Study of the 3-D response of 2-D scatterers: Earthquake engineering applications

The effects of topography and subsurface irregularities in a "2.5-D" elastic half-space for various types of incident seismic waves are investigated.;First a mathematical formulation of the "2.5-D" elastodynamic scattering problem is presented and validated. The formulation is a straightforward extension of the Discrete Wavenumber Boundary Integral Equation Method (DWBIEM). It is demonstrated that the Green's function which is appropriate for a boundary formulation of the "2.5-D" elastodynamic scattering problem is the one corresponding to a unit force moving on a straight line with constant velocity. Such a Green's function is derived in the present study.;The formulation may be used to study the wavefields in models of sedimentary deposits (e.g., valleys) or topography (e.g., canyons or ridges) with a 2-D variation in structure but obliquely incident plane waves. The advantage of a "2.5-D" formulation is that it provides the means for calculations of 3-D wavefields in scattering problems by requiring a storage comparable to that of the corresponding 2-D calculations.;Next the effects of topographic irregularities and alluvial valleys are studied for plane SH-, SV- and P-waves and a Rayleigh wave with different azimuthal angles. The general responses for different azimuthal angles seem to be the same but the relative amplitudes are azimuth dependent. Some of the time responses of scatterers are visualized by IBM's DataExplorer. This outstanding visualization tool allows us to see the details of waves propagating both on the surface and under the surface of scatterers.;Finally the motions recorded by the Gilroy array of instruments on the surface of the Santa Clara Valley, California, during the 1989 Loma Prieta and 1984 Morgan Hill earthquakes are analysed for evidence of valley induced surface waves.;The Santa Clara Valley extends in a NW-SE direction, south of the San Francisco Bay. The Gilroy linear array of instruments is an east-west alignment of stations crossing the Santa Clara Valley. Seismic refraction studies in the vicinity of the array indicate that the valley is wedge-shaped in cross-section with maximum thickness on the order of 1 km.;Analysis of the recorded motions of the 1989 Loma Prieta earthquake reveal clear evidence of the fundamental and first and second higher modes of Rayleigh waves, while analysis of the recorded motions of the 1984 Morgan Hill earthquake shows, in addition to the above surface wave modes, the presence of the fundamental Love mode. Motions generated by the latter event were more complicated due to the presence of the low-velocity zone of the Calaveras fault, which traps and focused seismic energy generated by slip on the fault, and leaks it to the surrounding medium in a rather complicated manner.;The observed valley-induced surface waves are simulated using a hybrid numerical technique which combines the Boundary Integral Equation Method with the Finite Element Method. The synthetic motions generated by the simulations, reproduce important aspects of the valley response, such as surface wave types and phase velocities. (Abstract shortened by UMI.).