This project is part of the NSF/USGS initiative within the National Earthquake Hazard Reduction Program (NEHRP) to investigate the effects of the Loma Prieta earthquake of 17 October 1989. The strong ground motions recorded in the San Francisco Bay Area during the Loma Prieta earthquake exceeded the levels predicted by standard empirical attenuation relations. A preliminary analysis of these records suggests that this may have been a result of critical reflections of the seismic waves from the base of the crust. The objective of this proposal is to perform further analyses and simulations using an expanded data base that includes recordings of other events of the earthquake sequence in order to rigorously test this and other explanations for the large ground motions observed. Evaluating the cause of this observed departure from predicted levels of ground motion has important implications for estimating ground motions from future large earthquakes on the San Andreas and Hayward faults. It also has implications for other regions in the United States such as the Seattle-Portland region where crustal earthquakes having deep focal depths may similarly elevate ground motions to damaging levels at distant locations; and in the Seattle, Los Angeles, Salt Lake City and other regions where basin response effects are expected to be significant.
