Site Response of the NARS-Baja and RESBAN Broadband Networks of the Gulf of California, México

Abstract

We studied the seismic response of broadband stations located around the Gulf of California, Mexico, using the horizontal to vertical component spectral ratio method (HVSR). We analyzed 92 earthquakes recor-ded by the NARS-Baja and RESBAN networks, operated by CICESE. The database consists of events recorded between 2002 and 2006, with magnitudes ranging from 3.2 to 6.6. We rotated the records to find radial and transversal ground-motion components and we calculated Fourier spectra of S-wave windows recorded for the three ground-motion components. Then, we calculated HVSR for the individual components and the average of both horizontal components for every event. We analyze records from 20 stations located on sites with different geologic characteristics and we find azimuthal dependence on six of them that have amplification factors varying from 1.5 to up to 13 times at narrow back-azimuth ranges. We also find that sites with significant amplification factors (above three) show increasing amplification with increasing source magnitude.