Attenuation of P and S waves in the Oaxaca, Mexico, subduction zone

Abstract

The quality factor Q and the spreading rate coefficient n (assuming that the geometrical spreading function approximates r − n , where r is the hypocentral distance) are estimated using spectral amplitudes of P and S waves from earthquakes recorded in the Oaxaca, Mexico, subduction zone. The effects of the source and site response are eliminated by taking spectral ratios from pairs of source-station combinations. We formed 23 combinations for the P waves and 31 for the S waves from seven events with magnitudes in the range 3.4 < M < 4.4 digitally recorded on hard-rock sites at hypocentral distances between 19 and 76 km. We found a frequency-independent geometrical spreading rate coefficient n = 0.5 for both P and S waves in the frequency band analyzed ( 1 < ⨍ < 25 Hz). When n is fixed at a value of one, Q increases with frequency, taking the form Q p = 22 ⨍ 0.97 for P waves and Q s = 56 ⨍ 1.01 for S waves. Thus the ratio of the quality factors Q p Q s is 0.4. This low ratio of Q, and the strong frequency dependence of Q observed, suggest that scattering is an important factor contributing to the attenuation of body waves in the Oaxaca, Mexico, subduction zone.