## 5 Conclusions  
In this study, we investigated shallow S-wave velocity structures (approximately 0-2000 m) beneath Taichung municipality in the west-central part of Taiwan. To accomplish this aim, we performed microtremor array measurements at 53 sites in the Taichung area. At each site, two different-sized arrays were deployed, and their radii ranged from 25 m to 400 m. At each array, 10 stations were used and deployed in the form of 3 triangles of differing apertures around a center station.  
For the analysis of microtremor array recordings, we first adopted the FK spectrum method to estimate the phase velocities of Rayleigh waves at each site. Therefore, we obtained dispersion curves at all 53 sites. The calculated phase velocities varied between sites. The acquired dispersion curves illustrate that the softer sites (e.g., WNE, western  
Figure 13: Contour map (**upper panel**) and 3D structure (**lower panel**) of the depth variations of the bedrock (Vs = 1500 m/s) in the Taichung area.  
coastal plain) had lower phase velocities than the harder sites (e.g., FNJ, near the Chelungpu Fault). We categorized similar dispersion curves into groups according to their amplitude and shapes. They were roughly divided into four groups. The sites of Group 1 (near or along the Chelungpu Fault) had the highest phase velocities, whereas those of Group 4 (at the western coastal plain) had the lowest values. The phase velocities of Groups 2 and 3 were between those of Groups 1 and 4.  
Furthermore, with reference to the dispersion curves, we calculated S-wave velocity structures by using the surface-wave inversion method at all sites. According to the 1D velocity profile, among these sites, site FNJ (near the Chelungpu Fault) had the highest S-wave velocities and thinnest alluvium, whereas site WNE (on the Western Coastal Plain) had the lowest S-wave velocities and thickest alluvium. If we assume that the S-wave velocity of the Tertiary basement is 1500 m/s, the thicknesses of Quaternary sediments are between 270 m (i.e., SGJ, at the eastern side) and 1550 m (i.e., WNE, on the Western Coastal Plain) in the Taichung area.  
To clarify the variations of V\({}_{\text{S}}\) structures in the area, we sketched 2D and 3D maps based on the inversion results. According to the V\({}_{\text{S}}\)-contour maps at different depths between 50 m and 1800 m, the lowest S-wave velocities were observed at the Western Coastal Plain, whereas the highest values appeared on the eastern side. The S-wave velocities gradually increased from the west to the east. According to the depth-contour map (Vs = 1500 m/s), the depths of the bedrock range from 250 m (e.g., FCE and FNJ, at the eastern part) to 1550 m (e.g., WNE and HCJ, at the western part). The thickness of the alluvium gradually increases from the east to the west. These results are consistent with the known geology of the Taichung area. These shallow S-wave velocity structures (0-2000 m) should be useful in future regional site-effect studies and ground-motion simulations or predictions.  
Conceptualization, H.-C.H.; measurement, H.-C.H., T.-H.S, C.-T.H. and C.-F.W.; data processing and analysis, T.-H.S. and C.-T.H.; validation, H.-C.H. and C.-F.W.; writing--original draft, H.-C.H.; project administration, H.-C.H.; writing and editing draft, H.-C.H. All authors have read and agreed to the published version of the manuscript. This research was supported by the Ministry of Science and Technology, Taiwan (MOST 104-2116-M-194-008). Not applicable. Not applicable. Not applicable. Not applicable. Not applicable. We thank the Engineering Seismology Laboratory, Department of Earth and Environmental Sciences, National Chung Cheng University, Taiwan, which conducted microtremor array measurements in the Taichung area. We also appreciate two anonymous reviewers for their valuable comments to improve the article. This research was supported by the Ministry of Science and Technology, Taiwan (MOST 104-2116-M-194-008). The authors declare no conflict of interest.