Finite difference calculations for SH-motion, as well as field observations, show that topography can have significant effects on seismic waves when the incident wavelengths are comparable to the size of the topographic features and the topographic slopes are relatively steep. These effects are frequency-dependent and can range from amplification to deamplification at a given site. For the models considered in this paper, amplifications of 75 per cent as compared to motions produced in regions of no topography were found. A qualitative discussion of the strong-motion accelerogram recorded at Pacoima Dam from the San Fernando earthquake of February 9, 1971, suggests that the recorded accelerations could, at any period, be expected to differ by 25 to 50 per cent from those that would have been recorded if no topographic features were present. The complex terrain precludes a prediction of the actual magnitude or sign of the difference. The present results are probably most significant from the viewpoint of engineering seismology, for the conditions required for a significant topographic effect are most likely to occur in the period range of engineering interest.

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