Finite element simulation of earthquake ground motion in realistic basins

The objective of this research is to develop a unified method based on the finite elements, for modeling three dimensional ground motion in large basins during strong earthquakes, and then to apply it to the analysis of local and global site effects in realistic sedimentary valleys. The work is performed in three stages: (1) developing an integrated 3-D FEM code for modeling elastic wave propagation on parallel computers with the aid of Archimedes, a toolset created for solving partial differential equations on parallel computers; the FE model includes absorbing conditions for minimizing the occurrence of spurious wave reflections at the boundaries of the computational domain, and effective forces for representing the seismic excitation, both for incident waves arriving from outside the domain and for point or extended fault sources generated within the domain; (2) simulation of ground motion in idealized and actual basins; and (3) analyzing local and global site effects. The particular cases considered include a hypothetical basin subjected to plane wave seismic excitation and the response of the San Fernando Valley in California to an aftershock and the main shock of the 1994 Northridge earthquake. In all cases, the results exhibit strong overall basin and local site effects in the form of large spatial variation of the response, large duration due to surface wave propagation, and large nodal amplification of the free-surface response within the sediments with respect to that of rock. These case studies serve to demonstrate the importance of including the effects of the source, propagation path, and local site effects in order to gain a complete understanding of the earthquake response in large basins.