Verification of Site Response Analysis Techniques for Soft Clay Sites Using Data from 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. The influence of local soft soil conditions on earthquake motions is often very dominant. Preliminary information suggests that this is particularly true for the Loma Prieta earthquake. The ability to confidently predict these effects is an important component in the effective design of structures, in determining the accuracy of seismic code provisions, and in estimating damage due to an earthquake. The goals of the project are (1) to determine the ability to predict the acceleration levels and response spectrum shapes of soft non-linear clay profiles using equivalent linear methods, and (2) to assess the adequacy of existing design codes for soft clay sites, using the damage potential index approach, based on the recorded and computed response. The specific objectives of this program are: (1) To acquire the available strong ground motion records on soft clay sites so as to determine a representative spectrum shape; (2) Compare this spectrum with spectrum shapes proposed by Seed and Sun (1989), the Structural Engineers Association of California (1988) and Borcherdt (1976); (3) Analyze three soft clay sites for which spectrum shapes have been predicted, using recorded rock motions as input values; (4) Compare ground motions computed using the computer program SHAKE with recorded values, in order to assess the accuracy of this program; (5) Analyze three additional clay sites for which the earthquake damage was heavy or where recorded motions are available, in order to provide additional verification; and (6) Calculate the damage potential index using the measured and computed response motions.