Ground Motion Prediction Under Uncertainty

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The objective of this project is to predict the strong ground motions at Treasure Island, in San Francisco Bay, resulting from postulated earthquakes on the Hayward and the San Andreas faults.Earthquake prediction exercises have been carried by various organizations of engineers and seismologists in recent years; for example for Turkey Flat, California (1990), the Ashigara Valley, Japan (1992) and most recently for the Kobe/Osaka region in Japan (1998). The basic idea was to attempt to model measured ground response and/or to predict response for sites at which recordings had not been made, permitting different methods of prediction to be compared. These exercises highlighted several difficulties that this project will attempt to avoid by focusing on one very well documented site - the National Geotechnical Experimentation Site at Treasure Island, California - and asking for predictions of motion on the Hayward and the San Andreas faults. A recent compilation of geologic data for the San Francisco Bay Area by researchers at the United State Geological Survey, combined with the intensive geologic studies by CALTRANS (California department of transportation) for the new Bay Bridge span have drastically reduced path uncertainty problems.The project involves a two-part prediction exercise. In the first part (this project), predictors will develop bedrock motion spectra under Treasure Island, for scenario earthquakes on the two faults. Predictors will be selected after a first pre-qualification phase, in which a selection of a range of approaches will be made. Once predictions for this well-documented setting are made, uncertainties implicit in the different methods will be easier to quantify at a final workshop. At least five student teams from different universities will be invited to present predictions as well, to be discussed at the final workshop, and included in the proceedings. A second phase is planned once this work is completed, in which predictors would then take the consensus bedrock motions under the island and predict soil response at various points in the 90-meter-deep soft soil profile, as well as at other points on the island. This second phase will be a follow-on project, intended to study the final cumulative uncertainties involved in source-path-site predictions by state-of-the art methods.