquake source models and parameters, serving as the basis of models and parameters for the study in this paper. The Long- menshan 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 MsS.0 earthquake recurring in the central fault of the Longmenshan fault zone, and set the models of earthquake sources of different high risks for compari- son, 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 380 cm/s2. Secondly, if there will be a MwT.5 earthquake occurring in the back range faults of the Longmenshan fault zone where the seismo- genic 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 earth- quake 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 impor- tance, the peak acceleration of the Shapai dam site is determined 735 cm/s2 by choosing the bigger ones.