震源破裂方式和断层性质对近场强地震动特征的影响

Influence of Seismic Source Mechanism and Fault Property on Near-field Strong Ground Motion

基于数值格林函数法的近场强地震动数值模拟方法,以1994年Northridge地震断层面上位错量的不均匀分布模式和该地区的地层剪切波速度结构为震源模型和计算模型,做了两个方面的模拟研究:(1)直立走滑断层(断层倾角为90°)情况下,模拟分析了有限断层单侧破裂模式和双侧破裂模式对强地震动特征——破裂方向性和上盘效应的影响;(2)对于倾斜断层(倾角为45°),模拟分析了正断层、逆断层和走滑断层情况下,单侧破裂模式对其强地震动主要特征——破裂方向性效应和上盘效应的影响。结果表明:断层的破裂方式直接影响着地表地震动峰值和矢量分布;在近场区无论直立断层还是倾斜断层,其地表地震动峰值分布所表达的破裂方向性效应显著,位于破裂传播前方的地震动强度大,反映了波前被压缩的趋势,破裂后方地震强度明显变小;倾斜断层引起的上盘效应明显,NS向分量和竖向分量的地表地震动峰值的最大值出现在上盘靠近断层迹线处,EW向分量的峰值在断层迹线两侧呈不对称分布,且逆断层引起的地震动峰值最大,走滑断层的次之,正断层的最小。

Based on the simulation of strong ground motion at near-field by numerical Green function method, take the seismic source with heterogeneous dislocation distribution and crustal velocity structures in 1994 Northridge earthquake as computing model, two studies are proceeded： （1） For vertical strike fault（the dip is 90°）, we simulate the strong ground motion by means of unilateral rupture and bilateral rupture, and analyze the influence on two major strong ground motion characters--the rupture directivity effect and hanging wall effect. （2） For oblique fault, （the dip is 45°）, we compute the strong ground motion of normal fault, thrust fault and strike fault with unilateral rupture mode, and analyze the influence on rupture directivity effect and hanging wall effect. The results reveal that in near field, the rupture directivity effect and hang- ing wall effect are obviously influenced by different fault properties and seismic source modes. The seismic source modes effect the ground motion peak values and vectors distribution directly. In near field, whatever vertical fault or inclined fault, the ground motion intensity ahead of rup-ture directivity is stronger than that of behind rupture directivity, which indicats that the wavefront are compressed during propagation. The hanging wall effect can be described by simulation results of inclined fault very well. The maximum values of NS component and UP component appear on hanging wall near the fault and the EW component maximum values are dissymmetry on both sides along fault. The peak values caused by thrust fault are greater than by strike fault and normal fault, the values cause by normal fault are the smallest of all.