Microtremor observations from the seismic network RESNOM of Baja California, Mexico

We evaluated the site and instrument response of eight stations of the seismic network RESNOM of Baja California, Mexico using spectral analysis of microtremors recorded by measurements made in situ. We determined the resonant frequency of the stations using spectral ratios between the horizontal and the vertical components of motion. We found that only one site shows a clear resonance frequency (PBX), at 2.5 Hz. The rest of the stations have a site response approximately constant between 0.4 and 25 Hz. Although PBX is on rock, topographic effects and the near surface characteristics of this site may influence the response. In general, the spectral ratios calculated separately for the N-S and E-W components have the same spectral shape and the amplification level is within the error bars of the estimates. We also used microtremors recorded by the permanent stations in combination with recordings in situ to test the instrument response of RESNOM stations. We show that since the spectral characteristics of the microtremors are approximately invariant with time, it is possible to use well-calibrated measurements of ambient noise to verify the instrument response of the permanent stations.