DIRECT BODY-WAVE Q-ESTIMATES IN NORTHERN BAJA-CALIFORNIA, MEXICO

We used direct arrivals of P and S waves recorded by the seismic netwo rk of northern Baja California (RESNOM) to estimate the quality factor Q in the Peninsular Ranges of Baja California, Mexico. Spectral ampli tudes from 34 earthquakes with magnitudes between 2.2 and 4.0 were ana lyzed 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 th e 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) = 14.4f(0. 9) for P waves and Q(S) = 28.3f(1.0) for S waves. The ratio of Q(P)/Q( S) approximate to 0.5 obtained in northern Baja California is similar to that obtained by Hough et al. (1988) [Attenuation near Anza, Califo rnia. Bull. Seism. Sec. Am. 78, 672-691] (Q(P)/Q(S) approximate to 0.5 4) for the Southern California Batholith near Anza, and the ratio repo rted 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 fre quencies (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. (C) 1997 Elsevier Science B. V.