The influence of critical Moho Reflections on strong ground motions recorded in San Francisco and Oakland during the 1989 Loma Prieta Earthquake

The amplitudes of strong ground motions from the Loma Prieta earthquake recorded in the San Francisco and Oakland areas exceeded the levels predicted by standard empirical attenuation relations. Preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust. These reflections, which are identified on the basis of their arrival times and phase velocity, and by comparison with simulated accelerograms, were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust. These motions were further amplified, presumably by impedance contrast effects, at soft soil sites in San Francisco and Oakland. The effect of the critical reflections in amplifying peak accelerations of the Loma Prieta earthquake in the San Francisco and Oakland regions was as large as the effect of soft soil site conditions. Focal depth has an important influence on strong motion attenuation at distances beyond about 40 km, and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range. Further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in San Francisco and Oakland, and the conclusions drawn from that explanation.