Seismic response of long-period sites: lessons from the September 19, 1985, Mexican earthquake

Abstract

The implications for seismic design in Canada, of the acceleration data recorded at a number of sites in Mexico City during the September 19, 1985, earthquake, were investigated. The conditions for amplification of low-level incoming ground motions at deep sites were identified, in particular, the critical role of variation in shear modulus with shear strain. The current procedure for determining site-specific ground motions was evaluated and major sources of uncertainty in the results were identified. Criteria for selecting representative input motions for site response studies are recommended based on results of analyses of Mexico City sites. A comparative study of sites in the Fraser Delta area of British Columbia showed that offshore subduction earthquakes should be considered when developing design spectra for deep sites in the delta. Key words: earthquakes, seismic design, ground motions, shear modulus, design spectra.