Abstract:

Besides the site condition,the distribution of sub-sources and expansion of sub-source rupture on finite fault effect the near-field ground motions strongly,too.In order to simulate strong ground motion effectively,we develop a simplified numerical simulation method by numerical Green's function's computation.The whole process can be divided into two steps.At first,in infinite uniform elastic medium,take the finite fault as an exterior source under asymmetry layers,the displacement time histories of overburden layers' bottom caused by all sub-sources can be synthesized by analytic solution.The results can be used as the input for the finite element numerical simulation.Secondly,in asymmetry medium,with local artificial transmitting boundary condition,the explicit decoupled finite element simulation approach is applied to model the near-field strong ground motions.Comparing with former source studies,two ideas are added into the source model.One is that,instead of simple couples source,the earthquake moment tensor is introduced to describe fault attitude and the rupture mechanism,which could express the relationships among force,slip direction and slip-time function of every sub-sources in detail.The dislocation could by equivalent to three-order earthquake moment tensor with 9 force couples and every couple expression includes source parameters,such as fault strike,dip angle and rake angle.The other idea is about the slip-time function.On a finite fault,the inhomogeneous characteristics can be expressed by the nonuniform dislocation distributions and the nonuniform rupture expanding in temporal and spatial space.In this paper,the Brune model is used to characterize the slip-time function.For a certain sub-source,three parameters can define the function,average dislocation,rising time and the rupture time delay.It is feasible of the simplified method of strong ground motion simulation by an example test.According to the source parameters and velocity structures of the 1994 Northridge M_W 6.7 earthquake,long period ground motions of four rock stations are simulated and compared with their records.The simulation acceleration histories and velocity histories are coincident well with the records in durations,amplitudes and envelops.The large acceleration pulse and velocity pulse in the simulation are clear.The simulation results also express engineering characters of hanging wall effect and rupture directivity effect distinctly.All above are well in accord with the ground motion records and earthquake investigation which prove that the approach is reasonable and applicable for seismic hazard evaluation and prediction.

Key words: near-field strong ground motion, numerical simulation, numerical Green’s function method, Northridge earthquake

摘要：

近场强地震动除受场地条件的影响外,还受到震源破裂面上子源的空间分布特点、子源破裂先后顺序的强烈控制,基于数值格林函数法的近场强地震动数值模拟方法可以综合考虑震源、传播途径及局部场地条件的影响,对计算过程进行合理简化,分2步完成地震动模拟:第1步,在介质均匀区采用矩张量的解析解计算所有子源在盖层底面的位移,形成下一步有限元计算的输入场;第2步,在盖层介质不均匀区,结合局部人工透射边界技术,采用时、空解耦的波动显式有限元方法计算地表强地震动.在有限断层模型中,采用具有9个力偶的等效地震矩张量表达断层产状、滑动方向等的影响,采用Brune模型定义各子源的滑动时间函数,描述滑动的时、空不均匀分布特征,从而细化震源模型.通过对Northridge地震中4个基岩台站地表地震动的模拟结果和强震记录,验证了此简化计算方法的可行性.

关键词: 近场强地震动, 数值模拟, 数值格林函数法, Northridge地震