汶川地震断裂带科学深钻WFSD-3附近上地壳S波分裂特征

S-wave splitting in upper crust near the scientific drilling WFSD-3 at the Wenchuan earthquake fault zone

为配合汶川地震断裂带科学钻探工程(WFSD),中国地震局地球物理研究所在四川省绵竹市天池乡和灌县—安县断裂附近分别架设了15套微震仪器和17个短周期地震台.基于WFSD-3附近的微震、短周期和区域台网的固定台站记录的近震数据,通过横波窗内S波分裂计算,得到其上地壳各向异性参数,即快波偏振方向和慢波的时间延迟,并分析了研究区的上地壳各向异性特征.结果显示,研究区大部分区域的快波偏振方向为NE向,与龙门山断裂带走向一致,但在研究区微震台阵布设小区域内,快波偏振方向表现出东西分区特征,东部为NE向,西部为NW向.上地壳各向异性主要是受到岩层中随应力分布排列的微裂隙和岩石或矿物结构的影响,研究区内快波偏振方向主要表现为NE方向,与断裂走向一致,反映了研究区上地壳各向异性主要受控于结构控制的各向异性,局部区域的快波偏振方向为NW向,与区域最大主压应力方向一致,说明区域应力场对研究区上地壳各向异性也有影响.通过分析微震台阵的归一化时间延迟随时间的变化情况可以反映区域应力场的变化情况.微震台阵的慢S波时间延迟在2012、2013年较为离散,在2014年有收敛的趋势,反映了强震后区域应力场逐渐稳定的趋势.

In order to cooperate with the Wenchuan Fault Scientific Drilling Program （WFSD）, Institute of Geophysics, China Earthquake Administration deployed 15 microearthquake instruments in Tianchi township, Mianzhu city of Sichuan Province and set up 17 short-period seismic stations near the Guanxian-Anxian fault. This paper studies anisotropic characteristics of the upper crust based on the seismic data recorded by thes microearthquake and short period seismic arrays and permanent stations near the WFSD-3. The dominate polarization directions of fast S-wave and delay times of slow S-wave are derived by analyzing S-wave particle motion in the S-wave window. The results show that the polarization directions of fast S-wave in most of the research area are in NE, consistent with the strike of the Longmenshan fault. However, the Tianchi microearthquake array shows partition characteristics, which is in NE for eastern part and NW for western part, respectively. S-wave splitting is mainly caused by stress-aligned microcracks in the rock and the structures of the rock and mineral. The polarization directions of fast S-waves show mainly NE, consistent with the strike of the faults, which reflects crustal seismic anisotropy in the study area is mainly controlled by the structure. In some places, the polarization directions are NW, consistent with principal compressive stress, which demonstrates crustal seismic anisotropy is also affected by the local stress field. The change of the regional stress field is studied by analyzing the normalized delay times changing time. The normalized delay times were discrete in 2012—2013 and tended to converge in 2014, implying the stress field of the region had a tendency becoming stable after the strong earthquake.