P波入射下断层参数对地表地震动的影响
Influence of Fault Parameters on Ground Motion under Incident P Waves

基于应用透射人工边界条件的显式有限元法计算断层破碎带宽度及力学参数变化、地震动入射角变化时二维断层场地模型P波入射下地表地震动场的分布。结果表明：（1）低速度破碎带的存在导致整个场地都有P波转换为SV波的分量，且在断层破碎带的区域出现断层陷波；（2）低速度破碎带的存在使输入场地恒定的能量向破碎带集聚放大，随着破碎带宽度增大或其介质波速降低集聚放大效应增大；（3）场地放大效应是频率相关的，宽度较宽或介质波速较低的断层破碎带对输入地震动中较低的频率成份放大显著；（4）竖向断层破碎带能阻隔斜入射地震P波，随着入射角增加隔震效应更显著。
In this study, a numerical solution for a 2-D fault site model under incident P waves using the explicit finite element method combined with transmission of artificial boundary condition was derived. The distribution rules of peak ground acceleration (PGA) and the spectrum characteristics influenced by fault characteristics such as width and shear wave velocity of the fault and the incidence angle of the seismic waves were discussed. Numerical simulations shown that transform SV waves developed in the low velocity fault zone (LVFZ) when it struck by external incident P waves, and many trapped waves obviously developed in it. This result implied that the external incident seismic radiation congregated in the LVFZ, and the agglomeration effect increased with the increased width and decreased shear wave velocity of the LVFZ. Vertical LVFZs blocked the spread of oblique incident P waves, and the isolation effect decreased with the increased incidence angle of P waves. The seismic response of a variety of LVFZ models with different width and shear wave velocity struck by external vertically incident P waves was determined, and the results implied that the ground motion amplification characteristics in the near fault zone site are frequency dependent. The higher frequency components of ground seismic waves are obviously amplified by LVFZs with narrower widths and higher shear wave velocities, while the lower frequency components of ground seismic waves are obviously amplified by LVFZs with wider widths and lower shear wave velocities. In other words, the site amplification coefficient is frequency dependent, such that the widening the width or decreasing the shear wave velocity of fault fracture zone, the predominant frequency of a larger site amplification coefficient is becoming lower.