Characterizing ground response using HVSR analysis at hard sites: a case study from eastern Taiwan

This study examines ground responses at five stations in eastern Taiwan (EYUL, TTN020, ECS, EHD, and TTN023) where borehole data indicate hard geological materials. The horizontal-to-vertical spectral ratio (HVSR) method, commonly used to assess site characteristics in soft-soil areas, is applied. Ground motion data are obtained from microtremors, the 2016 Meinong earthquake (far-field), and the 2022 Kuanshan-Chihshang earthquake (near-field). Microtremor HVSR analysis reveals predominant ground frequencies of 2.35 Hz at EYUL, 2.84 Hz at EHD, and 0.63 Hz at ECS, which align with the frequencies identified using earthquake HVSR at these sites. However, both microtremor and earthquake HVSR analyses are less effective at TTN020 and TTN023: the microtremor HVSR curves lack clear peaks, and the earthquake HVSR peaks differ because of variations between near- and far-field seismic characteristics. These results indicate that microtremor and seismic motion data do not always correspond. Furthermore, HVSR analysis does not effectively capture seismic velocity pulses associated with near-field effects. Consequently, the HVSR method should be used cautiously at hard sites, and a comprehensive evaluation of spectral variations is necessary for effective seismic mitigation engineering.