Near-surface seismic anisotropy, attenuation and dispersion in the aftershock region of the 1999 Chi-Chi earthquake

Seismograms from local aftershocks of the 1999 Chi- Chi, Taiwan, earthquake recorded at a 200-m-deep downhole station CHY of the Taiwan Central Weather Bureau Seismic Network (CWBSN) have clear direct upgoing shear waves and their surface-reflected downgoing phases. Measurements of time difference between the direct and reflected phases of the fast and slow components of split shear waves show approximately 8 per cent velocity anisotropy in the top 200 m of the crust. The phase velocities extracted from the direct and reflected waveforms display clear evidence of attenuation-related dispersion. Taking the dispersion and geometrical spreading factor into account, we estimate the Q value of the shear waves by fitting calculated results to the observed reflected waveforms. The amplitude spectral density ratios between the direct and reflected phases are approximately linear within the frequency range 2-15 Hz. This allows us also to estimate the Q value from the slope of the amplitude spectral ratio (in dB Hz 1) in this range. The estimated Q values with both methods, based on a set of similar waveforms and additional 156 high-quality records, are 61-68 for the fast components and 43-52 for the slow components. The observed attenuation anisotropy may be, similarly to velocity anisotropy, a manifestation of microcracks alignment and their response to in situ stress. Strong attenuation anisotropy (23-30 per cent in this study) along with attenuation-related dispersion in the shallow crust can affect significantly the properties of shear waves and should be taken into account in studies employing surface and shallow borehole records of shear waveforms.