Abstract

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) ∼ exp(–πκf), defining the observed rate of fall-off, κ, as the spectral decay parameter. It has been proposed that variability in κ results from variable near-surface attenuation and that estimates of κ from small- to moderate-magnitude events apply to the largest earthquakes. No systematic investigation into the behavior of κ 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 κ values are estimated from the high-frequency slopes alone. This study finds that the terms in the model κ = κsite + κevent reduce the misfit significantly, with the κsite and κevent contributing similarly to the total κ value. Evidence suggests that κevent results primarily from source characteristics as opposed to propagation path effects. The absence of a statistically significant dependence of κ on epicentral distance is observed in the Guerrero region. Measuring κ over the frequency bands from 10 to 30 Hz and 10 to 45 Hz reveals increasing κevent with magnitude. When κ is measured from 25 to 45 Hz, κevent is not correlated with magnitude, although its role in error reduction remains significant. κevent also varies systematically with focal mechanism: of all M ≥5.1 earthquakes with available focal mechanisms, κevent is on average 4.5 msec less in normal-faulting earthquakes than in thrusting events.

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