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A Comprehensive Study of the Observed Spectral Decay in Strong-Motion Accelerations Recorded in Guerrero, Mexico

Matthew D. Purvance, & John G. Anderson

Published April 2003, SCEC Contribution #617

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(–{pi}{kappa}f), 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 ≥5.1 earthquakes with available focal mechanisms, {kappa}event is on average 4.5 msec less in normal-faulting earthquakes than in thrusting events.

Key Words
10 hz, attenuation, earthquakes, lithosphere, model, parameter, shear waves, southern California, subduction zone, s-waves

Purvance, M. D., & Anderson, J. G. (2003). A Comprehensive Study of the Observed Spectral Decay in Strong-Motion Accelerations Recorded in Guerrero, Mexico. Bulletin of the Seismological Society of America, 93(2), 600-611. doi: 10.1785/0120020065.