Direct body-wave Q estimates in northern Baja California, Mexico

Abstract

We used direct arrivals of P and S waves recorded by the seismic network of northern Baja California (RESNOM) to estimate the quality factor Q in the Peninsular Ranges of Baja California, Mexico. Spectral amplitudes from 34 earthquakes with magnitudes between 2.2 and 4.0 were analyzed to determine attenuation functions at 17 frequencies (0.3 < f < 12.6 Hz). The estimates of Q were retrieved from the attenuation functions obtained assuming a geometrical spreading function of 1 r ( r being the hypocentral distance) for r < 80 km and (80r) −1 2 for r > 80 km. Q for both P and S increases with frequency taking the form Q P = 14.4 f 0.9 for P waves and Q S = 28.3 f 1.0 for S waves. The ratio of Q P Q S ≈ 0.5 obtained in northern Baja California is similar to that obtained by Hough et al. (1988) [Attenuation near Anza, California. Bull. Seism. Soc. Am. 78, 672–691] ( Q P Q S ≈ 0.54 ) for the Southern California Batholith near Anza, and the ratio reported by Taylor et al. (1986) [Attenuation and scattering of broadband P and S waves across Noth America. J. Geophys. Res. 91, 7309–7325] for the crust and upper mantle in the Basin and Range region for high frequencies ( f > 2 Hz). This result suggests that scattering effects are an important factor affecting the decay of the attenuation functions in the Peninsular ranges of Baja California.