Exploring site characteristics in Tainan City, Taiwan, using shallow S-wave velocity structures

Abstract Shallow S-wave velocity (VS) structures in Tainan City, Taiwan, were characterized using microtremor array data for 18 sites compiled by Huang et al. (Terr Atmos Ocean Sci 35:20, 2024). Theoretical transfer functions were calculated at the surface relative to six depth formations (50, 100, 200, 400, 700, and 1,000 m) and five VS formations (350, 550, 750, 1,000, and 1,500 m/s) by using the Haskell method. Higher predominant frequencies were observed in the Tainan tableland (central part of the study area), whereas lower predominant frequencies were observed in the Anping plain (western part of the study area). If the S-wave velocity in the Tertiary basement is 1,500 m/s, the predominant frequencies of Quaternary sediments range from approximately 0.2 Hz (in the Anping plain) to 0.65 Hz (in the Tainan tableland), decreasing radially from the central to surrounding sites. We compared the horizontal-to-vertical (H/V) ratios of microtremors with the predominant frequencies calculated using theoretical transfer functions. Our findings suggest that site amplifications derived from H/V ratios are primarily attributable to alluvium from the surface to depths of 50–100 m at most sites. For sites in the Tainan tableland, amplifications derived from H/V ratios are primarily attributable to alluvium between the surface and the formation of VS = 550–750 m/s. For sites in the Anping plain and the Dawan lowland, amplifications are primarily attributable to alluvium between the surface and the formation of VS = 350–550 m/s.