卢龙地区S波偏振与上地壳裂隙各向异性

SHEAR-WAVE POLARIZATION AND CRACK INDUCED ANISOTROPY OF UPPER CRUST IN LULONG, NORTH CHINA

由三分量数字地震仪组成的小孔径流动台网记录了1982年10月19日河北卢龙M_s=6.1级地震的部分余震.用质点运动图的方法对横波的偏振进行了分析。研究结果表明,在横波窗内的各观测点都存在横波的分裂现象.不同离源角和方位角快波偏振的水平投影都具有近NE40°方向的优势取向,与根据卢龙地震两组断层错动在各向同性介质中所辐射的横波的偏振方向不一致.这可以由传播介质中应力所导致裂隙的定向排列来解释.这一观测结果提供了卢龙地区脆性上地壳大范围膨胀各向异性(EDA)的证据,并表明这一地区直立平行排列裂隙取向和水平主压应力的方向为NE40°.

We have used a temporal network consisting of three-component digital seismographs to record aftershock sequence of the Lulong, Hepei Province, China, earthquake of October 19, 1982, (Ms = 6.1). Polarization of shear-wave is analysed by using particle motion diagram. It is demonstrated that the shear wave splitting exists at all seismic stations within shear-wave window. For different azimuths and take-off angles, the horizontal projections of faster shear-wave polarization present a dominent azimuth of polarization of about NE40°. This azimuth is different from the shear-wave polarization pattern determined from two groups of fault planes in Lulong earthquake sequence if the anisotropy is not taken into account. The stress-induced crack alignments in the propagation media can be used to explain this discrepancy. The observation presented in this study exhibits the existance of extensive dilatancy anisotropy (EDA) in upper crust of the Lulong, North China, area and demonstrates that the orientation of the vertical parallel-aligned cracks and the regional horizontal maximum compression is about NE40°.