Abstract: Based on waveform data recorded by Zipingpu Reservoir Digital Seismic Network and Chendu Digital Seismic Network from August 2004 to May 2008 before Wenchuan MS 8.0 earthquake,and using ratio of amplitude,we calculated the focal mechanism parameters of 486 earthquakes with magnitude above 1.6 in Zipingpu area,and analyzed the temporal-spatial variation characteristic of focal mechanisms and stress.Results show:from 2006 to 2008,there were seldom thrust-type earthquakes,and strike-slip earthquakes increased in Zipingpu reservoir water field.The ratio of thrust earthquake was high,and strike-slip earthquakes' was low in areas far away from water field.Since the impoundment of the reservoir in October,2005,the thrust earthquakes in area A reduced significantly.The seismicity did not decrease in areas B and C,but showed a trend of increase.The strike-slip faulting earthquakes increased significantly in area A,but in areas B and C,they did not show an increasing trend.Under the background of increased thrust earthquakes,strike-slip earthquakes decreased in area C.The azimuth of mean principal compression stress fields deflected and disturbed to a certain extent before the M_S 8.0 Wenchuan earthquake in all areas.From the analysis of temporal variation of mean principal compression stress in all A,B and C areas,the variation of stress field of the whole study area is similar to that of Sichuan-Qinghai block,i.e.deflection and disturbance of mean stress field occurred in 2006.The variations in area A,B and C were similar,but deflection and disturbance in areas B and C were more distinct in scope.Analyses on the ratio between the thrust,strike-slip and normal earthquakes and on the temporal variation of stress fields of areas A,B and C show that,after the impoundment,the relative change of ratio of earthquake types of area A is the smallest,that of C the biggest,and that of area B is between A and C.This may be because that area C is right in the water area of the reservoir,B is farther,and A is the farthest from reservoir waters.

Key words: Zipingpu reservoir, Wenchuan earthquake, focal mechanism, stress field

摘要： 利用2004年8月16日到2008年5月12日汶川8.0级地震前紫坪铺水库数字地震台网和成都数字地震台网的地震波形资料,运用振幅比方法计算了紫坪铺库区486次1.6级以上地震的震源机制参数;根据得到的震源机制参数分析了紫坪铺库区震源机制和应力场的时空变化特征。结果显示:2006—2008年紫坪铺水库水域范围内逆冲型地震很少,走滑型地震增加;距水域较远的区域逆冲型地震的比例偏高,走滑型地震没有增加;各区域的平均主压应力场的方位在汶川8.0级地震前出现了不同程度的偏转和扰动。

关键词: 紫坪铺水库, 汶川地震, 震源机制, 应力场