A comprehensive study of the observed spectral decay in strong-motion accelerations recorded in Guerrero, Mexico

The Fourier spectra of S waves on strong-motion accelerograms fall off rapidly at the highest frequencies. Anderson and Hough (1984) suggested characterizing the high-frequency fall-off as A(f) similar to exp(-pikappaf), defining the observed rate of fall-off, kappa, as the spectral decay parameter. It has been proposed that variability in kappa results from variable near-surface attenuation and that estimates Of kappa from small-to moderate-magnitude events apply to the largest earthquakes. No systematic investigation into the behavior Of kappa over a broad magnitude range has been previously undertaken. The Guerrero Accelerograph Array has recorded enough strong-motion data from earthquakes with magnitudes ranging from under 3.0 to 8.0 to achieve a high level of statistical significance. The records are chosen to minimize the influence of the corner frequency on the spectral shape, and the kappa values are estimated from the high-frequency slopes alone. This study finds that the terms in the model kappa = kappa(site) + kappa(event) reduce the misfit significantly, with the kappa(site) and kappa(event) contributing similarly to the total kappa value. Evidence suggests that kappa(event) results primarily from source characteristics as opposed to propagation path effects. The absence of a statistically significant dependence Of kappa on epicentral distance is observed in the Guerrero region. Measuring kappa over the frequency bands from 10 to 30 Hz and 10 to 45 Hz reveals increasing kappa(event) with magnitude. When kappa is measured from 25 to 45 Hz, kappa(event) is not correlated with magnitude, although its role in error reduction remains significant. kappa(event) also varies systematically with focal mechanism: of all M greater than or equal to 5.1 earthquakes with available focal mechanisms, kappa(event) is on average 4.5 msec less in normal-faulting earthquakes than in thrusting events.