Estimations of the S-wave velocity structures in Chia-Yi City, Taiwan, using the array records of microtremors

Shear-wave velocities (V_S) have been widely used for the site characterization of earthquake ground motion. We report here our investigation of the S-wave velocity structures of Chia-Yi City, Taiwan using the array records of microtremors at seven sites. The dispersion curves at these sites were first calculated using the F-K method proposed by Capon (1969); the S-wave velocity structures in Chia-Yi City were then estimated by employing the surface wave inversion technique (Herrmann, 1991). At frequencies lower than about 1 Hz, the propagation directions are concentrated in the northwest and southwest quadrants. The generation of these may be attributed to the ocean waves of the Taiwan Strait. The harder site (CBA) has higher phase velocities, while the softer sites (CWB, SHP and YRU) have lower phase velocities, especially at frequencies between 1 and 5 Hz. The shallow velocity structures (0–1,500 m) can be roughly divided into four to five layers. The depth of the alluvium gradually increases from east to west and from north to south.