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 Rayleigh-wave 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 VS 1100 m/sec exists in the east-central part of the Taichung basin. We estimate the thicknesses of sedimentary layers above the pre-Tertiary 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 VS = 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).

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