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

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