Effect of topography on surface motion

Abstract

The effect of topography on surface motion is investigated in the cases of incident SH, P and SV waves. Several types of topography ranging from a ridge to a valley are used. Different incidence angles are considered for a wavelength interval extending from 2h to 20h, where h is the vertical dimension of the anomaly. The computation is made by using a single frequency method developed by Aki and Larner and by synthesizing the results in order to get a solution in the time domain. A method of correction taking into account the residual stress is added. The surface displacement appears to be very much influenced by surface irregularities. In the case of a ridge, a zone of amplification takes place near the top, whereas, for a depression, a zone of attenuation occurs near the bottom. An application to the Pacoima Dam accelerograph site suggests that the high accelerations recorded there during the San Fernando earthquake could have been amplified between 30 and 50 per cent by the uneven topography.