Crustal velocity and attenuation anisotropy and their
usefulness in earthquake forecasting research
Yunfeng Liu, Ta-Liang Teng, and Yehuda Ben-Zion
Department of Earth Sciences, University of Southern California, Los Angeles,
CA, 90089-0740, USA
There are a number of recent papers claiming the usefulness of shear-wave splitting
data to the potential earthquake forecasting application. We report our results
obtained in a set of perhaps most ideal experimental conditions: excellent near-field
data quality, dense spatial and temporal coverage with time spanning 3 years before
and 2 years after one of the most violent crustal disturbances – the 1999 M7.6 Chi-Chi
earthquake and all its large aftershocks. Seismograms from local events and
aftershocks of the 1999 Chi-Chi earthquake recorded at a 200 m deep downhole
station CHY, one of the Taiwan Central Weather Bureau Seismic Network (CWBSN)
stations that is close to the epicenter of the Chi-Chi mainshock. The downhole
records have clearly the direct up-going phases ( S/ ) and the surface-reflected
down-going phases ( S\ ). We project the horizontal waveforms in 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 time difference between
the S / and S \ phases of the fast and slow components, respectively, give a
approximate 8% of velocity anisotropy in the top 200 m of the crust.
We taper both the S / and S \ phases with a window that consists of a pair of cosine
bell and calculate their amplitude spectrum by FFT. The amplitude spectral density
ratios between the S / and S \ phases demonstrate a reasonable linearity in the range
1~17 Hz. We then estimate Q values from the slope of the amplitude spectral ratio
(dB/Hz) in this range. The estimated Q values from the staked amplitude spectral
density ratios of about 100 high-quality records are 55 for the fast components and 40
for slow components respectively. The results show clear evidence of attenuation
anisotropy in the near-surface structure. More importantly, the results have not
detected any noticeable variations of the shear-wave splitting, neither in splitting time
nor in polarization over the entire period in the part of crust which has under some of
perhaps the most violent disturbance during the occurrence of the M7.6 Chi-Chi
sequence (with 6 after events larger than M6.5). This finding put a serious question
mark on the usefulness of shear-wave splitting data as an indicator that can be applied
to the effort of earthquake forecasting.