Earthquake site effect modeling in sedimentary basins using a three-dimensional indirect boundary element-fast multipole method

The Boundary Element Method has been applied to simulate seismic wave propagation in three-dimensional sedimentary basins in recent years. However, the structure and size of the large, realistic, three-dimensional sedimentary basin and the frequency range of analysis are limited. We develop the Indirect Boundary Element Method with the Fast Multipole Method in order to overcome this computational limit and cost, and simulate the site response in a realistic three-dimensional sedimentary basin.; The constant circular elements of the boundary surface, which apply to IBEM, are generated automatically from contours of discrete bedrock elevation data by a kriging method and Delaunay triangulation. Most previous applications of BEM have used full space Green's functions for wave propagation between element points. We use halfspace Green's functions which include the seismic wavefield interactions at the free surface and need only the boundary elements of the basin interface. In this way the size of matrix equation can be reduced to about a quarter of that using full space Green's functions. Finally, the reduction in memory requirements and computation time are achieved with the FMM.; We validate the IBEM-FMM by computing the site response in the croissant-shaped model. We compare the IBEM-FMM synthetics with those computed with the Finite Difference Method by Pitarka and find a very good level of agreement. A parallel version of IBEM-FMM has been implemented for large-scale computations. A parallel iterative linear solver based on the Generalized Minimal Residual method using the MPI library has been developed for IBEM-FMM.; IBEM-FMM succeeds in modeling earthquake site responses in a three-dimensional basin. The applications to the real sedimentary basin model in Caracas and in Granada will be shown. The basin-generated scattering waves can be identified propagating back and forth inside the basin and these also generate surface waves weakly propagating outside the basin. The scattering wave propagations inside and outside the basin show different patterns. We can see the scattering wave is locally amplified and its propagation direction is deviated from the incident wave's propagation direction. The computed seismic response in the basin could provide us with good estimations of seismic motion.