###### Abstract  
Near-surface S-wave velocity structures (V\({}_{\rm S}\)) are crucial in site-effect studies and ground-motion simulations or predictions. We explored S-wave velocity structures in Taichung, the second-largest city in Taiwan by population, by employing array measurements of microtremors at a total of 53 sites. First, the fundamental-mode dispersion curves of Rayleigh waves were estimated by adopting the frequency-wavenumber analysis method. Second, the surface-wave inversion technique was used to calculate the S-wave velocity structures of the area. At many sites, observed phase velocities were almost flat, with a phase velocity of approximately 800-1300 m/s in the frequency range of 0.6-2 Hz. A high-velocity zone (V\({}_{\rm S}\) of 900-1500 m/s) with a convex shape was observed at the shallow S-wave structures of these sites (depths of 50-500 m). On the basis of the inversion results, we constructed two-dimensional and three-dimensional contour maps to elucidate the variations of V\({}_{\rm S}\) structures in Taichung. According to V\({}_{\rm S}\)-contour maps at different depths, lowest S-wave velocities are found at the western coastal plan, whereas highest S-wave velocities appear on the eastern side. The S-wave velocity gradually decreases from east to west. Moreover, the S-wave velocity of the Tertiary bedrock is assumed to be 1500 m/s in the area. According to the depth-contour map (V\({}_{\rm S}\) = 1500 m/s), the depths of the bedrock range from 250 m (the eastern part) to 1550 m (the western part). The thicknesses of the alluvium gradually decrease from west to east. Our results are consistent with the geology of the Taichung area. F-K method; dispersion curve; surface-wave inversion method +
Footnote †: journal: Applied  
0000-0002-2300-2300]T.H.H.Shih  
0000-0002-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-2300-23000-2300-23000-2300-2300-2300-2300-23000-230 during large earthquakes. Understanding the S-wave velocity structures of an area is key to performing seismic hazard assessments and ground-motion simulations to improve earthquake preparedness. Near-surface S-wave velocities are widely used for site characterization. S-wave velocity data are generally obtained using the well-logging method (direct borehole method), but this approach is costly and time-consuming. Compared with the well-logging method, nonintrusive surface methods can be used to obtain data at a considerably lower cost. In [1], compared with other non-intrusive surface methods, microtremor array measurements have several advantages (e.g., no limitation of time and location, easy observation, no environmental problems, etc.). In terms of economical and practical considerations, microtremor array measurements provide a favorable alternative in estimating S-wave velocity structures. Many studies have employed microtremor array measurements to examine material properties [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. [PERSON] et al. [15] estimated the site characteristics of different geological formations in the Kachchh seismic zone of western India by conducting microtremor array measurements at 7 sites and single-station microtremor measurements at 36 sites. The results demonstrated diverse response patterns with varying predominant frequencies, amplitudes, and shear-wave velocities. [PERSON] et al. [16] explored shallow S-wave velocity profiles at 28 sites and provided the fundamental data to assess seismic hazards in Hatay and Kahramanmaras Provinces in southern Turkey. [PERSON] et al. [17] estimated the V\({}_{\text{S}}\) structure (~40 m depth) of the subsoil throughout the city of Jakarta, Indonesia by using microtremor array observations at 55 sites. The spatial distribution of V\({}_{\text{S}30}\) in the city revealed low velocity in the northern part that gradually increased toward the south. This closely matched a map of V\({}_{\text{S}30}\) converted from the SPT30 (standard penetration test) map.