紫坪铺水库水位变化对剪切波分裂参数的影响

本文通过对2006—2009年四川紫坪铺水库库区8个地震台站记录的地震事件,采用剪切波分裂方法获得了水库库区剪切波分裂参数,并结合地震活动性与水库水位之间的变化关系,分析了紫坪铺水库库区地壳应力的变化特征.剪切波分裂结果显示该研究区域快波偏振方向有两个,分别为北东向和北西向,充分体现了紫坪铺水库地区地壳应力是由北西向的区域主压应力与南东走向的龙门山断裂带综合作用的结果.慢波延迟时间平均值为5.8ms·km-1,慢波延迟时间较大的地区位于库坝和库尾,分别是水库蓄水排水引起地壳应力变化最大的区域.对比慢波延迟时间的变化和水库水位的变化显示了慢波延迟时间与水库水位之间的一致变化关系,揭示了水库的蓄水排水对地壳应力的影响.

The effect of water level changes in Zipingpu reservoir on the parameters of shear wave splitting

The water storage and drainage of the reservoir usually cause the stress change in the crust, which can induce earthquakes. The Zipingpu reservoir is located in the middle section of Longmenshan fault with complex structure. The digital seismic network has been established in the reservoir to monitor moderate and small earthquakes, and accumulated a wealth of data, facilitating the study of earthquakes in the reservoir. #br#Shear wave splitting can be used to detect the stress change in the crust, and obtain the anisotropic static and dynamic gauge. The polarization direction of fast shear waves is consistent with the direction of cracks in the crust, indicating the regional stress direction in the crust. The time delay of the slow shear waves is related to the distribution of the micro-cracks in the crust which implies the change of water pressure in the crust. And the delay time can be used to study the effect of reservoir water level on the crustal anisotropy. This paper obtained the shear wave splitting parameters by the polarization analysis with high accuracy and strong visuality, using seismic data recorded by 8 seismic stations in the Zipingpu reservoir area from 2006 to 2009. According to the velocity model of the shallow exploration, the shear wave window chosen in this paper is 58 degree. The focal depths of the earthquakes are in the range of 5~12 km, and the signal to noise ratio of shear waves is greater than 6.0. 512 seismic events are justified by the data selection criteria. On the basis of the particle moving figures, we choose the position where the particle trajectory deflects vertically to ascertain the arrival time of fast and slow shear waves. #br#Every seismic station recorded more than 30 events in the study area, and the polarization direction at each station is obvious. Combining with the relationship between the seismic activity and water level of the Zipingpu reservoir, this paper obtains the variation characteristics of the stress in the crust. The results of shear wave splitting show two polarization directions of fast shear waves, which are northeast and northwest. The northeast polarization direction of fast shear waves is consistent with the strike of the Longmenshan fault. And the northwest direction agrees with the direction of the regional principal compressive stress. The average delay time of the slow shear waves in the reservoir is 5.8 ms·km^-1. The large delay time of the slow shear waves is from the station BAJ and LYS at the dam and reservoir tail, respectively, where the stress change is large because of the water storage and drainage in the reservoir. The small delay time is from the station YZP far from the reservoir, where the stress in the crust is affected least by the water level of the reservoir.#br#Comparison with the previous results of shear wave splitting in the Zipingpu reservoir shows agreement with our results. The polarization direction of fast shear waves is consistent with the direction of the regional principal compressive stress and the strike of active faults. The relationship between the delay time of the slow shear waves and the water level of the reservoir suggests the increase of water level in the reservoir is often accompanied with the increase of the delay time. The reservoir storage causes the diffusion of the pore pressure in the crust, which makes the aspect ratio of the microcracks in the crust increase.