[S-GD01] Geodetic Advances in Crustal Dynamics and Environmental Change for Taiwan and Neighboring Regions

[SGD01-02] Directional Resonance Variations Across the Highly Deformed Fault-bounded Mudstone Area in Southwestern Taiwan

*Ruey-Juin Rau¹, Cheng-Feng Wu¹, Ting-Yu Lin¹ (1.National Cheng Kung University)

Keywords:ChungLiao tunnel, microtremor, horizontal-to-vertical spectral ratio, time-frequency polarization analysis

The ChungLiao tunnel is perpendicularly located across the NE-SW-trending fault-bounded, with the Chekualin Fault on the west and the Chishan Fault on the east in the mudstone area in southwestern Taiwan. The fault-bounded area is characterized by about 50 mm/yr fault-perpendicular WNW-striking horizontal motion and 20-80 mm/yr vertical uplift in the overall NW-SE tectonic convergence regime based on GNSS and precise leveling measurements. We investigate the seismic response of the area by using microtremor horizontal-to-vertical spectral ratios (MHVR) and time-frequency polarization analysis on three fault-perpendicular profiles to reveal the seismic characteristics of the highly deformed fault-bounded mudstone area. Each station is equipped with a DATA-CUBE³ recorder and an HG-6 geophone, and each profile contains 20 stations within a distance of less than 1000 meters. The MHVSR profile indicates notable subsurface structural discontinuities influenced by the Chekualin and Chishan Faults, exhibiting steep dip angles corresponding to the region's established high-angle stratigraphic features. Additionally, there is no apparent amplification ratio between the two faults, suggesting a lack of pronounced velocity layering in the underlying strata. This implies that the strata within the fault zones have experienced significant fracturing due to fault activity, leading to a highly disrupted geological structure. This interpretation is consistent with Eikonal Tomography findings, which reveal a distinct low-velocity zone between the two faults. Polarization results show that near the fault, the direction is at a high angle or perpendicular to the strike (NE-SW) of the faults. In contrast, the polarization direction within the two faults is mainly parallel to the fault strike, matching characteristics of fault-trapped waves in low-velocity zones. The study found that microtremor variations do not completely align with inferred fault locations, indicating a more complex subsurface structure or a need for fault location refinement.