P- and S-wave attenuation in the northern region of the Gulf of California, Mexico

Abstract

We determine average attenuation functions and estimates of the quality factor Q for both P- and S-waves in the northern Gulf of California, Mexico. We use seismograms from the Broadband Seismological Network of the Gulf of California (RESBAN) operated by the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE). The database consisted of 64 earthquakes with Mw between 4.5 and 6.6 and hypocentral distances between 30 and 350 km. Attenuation functions were determined from a nonparametric model obtained by inverting observed spectral amplitudes of 25 frequencies between 0.1 and 25.12 Hz. To estimate \({Q}_{p}\) and \({Q}_{s}\), three geometric dispersion functions were defined: one frequency dependent and two frequency independent. We find that the estimates of \(Q\) depend strongly on the geometric dispersion function adopted. Estimates of \(Q\) obtained for the hypocentral distance from 30 to 350 km indicate that P-wave attenuation is larger than S-wave attenuation regardless of the geometrical spreading function used. When using the frequency-dependent geometric dispersion, we estimate that \({Q}_{P}=224.6{f}^{1.10}\) and \({Q}_{S}=244.7{f}^{1.17}\). In general, the high values of \({Q}_{p}\) and \({Q}_{s}\) suggest that the northern Gulf of California consists of a continental crust, possibly containing rocks with low fluid content and that the likely high pressure present in that region could generate a decrease in attenuation due to the closure of pores in the rock. In addition, the values of the \({Q}_{s}/{Q}_{p}\) ratio suggest that the rocks in that region must have a low fluid content.