Estimation of S-wave velocity structures of Tainan area, Taiwan, using array records of microtremors

In this study, the shallow S-wave velocity structures beneath the Tainan area, Taiwan are investigated using microtremor array measurements at 18 sites. An F-K spectrum method was used to calculate the dispersion curves for these sites, and surface-wave inversion was used to estimate the S-wave velocity structures beneath the Tainan area. The results indicated that, at frequencies between 0.2 and 1.1 Hz, propagation is concentrated between the northwest and south quadrants, corresponding to the coast of the Taiwan Strait. CHM, a hard central site on the Tainan tableland, was found to have higher phase velocities than the soft sites do, such as site SIM, on the Anping Plain, particularly at frequencies of 0.3–5.0 Hz. Given that the tertiary basement exhibits an S-wave velocity of 1500 m/s, the depth of the Quaternary alluvium in the Tainan area ranges from 400 m at site CHM to 1350 m at site FUP on the Anping Plain. In addition, the depth of the alluvium gradually increases radially from the central CHM site (on the Tainan tableland) to the surrounding areas. These results are consistent with the geolpogy of the Tainan area. Because our shallow S-wave velocity structures are comparable to those proposed in three other studies, our inversion results are concluded to be reasonable and valid. According to the horizontal/vertical (H/V) ratios of our microtremor array data from 18 sites, the predominant frequencies range from 1.0 to 3.8 Hz. Compared with the eastern part (i.e., Dawan lowland), the central part (i.e., Tainan tableland) has higher predominant frequencies.