Observed effects of topography on ground motion

abstract Field experiments were performed to obtain data needed for studying the effects of topography on seismic motion. L-7 instruments deployed at the crest and base of Kagel Mountain and Josephine Peak, California, recorded several aftershocks of the February 9, 1971, San Fernando earthquake. L-7 instruments deployed at the crest, middle, and base of Butler Mountain near Tonopah, Nevada, recorded the seismic signal generated by the cavity collapse following the Nevada Test Site detonation, ALGODONES. Frequency-dependent amplification of the motion at the crest relative to the motion at the base was observed at all three mountains. Factors as large as 30 in the frequency domain were seen at Kagel Mountain with lesser amounts at the other two locations. The three mountains are similar in shape, but different in size. The effect of shape is not clear. The smallest mountain amplified the motion in a narrow range of periods (peaking around 0.3 to 0.5 sec), the medium-size mountain showed amplification over a slightly broader range (peaking at periods of 0.4 to 0.5 sec), whereas the largest mountain showed less amplification, but it occurred over a broader range of periods. Effects in the time domain are generally comparable to, or less than, the effects in the frequency domain. The ratios of peak amplitudes (crest to base) are usually less than the spectral ratios. It is concluded that topography plays a significant role and is an important consideration in determining the seismic motion that a particular site receives.