Investigation of shallow S-wave velocity structure and site response parameters in Taiwan by using high-density microtremor measurements

Site effect is considered a critical component affecting ground motions, and the shallow velocity structure is a primary factor for determining the site effect. The shallow velocity structure should be carefully evaluated to mitigate earthquake hazards. Thus, we use microtremor array measurements as well as the inversion of microtremor horizontal-to-vertical spectral ratios (MHVSRs) to investigate shallow S-wave velocity (Vs) profiles in Taiwan, which is one of the most seismically active areas of the world. On the basis of the diffuse field assumption, Vs profiles can be efficiently inverted. To reduce the nonuniqueness of MHVSR inversion, Vs profiles obtained using microtremor array measurements and from an engineering geological database were adopted as model constraints. Finally, this study included 3587 inverted MHVSR Vs profiles and 65 inverted Vs profiles from microtremor arrays. The results were used to create a detailed updated V_S30 map of Taiwan and to map the depth contours of Z_1.0; thus, the relationship between V_S30 and Z_1.0 was determined. We proposed a new parameter, H_R, as a proxy for V_S30, that is defined as a ratio of the average MHVSR across high- and low-frequency bands. This parameter was correlated to the predominant frequency without subjective selection. Moreover, we constructed a pseudo-three-dimensional shallow Vs model of Taiwan, which describes main shallow structural features and provides complete details for plain areas.