紫坪铺水库地区震源位置和速度结构的联合反演

利用紫坪铺水库7个库区地震台站和10个区域地震台站记录的2004年8月至2008年5月的地震震相观测报告,通过震源位置和速度结构联合反演的方法,采用Simulps14软件对紫坪铺水库地区进行了小震精定位及速度结构反演。通过计算得到了紫坪铺水库地区记录到的几乎所有地震的精定位结果,以及0km、3km、6km和10km几个层面上较好的P波速度和波速比分布情况。精定位结果显示,紫坪铺水库地区的地震活动主要集中在虹口、玉堂镇和水磨3个地区。层析成像和波速比结果则较好地反映了紫坪铺地区受到水库渗水的影响范围及紫坪铺水库对汶川地震的影响。整体上来讲,紫坪铺水库的西南端水库渗水作用最大深度≤8km,汶川主震深度上没有明显的P波低速异常,也没有明显的波速比高值异常,说明水的渗透作用并没有达到汶川主震位置深度,即水对汶川地震的发生没有直接作用。

SIMULTANEOUS INVERSION OF EARTHQUAKE HYPOCENTERS AND VELOCITY STRUCTURE IN ZIPINGPU RESERVOIR AREA

In this study, using the earthquake observation reports recorded by 7 local earthquake stations in Zipingpu reservoir area and 10 regional seismic stations during the period from August 2004 to May 2008, and with the method of simultaneous inversion of earthquake hypocenters and velocity structure, we invert the velocity structure and conduct the relocation of small earthquakes in the Zipingpu area using the software Simulps14. Nodes method is used in model parameterization and ART-PB is used in forward calculation. Damped least square method is used in inversion. Regarding the evaluation of solutions, the RDE and DWS are both given in this study. By calculating, we determined the precise locations of almost all earthquakes in Zipingpu area and the distributions of P wave velocity and V_P/V_S ratio on layers 0km, 3km, 6km and 10km. The relocations of earthquakes show that the seismicity mainly concentrates in the three areas of Hongkou, Yutang town and Shuimo.Tomographic imaging result of P wave velocity structure and wave velocity ratio elaborated the effect of reservoir seepage on Wenchuan M_S8.0 earthquake. We find the effect of reservoir seepage in the southwest of Zipingpu reservoir, which is closest to the epicenter of Wenchuan earthquake, is not more than 8km in depth, and there is no obvious low velocity anomaly at the depth of the Wenchuan main shock, nor obvious high value anomaly of V_P/V_S ratio. This means penetration of water didn't reach the depth of Wenchuan main shock, that is, water was not the direct factor causing the Wenchuan earthquake.