四川宜宾地区S波分裂特征

本文采用纵横比与偏振分析相结合的方法测定了2013年4月25日~2015年12月31日四川宜宾地区10个台站S波分裂参数,即快波偏振方向和慢波延迟时间。结果表明,华蓥山断裂两侧台站呈现不同的快波偏振优势方向,断裂带以西的台站偏振优势方向为NW向,与区域应力场方向一致;位于断裂带以东的台站优势偏振方向为NE向,与断裂走向一致。在地震密集分布区域内的CNI台的优势偏振方向为NE向,与台站附近的断裂带走向基本一致。研究区域南段的3个台站(JLI、YAJ、XWE)优势偏振方向近NS向。各个台站平均慢波延迟时间在3.07~11.95ms/km范围内,慢波延迟时间最大的台站是CNI台,距离2013年4月25日06时10分M_L5.2地震震中位置最近,这反映出震源区地震波各向异性程度较强。CNI台站的慢波延迟时间显示,在2015年2月7日M_L4.8地震前观测到慢波延迟时间有明显的上升趋势。

Characteristic of shear-wave splitting in the Yibin area, Sichuan Province

In this paper,the methods of aspect ratio and polarization analysis were used to determine the S-wave splitting parameters of 10 stations in Yibin,Sichuan from April 25,2013 to December 31,2015,i.e.,fast wave polarization direction and slow wave delay time. The results show the polarization directions of fast shear waves on both sides are different. In the west of Huaying Fault the dominant polarization directions of fast shear waves are NW in agreement with the directions of the regional principal compressive stress. In the east the dominant polarization directions of fast shear waves are NE,which is consistent with the strike of Huaying Fault. The dominant polarization directions of fast shear waves at the station CNI in the earthquake concentrated area are NE,which is almost consistent with the strike of faults near the stations. The dominant polarization directions of fast shear waves at the 3 stations(JLI,YAJ,and XWE)in the southern section of the study area are near NS. The average delay time of slow shear waves at each station is in the range of 3.31-11.95ms/km,the maximum delay time of slow shear wave is at the station CNI which is the nearest station from the epicenter of M_L5.2 earthquake,reflecting the strong anisotropy in the source area. The delay times of slow shear waves at the station CNI are in disorder in the period after the two M_S4.0 earthquakes. Characteristic increases of time-delays is observed before the M_L4.8 earthquake on February 7,2015.