We examine differences of empirical site characteristics among S waves, P waves, coda, and microtremors using records at 20 sites in and around the Sendai basin, Japan, and interpret the differences theoretically. At soft soil sites the horizontal-to-vertical spectral ratios (HVRs) for early P coda become different from HVRs for a P wave with increasing time and eventually converge on HVRs for microtremors. The HVRs for an S coda become similar to HVRs for microtremors with increasing time in the frequency range lower than 3 Hz at soft soil sites. By contrast, at a rock site and two hard soil sites, HVRs for S coda agree well with HVRs for an S wave. The soil-to-rock spectral ratios for horizontal (HHRs) and vertical (VVRs) components for early S coda are larger than those for an S wave at soft soil sites. When we use the deep sedimentary structures above the bedrock before Tertiary age, theoretical HVR for the fundamental mode of Rayleigh waves is consistent with observed HVR for microtremors and theoretical HVR for an obliquely incident SV wave is consistent with observed HVR for an S wave. Theoretical S-wave site amplification factor explains well observed HHR for S wave but does not coincide with HVR for microtremors. In general the frequencies of maximum peaks of HVRs for microtremors do not coincide with those of HVRs and HHRs for S wave. However, if we select HVRs with peak frequencies lower than 1 Hz and peak amplitudes larger than three, the peak frequencies of HVRs for microtremors roughly coincide with those of HVRs and HHRs for S wave. Even under these constraints, their amplitudes do not coincide with each other. Concerning coda, we conclude that the Rayleigh wave contamination in coda is significant in the frequency range lower than 3 Hz at soft soil sites.

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The objective of this study is to estimate the S-wave velocity structure of the Taichung basin in a near-fault region, which is needed for strong-motion evaluation for the 1999 Chi-Chi earthquake. We have conducted array measurements of microtremors with a total of 12 arrays at four sites and single-station measurements of microtremors at 48 sites in and around the Taichung basin. Based on the Rayleighwave inversion technique using phase velocities estimated from array records of microtremors, we find that a thick layer (the thickness of about 1000 to 1400 m) with an S-wave velocity of V S 1100 m/sec exists in the east-central part of the Taichung basin. We estimate the thicknesses of sedimentary layers above the preTertiary bedrock at 48 sites to fit calculated peak and trough frequencies of horizontal-to-vertical spectral ratios of Rayleigh waves to observed peak and trough frequencies, assuming the same S-wave velocities estimated using array records. The pre-Tertiary bedrock depth was estimated to be about 5 to 6 km in this region. The estimated thickness of the layer with V S ‫ס‬ 1100 m/sec is largest in the east-central part of the basin and rapidly decreases to less than 400 m in the northeastern and western parts inside the basin. The estimated S-wave velocity structures reasonably explain arrival time of initial P and S waves of aftershock records observed by Higashi et al. (2001).

Previous studies (e.g., Satoh et al., 1995a) on inversion of quality factor for S waves, Q S , of sediments using surface-to-borehole spectral ratios of seismograms, are mostly based on one-dimensional wave propagation of vertically incident SH waves. In this study a method is proposed for inverting frequency-dependent Q S , S-wave velocity V S , P-wave velocity V P and incidence angle by considering obliquely incident SH and SV waves at the bedrock. The method accounts for S to P conversion waves as well as direct and reflected SH and SV waves in the multilayered structure. This method is tested using seismograms recorded on ground surface and deep (GL-1206m) boreholes in the Sendai basin, Japan. Surface-to-borehole spectral ratios of transverse, radial, and vertical components for S-wave windows in the frequency range from 0.5 to 10 Hz are used in the inversion. The peak frequencies of observed spectral ratios are typically different between radial and transverse components, and this difference is reproduced reasonably well by the computed spectral ratios as a result of considering obliquely incident SH and SV waves with the inverted incidence angle. In addition, three components of observed seismograms are also matched well by the synthetics in the time domain. The inverted Q S is modeled as Q S ‫ס‬ V S f 0.63 / 28 (V S in m/sec and frequency f in Hz) for soft soil with V S of about 200 m/sec above 50 m and as Q S ‫ס‬ V S f 0.63 /54 for hard soil and sedimentary rock with V S between 500 m/sec and 3,000 m/sec.