Site Response and Soil Liquefaction in San Francisco Due to the Loma Prieta Earthquake

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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. San Francisco offers a unique opportunity to compare the effects of two earthquakes through detailed and comprehensive observations which now exist for the 1906 and the 1989 earthquakes. By studying different locations within the city, it is possible to evaluate how variations in fill thickness and density, the thickness and consolidation state of underlying soft clay, the water table, and the depth to bedrock affect the ground surface response and the development of soil liquefaction for similar levels of bedrock seismic motion. Moreover, ground response coupled with the performance of buried lifeline systems provides an additional chance to quantify the effect of soil-structure interaction on complex networks that are critical for emergency operations. To take advantage of the knowledge gained from the 1906 and 1989 earthquakes, this project is structured according to a three- fold set of objectives: (1) to characterize subsurface ground conditions and site response at select locations in San Francisco - this requires a comprehensive data base on soil conditions, as well as the familiarity and the site-specific knowledge gained from previous investigations and post-earthquake reconnaissance in the city, (2) to model analytically the site response at select locations, and to compare these analytical results with field observations and with seismic data in order to substantiate and refine the analytical models, and (3) to use analytical models to predict the effects on the city of San Francisco of possible future earthquakes that are larger and closer than was the Loma Prieta earthquake. This project involves cooperation with the U.S. Geological Survey (USGS) and San Francisco geotechnical firms and city agencies. The USGS is performing soil boring at selected locations; geotechnical firms are providing soil boring logs (data) and geotechnical investigation reports in order to expand the available data base on subsurface conditions and relevant soil properties. In addition, city agencies, such as the San Francisco Fire Department, are involved in evaluating how the site response characteristics might affect the water supply, and how best to design and operate the pipeline system so as to minimize the effects of locally severe ground shaking and liquefaction.