Microtremor analysis in the central Longitudinal Valley, eastern Taiwan

Eight days after the 2022 M_W 7.0 Chihshang earthquake, we installed 58 short-period seismic stations in Yuli, Longitudinal Valley for a month-long measurement, and 72 stations around the earthquake rupture zones for short-term measurements. The region covers approximately 35 square kilometers, and encompasses the structures from west to east: Central Range Fault, Yuli Fault rupture zone and Chihshang Fault, a segment of the Longitudinal Valley Fault. An alluvial plain covers the topographic lows from the Central Range to Longitudinal Valley while terraces surround the river west of the Central Range Fault. We used horizontal-to-vertical spectral ratio (HVSR) analysis within the frequency range of 0.2 to 20 Hz for data processing, the region can be classified into six major categories from west to east: Category A, located in Central Range; Category B, at the terraces; Category C, situated in the plain west of the Yuli fault rupture zone; Category D, located proximal to the Yuli Fault rupture zone; Category E, located in the plain east of Yuli fault; and Category F, found in the plain east of Chihshang Fault and in the Coastal Range. Our results show that B, C and F have peak amplitudes of around 4.5 within dominant frequency (f₀) range of 1.5-2.5 Hz, 2-3 within f₀ range of 0.7-1.5 Hz and 3-5.5 within f₀ range of 5-13 Hz, respectively. The locations of Categories B and C align with the position of Central Range fault line. Category F, located along the western boundary along the Chihshang Fault separated from the other five categories and exhibits a noticeable f₀. On the other hand, A, D, and E exhibit less pronounced dominant frequencies. Category D is positioned at the center of the alluvial layer and is distributed around the rupture zone of Yuli Fault, showing high similarity in HVSR curve across numerous stations, with unclear dominant frequencies. As a result, the HVSR results in the central Longitudinal Valley are mostly related to the tectonics and topographic units in this region.