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Apparent Attenuation at High Frequencies in Southern California

Yu-Pin Lin, & Thomas H. Jordan

Published August 15, 2016, SCEC Contribution #6828, 2016 SCEC Annual Meeting Poster #228

Accurately simulating strong motions for seismic hazard analysis requires accurate 3D models of crustal structure. At low frequencies (< 1 Hz), the amplitude reduction of seismic wavefields due to anelastic attenuation is relatively minor, and available tomographic models for Southern California (e.g., CVM-S4.26 of Lee et al., 2014) do a pretty good job of accounting 3D elastic scattering on wavefield amplitudes. At higher frequencies, however, anelastic attenuation becomes more important, and the elastic scattering depends on unresolved small-scale heterogeneities, giving rise to a complex apparent attenuation structure that depends on both position and frequency. We place constraints on this structure in the band 1-10 Hz through the analysis of earthquake waveforms recorded by the Southern California Seismic Network (SCSN). We localize signals in frequency and time using wavelet transforms, and we account for source structure and geometrical spreading by referencing the spectral amplitudes to values computed from synthetic seismograms. This approach is examined by the synthetic test which is accomplished by F-K method. We directly invert narrow-band spectral amplitudes rather than the t* measurements commonly used in previous studies, which allows better resolution of frequency-dependent effects. Inversions of datasets recover an attenuation structure that, when averaged laterally and over frequency, is generally consistent with the tomographic study of Hauksson & Shearer (2006). In particular, we find that the apparent quality factor for P waves (QP) is less than the apparent quality factor for S waves (QS), in contradiction with the classical relation QP ~ 2QS that has been used for most wavefield modeling at low frequencies. The data are consistent with QP anomalies being strongest in the low-Q, near-surface waveguide, suggesting that strong scattering from small-scale heterogeneities may play a role in explaining this discrepancy. The data also require that the apparent attenuation be strongly frequency dependent across the 1-10 Hz band.

Key Words
Amplitude spectrum, Seismic attenuation, Southern California

Lin, Y., & Jordan, T. H. (2016, 08). Apparent Attenuation at High Frequencies in Southern California. Poster Presentation at 2016 SCEC Annual Meeting.

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