Foam rubber modeling of topographic and Dam interaction effects at Pacoima Dam

A study of the topographic and dam interaction effects was made using a 3-D foam rubber model of the actual topography around the Pacoima Dam accelerograph which recorded over 1 g high-frequency horizontal ground accelerations during the 1971 San Fernando earthquake. Scaling of frequency from the model to the earth depends on the average value of shear-wave velocity in the upper few hundred meters. Assuming βe = 2 km/sec, for vertically incident SH waves, the spectral ratio of the ground acceleration on the ridge to the free field (flat surface) indicates an amplification of about 60 per cent around 6.5 Hz on the N76°W component. Topography has little effect upon the motion recorded on the S14°W component. Motion on the ridge is lower than the free-field motion on both horizontal components for frequencies above 9 Hz. Amplification peaks shift to higher or lower frequencies depending on the assumed shear-wave velocity in the upper few hundred meters.

Results from nonvertically incident SH waves show that the topographic effect is dependent on the direction of approach of the seismic energy. The effect is either de-amplification (in part by shadowing) or amplification (relative to the case where no topography is present), depending on whether the canyon is on the ray path or not. The Fourier spectrum of the ground motion at the dam crest shows peak frequencies at about 5 Hz and 10 Hz (resonance), which correspond to the normal modes of the dam. A study of dynamic interaction between the Pacoima Dam and the ridge shows that the coupling is less than 2 per cent at about 10 Hz and less than 12 percent at about 5 Hz.