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Site response, attenuation and source spectra of S waves along the Guerrero, Mexico, subduction zone

SEISMOLOGICAL LABORATORY UNIVERSITY OF NEVADA, RENO, NEVADA 89557
INSTITUTO DE INGENIERÍAandINSTITUTO DE GEOFISICA UNAM, CU, COYOACÁN, 04510 México, DF

Abstract

Acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra, the site response, and the quality factor Q of S waves. This method is applied to moderate earthquakes (4.0 < M < 7.0) digitally recorded on an accelerograph array installed since 1985 above the subduction zone of Guerrero, Mexico. Results obtained using a two-step inversion and a complete parametric approach are not significantly different. If we assume that geometrical spreading is proportional to 1/r, we find that the frequency dependence of Q for f > 0.8 Hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions. When the source spectra are interpreted in terms of Brune's model, the stress drops obtained are less than 20 bars for the four larger events analyzed. The site functions obtained indicate that although all the stations used are on rock, the effect of the near surface geology caused important amplifications at some stations.