HR: 0830h
AN: S11C-0311    [PDF]
TI: Near-Surface Anisotropy of Crustal Velocity and Attenuation
AU: * Liu, Y
EM: yunfengl@terra.usc.edu
AF: Dept. of Earth Sciences, University of Southern California, University Park, Los Angeles, CA 90089 United States
AU: Teng, T
EM: lteng@terra.usc.edu
AF: Dept. of Earth Sciences, University of Southern California, University Park, Los Angeles, CA 90089 United States
AU: Ben-Zion, Y
EM: benzion@terra.usc.edu
AF: Dept. of Earth Sciences, University of Southern California, University Park, Los Angeles, CA 90089 United States
AB: Seismograms from local aftershock events of the 1999 Chi-Chi earthquake recorded at a 200 m deep downhole station CHY of the Taiwan Central Weather Bureau Seismic Network (CWBSN) have clearly direct up-going shear waves ($S^/$) and their surface-reflected down-going phases ($S^{\backslash}$). The horizontal waveforms are projected into the fast and slow polarization directions, which are determined from direct up-going phases using the aspect ratio method of shear-wave splitting. Measurements of the time difference between the $S^/$ and $S^{\backslash}$ phases of the fast and slow components give approximately a value of $8 %$ velocity anisotropy in the top 200 m of the crust. Both the $S^/$ and $S^{\backslash}$ phases are windowed with a cosine taper and their amplitude spectrums are calculated. The amplitude spectral density ratios between the $S^/$ and $S^{\backslash}$ phases are approximately linear in the range 3 - 15 Hz. We then estimate $Q_{\beta}$ from the slope of the amplitude spectral density ratios (in dB/Hz) in this range. The results show clear evidence of attenuation anisotropy in the near-surface structure. The estimated values from stacked amplitude spectral ratios of about 180 high-quality records are $Q_{\beta f} = 60$ for the fast components and $Q_{\beta s} = 45$ for slow components. The observed attenuation anisotropy may be a manifestation, similarly to velocity anisotropy, of aligned microcracks and macro-fracture related to the in-situ stress. Strong attenuation anisotropy ($25%$ in this study) will significantly affect the properties of shear-wave seismograms and therefore should be taken into account in studies employing shear-wave amplitude data.
DE: 5144 Wave attenuation
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting