Abstract

The city of Acapulco is located near or above the mature seismic gap of Guerrero along the Mexican subduction zone. With the purpose of studying the character of strong ground motion on soft sites, four digital accelerographs have been installed in the city on such sites. These instruments have been in operation since 1988. Two additional instruments, part of the Guerrero Accelerograph Array, are located on hard sites in the area. One of these, VNTA, has been in operation since 1985 and the other, ACAN, since 1989. These stations have recorded several earthquakes. We use data from eight events (4.2 ≤ M ≤ 6.9) to study spectral amplification of seismic waves at the soft sites with respect to VNTA. The S waves are amplified by a factor of 6 to 25 at the soft sites in a fairly broad range of frequencies; both the amplification and the frequency band over which it occurs depend upon the site. Although the largest earthquake in our data set (M = 6.9) gave rise to a peak horizontal acceleration exceeding 0.3 g at one of the soft sites, no clear evidence of nonlinear behavior of the subsoil is found. Spectral amplifications of S-wave coda are very similar to those of S waves. We also measured microtremors at the strong-motion sites. The microtremor spectra were interpreted, using reasonable assumptions, to test the feasibility of this technique in reproducing the spectral amplifications observed during earthquakes. Our results show that only a rough estimate of site response can be obtained from this technique, at least in Acapulco; caution is warranted in its use elsewhere.

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