Abstract: To investigate the mechanism of seismicity due to impoundment of a reservoir,we propose a method for integrated study on regional/local seismicity and stress by both surface loading and pore pressure diffusion.As an example,possible role of Zipingpu Reservoir on nearby seismicity was studied in detail.The epicenter of the M_S 8.0 Wenchuan earthquake on May 12 of 2008 is very close to the Zipingpu Reservoir.On one hand,several statistical properties including event rate(n),cumulative energy release(E),seismic b-value in the magnitude-frequency relation,and spatial correlation length(SCL)of earthquakes occurring in Zipingpu area from 2004.8 to 2008.5.11 were estimated in detail.On the other hand,we quantitatively examined change of Coulomb Stress(ΔCFS)due to the impoundment of the reservoir.Both weight loading and pore pressure diffusion resulted in significant ΔCFS on the underlying Yingshu-Beichuan and Guanxian-Mianzhu Faults,which are considered as the source faults of the Wenchuan earthquake.Some clear correlations were verified between the local seismicity and stress change,thus we concluded that the impoundment of Zipingpu Reservoir clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake.

Key words: Coulomb failure stress, M_S 8.0 Wenchuan earthquake, dam, seismic activity, statistic model

摘要： 为系统研究水库对附近地区地震活动的影响,提出了一个综合分析地震活动性与水库蓄水引起的应力变化的方法并开发了通用性较强的软件。以紫坪铺水库为例,对水库引起的应力变化与库区附近地震活动性变化的关系进行了详细分析。一方面,定量地分析了水库蓄水过程在附近断层附加的库仑应力变化;另一方面,用紫坪铺水库地震观测台网的地震目录,对包括地震发生频度、能量释放速率、b值、空间相关距离等统计参数的时间演化进行了详细解析。数值计算结果发现,无论是荷重还是孔隙压的扩散都在被认为是汶川大地震发震断层的映秀-北川断层和灌县-江油断层的库区下伏段引起了明显的库仑应力变化。地震活动统计特性的时间演化与主要断层的应力变化之间存在明确的且符合岩石力学破坏准则的相关性。据此,初步认为紫坪铺水库在其蓄水过程中对其地下的龙门山中央断层和山前断层有明显的作用。

关键词: 库仑应力, 水库, 汶川M_S 8.0地震, 地震活动, 统计模型