Abstract

Spectra from moderate earthquakes are used to investigate shear-wave attenuation and site response for accelerographs located on rock outcrops in Guerrero, Mexico. A least-squares method is used to model the strong-motion spectra assuming an ω−2 source and an exponential decay term, e−πκf, to describe the high-frequency spectral shape. The spectral decay variable, κ, associated with attenuation, is parameterized as the sum of a distance dependent component, κ˜(r), and a site term, κ0(S), which takes a different value for each site, S. Frequency-dependent site response is determined by comparing observed spectra to model event spectra corrected for κ and geometrical spreading. Averaging these residuals over several earthquakes results in an estimate of site response that is generally free of source and path effects.

The distance dependence of the spectral decay parameter as expressed by κ˜(r) is very weak in the Guerrero region in contrast to that in southern California (Anderson, 1991b). This difference may be due to greater shear strength of the top 20 km of crust in the Guerrero subduction zone. Near-site attenuation, represented by κ0(S), is greater in Guerrero compared to the stations in the Anza array of southern California. No obvious correlation with site geology was found. A combination of a deeper weathered layer of crystalline rock and varying subsurface lithology may be responsible for the greater mean values of κ0(S) in Guerrero. The frequency-dependent site functions show significant amplification and deamplification effects for the hard-rock sites. No direct correlation with local topography is observed to explain these effects. The site functions obtained here by constraining the source shape and geometrical spreading closely resemble those derived for Guerrero stations by Castro et al. (1990) using much less restrictive assumptions about the source and propagation effects.

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