S-wave velocity structure of the Taichung Basin, Taiwan, estimated from array and single-station records of microtremors

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-moti on evaluation for the 1999 Chi-Chi earthquake. We have conducted array meas urements of microtremors with a total of 12 arrays at four sites and single -station measurements of microtremors at 48 sites in and around the Taichun g basin. Based on the Rayleigh-wave inversion technique using phase velocit ies estimated from array records of microtremors, we find that a thick laye r (the thickness of about 1000 to 1400 m) with an S-wave velocity of V-s 11 00 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-verti cal spectral ratios of Rayleigh waves to observed peak and trough frequenci es, 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 regi on. The estimated thickness of the layer with V-s = 1100 m/sec is largest i n 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).