Abstract: In this paper, the indirect boundary element method is extended to simulate near-fault ground motion of complex sites. Seismic amplification effect of alluvial valley under the successive dislocation of a strike-slip fault is quantitatively analyzed by a two-dimensional finite-fault kinematic model. First, hanging wall, footwall of the fault and alluvial valley are divided into different calculation domains. Second, the boundary integral equation is established by using the stress and displacement boundary conditions of different interfaces in the frequency domain. The virtual load density is obtained by discretization, and the seismic response in the frequency domain is solved. The time domain results can be obtained by Fourier transform. Further, the accuracy of the proposed method is verified by comparing with the analytical results. Finally, the amplification characteristic of near-fault ground motion of the alluvial valley is investigated. The influence of the varying parameters, such as the buried depth of the fault upper boundary, the dip angle of the fault, the fault distance of alluvial valley, and the fracture velocity of fault element, on the seismic response of the model are studied. The results show that the alluvial valley has an obvious amplification effect on the amplitude of near-fault ground motion, and the peak value of acceleration response spectrum of the analyzed model can be magnified by 4.64 times. In the interior of alluvial valley, the duration of ground motion is prolonged obviously, and long-period velocity pulse with larger amplitude appears. The near-fault ground motion damage has a typical concentration-effect: when the increase of the fault distance is 10 km, the half-space surface displacement approximately attenuates 50%. This study can provide a new and effective method for simulating near-fault ground motion of complex sites, and it is of great significance for seismic zoning of the complex site with near fault and seismic design of engineering structure.

Key words: indirect boundary element method, seismogenic fault, alluvial valley, seismic ground motion, site effect