Attenuation Tomography at High Frequencies in Southern California

Yu-Pin Lin, & Thomas H. Jordan

Submitted August 15, 2017, SCEC Contribution #7739, 2017 SCEC Annual Meeting Poster #038

Accurate 3-D models of crustal attenuation structure are important for simulating seismic wavefields at high frequencies (f > 1 Hz). Southern California is an excellent natural laboratory for studying crustal attenuation, owing to its high seismicity and the dense coverage of Southern California Seismic Network (SCSN). In this study, we collected P and S waveforms from 160 regional earthquakes (3.3 ≤ M ≤ 5.7) recorded at 265 broadband SCSN stations, and we measured spectral amplitudes in the 1-10 Hz band using integrals over wavelet transforms. We accounted for source structure and geometrical spreading by referencing the spectral amplitudes to values computed from 1-D synthetic seismograms. We first inverted the spectral amplitude data for a 1-D, frequency-dependent crustal attenuation model, obtaining a consistent logarithmic derivative γ=dlnQ/dlnf ≈ 0.4±0.1 for both P and S waves, which is on the low side of that found in most previous studies. Consistent with previous work (e.g., Hauksson & Shearer, 2006), the best-fitting model shows QP/QS < 1 throughout the crust, and this relation plus the frequency dependence suggests that the attenuation at high frequencies is dominated by strong scattering effects (Sato & Fehler, 1998). The inversions accounted for frequency-dependent variations in the source spectrum and site response. The source spectra roll off at an average rate n of about 2.3 for both wave types. The station residuals reflect the unresolved attenuation and scattering structure near the surface, and they show stronger attenuation in the Los Angeles region and weaker attenuation in the Peninsular Ranges and Mojave block. In a second set of experiments, we explored the 3-D attenuation structure of Southern California using ray-theoretic tomographic inversions. We initialized the inversions with our 1-D frequency- and depth-dependent model, correcting the spectra for a source roll-off rate of n = 2.3. Our preliminary 3-D model shows some correlations of attenuation with velocity variations.

Key Words
Seismic attenuation, Frequency dependent Q, Southern California

Lin, Y., & Jordan, T. H. (2017, 08). Attenuation Tomography at High Frequencies in Southern California. Poster Presentation at 2017 SCEC Annual Meeting.

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