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 of 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 focuses 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 mathematical formulation that we use has been developed for a class of cylindrical inclusions of infinite length, having an arbitrary cross-section, embedded in a homogeneous (or layered) half-space, subjected to plane waves impinging at an oblique angle with respect to the axis of the inclusion. Even though the model of the valley is two-dimensional, the response is three-dimensional and has the particular feature of repeating itself with a certain delay for different observers along the axis of the valley. This feature leads to a considerably simpler solution than that for a valley with a 3-D geometry.
