Inslab earthquakes of Central Mexico:: Q, source spectra, and stress drop

We analyze 17 intermediate-depth, normal-faulting, inslab earthquakes of Mexico (4.1 less than or equal to M-w less than or equal to 7.4; 35 km less than or equal to H less than or equal to 118 km), recorded on hard sites at local and regional distances (R :! 600 km), to study spectral attenuation of seismic waves, quality factor Q, source spectra, and Brune stress drop. Assuming l/R geometrical spreading, the quality factor is given by Q(f) = 251f(0.58). Although there is considerable uncertainty in Q due to the trade-off between geometrical spreading and Q, this uncertainty does not influence strongly the estimation of source spectra and stress drops. We find that source spectra of nine events (4.1 less than or equal to M-w less than or equal to 6.4) follow the omega(2) model, while those of the other eight (5.8 less than or equal to M-w less than or equal to 7.4) significantly deviate from it. Interpreting the high-frequency level of the source spectra with the omega(2) model yields a nearly constant stress drop, Deltasigma, with a median value of 304 bars. This is more than 4 times greater than the corresponding value for interplate earthquakes in central Mexico. The observed source acceleration spectra, S(f), is, however, better fit by an empirical source spectrum characterized by two corner frequencies, fa and f(b), such that S(f) = f(2)M(0)/[{1 + (f/f(a))(2)). {1 + (f/f(b))(2)}](1/2), where f(a) = 4.962 X 10(10)/M-0(0.454), f(b) = 4.804 X 10(5)/M-0(0.213), and M-0 is in dyne centimeters. This empirical source spectrum may be useful in predicting ground-motion parameters using stochastic methods.