Effect of Subsurface Conditions on Earthquake Ground Motions

摘要

The revised version of the SHAKE program was prepared and used to study theeffects of subsurface conditions on the earthquake ground motion in the Loma Prieta earthquake. Preliminary soil profile data from the sites of the Oakland Outer Harbor Wharf and Apeel 2 strong motion stations are used to calculate ground motions, which are then compared with the recorded ground motions using response spectra calculated for a 5% damping ratio. Parameters affecting the amplitude of the calculated ground motion are examined. Response Spectra for recorded and calculated ground motions are compared with recommended design spectra (NEHRP, 1988). It is shown that for periods less than 1.4 s the response spectra for recorded far source earthquake motions at Oakland Wharf and Apeel 2 fall outside the envelope of the applicable design spectra, and that response spectra for deeper soil profiles calculated for near source conditions exceed the design spectra by a considerable margin. Design spectra for the San Francisco Bay region, recently proposed in a USGS study, are reasonably close to the calculated near source spectra for deeper soil profiles for periods less than 1 s, but they are conservative for the bedrock motion, and extremely conservative for longer period structures.