Abstract

We performed a seismic noise‐level analysis from records of seismic stations located in northern Baja California, Mexico. We used data from stations of the Northwest Mexico Seismic Network, with the goal to characterize the noise spectrum for each station as a function of time. The seismic stations are located in the sedimentary environment of the Mexicali Valley (MV) and the granitic Peninsular Ranges of Baja California (PRBC). The ambient seismic noise was characterized using the power spectral density (PSD) technique (Peterson, 1993). We found that, at periods between 0.1 and 2 s, the median noise levels (MNLs) of stations in the MV are up to 25 dB higher, and near the new high‐noise model (NHNM), than MNLs of PRBC stations. For periods >2  s, the MNLs are similar for both regions and are between the NHNM and new low‐noise models. We found differences in the noise levels when seismic sensors (different brands but same bandwidth) were interchanged. For periods <1  s, the MNLs computed from records of Güralp sensors are 20  dB higher than MNLs from Nanometrics sensors; for periods >10  s the MNLs of Nanometrics sensors are 20  dB higher than the MNLs from Güralp sensors. We observed daily variations in short‐period noise, related to human activity, such as higher noise levels for periods <1  s at daylight in a station in the city of Mexicali. No influence of variations of the sea level of the Pacific Ocean on the PSD of stations of north Baja California was observed. At least for two sites, in and south of MV, there is a direct relationship among variations of pressure and temperature with seismic noise: high pressure and low temperature are related with high‐noise levels in the 4.0–8.5 s period band and vice versa.

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