孕震过程中波速比及S波分裂时空变化的数值模拟

本文利用有限元方法模拟计算了强震发生前孕震区的波速比及Ｓ波分裂的时空变化，结果表明，孕震过程中，随着应力的局部集中，岩体发生扩容，引起裂隙的增生及定向排列，从而使孕震区产生各向异性，引起波速比的变化及Ｓ波分裂现象，在不同阶段，穿过同一路径的地震射线其波速比和Ｓ波分裂特征不同；在同一时刻，沿不同路径传播的地震射线其波速比及Ｓ波分裂也不同。总体看来，平行于孕震断层传播的地震波速比变化比垂直于其波速比及

NUMERICAL SIMULATION OF SPATIAL AND TEMPORAL VARIATION OF V_P/V_S AND SHEAR WAVE SPLITTING IN EARTHQUAKE PREPARATION

In this paper, spatial and temporal variations of VP/ Vs and shear wave splitting in seismogenic zone before strong earthquake occurrence were simulated by using finite element method. The results show that stress concentration in earthquake preparation leads to rock dilatancy and development of oriented cracks. Therefore, the seismogenic zone turns to be anisotropic and causes changes of P/ Vs and shear wave splitting. In different time stages, characteristics of VP / Vs and shear wave splitting are different for the same ray path; while in the same time, characteristics of VP/ Vs and shear wave splitting are different along different paths. Generally, variation of VP/ Vs is bigger when the wave propagates parallel to the strike of the fault, and that is smaller when the wave propagates perpendicular to the fault strike; the change of VP / Vs is more obvious when the wave propagates near the fault or through the ends of the fault. For time delay of shear wave splitting, it is bigger when the wave goes through the source area while it is smaller when the wave goes through the periphery area in early stage of earthquake preparation. In later stage of earthquake preparation, time delay of shear wave splitting is smaller when the wave goes through the source area while it is bigger when the wave goes through the periphery area. The result also show that time delay of shear wave splitting tends to grow up before earthquake occurrence.