Investigation of shear wave velocity structures in the western Taiwan using array measurement of microtremors

Western coastal plain in Taiwan is an important economic zone with a lot of population. This area borders on the east by the western foothills which is a fold-and-thrust zone and one of the major seismic zones in Taiwan. The earthquakes from western foothills often cause a lot of disaster in coastal plain, like Chi-Chi earthquake. We conducted array measurements of microtremors at seven sites including Lugang, Erlin, Huwei, Dongshih, Taibao, Yijhu and Jiali in western coastal plain in Taiwan. At each site, we performed four arrays with difference apertures. Based on F-K analysis we obtained the phase velocity dispersion curves. After inversion of dispersion curves, the shear wave velocity structures under all sites were estimated. We could roughly describe the variation of shallow shear wave velocity structures in the western coastal plain of Taiwan. According to the comparison between our results and previous studies, the inversion of dispersion curves in the frequency range from 0.1 to 5 Hz can completely estimate shear wave velocity structures down to the depth of 3 km. The shear wave velocity structures of all sites which we desired correctly reflect the velocities and depths of main interfaces including the tops of Pliocene and Miocene. Peikang high apparently dominates the S-wave velocity structure in western Taiwan. Therefore, the shear wave velocity of Dongshih in the depth about 1 km is higher than that of other sites. And the depths of major interfaces are shallowest near Peikang high and dip to north, east and south. Besides, the variation of S-wave velocity at Yijhu is similar to Dongshih, because the Yijhu hinge fault which is the southern boundary of Peikang high cross this area. According to residual gravities and seismic surveys, there are a lot of complex structures including normal faults and anticlines.