Land Subsidence-Induced Horizontal Displacement Along the High-Speed Rail in Central Taiwan: An Integrated Multi-Temporal InSAR, GNSS, and Leveling Approach

Land subsidence driven by excessive groundwater extraction in the Choushui River alluvial fan of central Taiwan poses a significant threat to the structural integrity of the Taiwan High-Speed Rail (THSR). This study presents an integrated approach combining multi-temporal Interferometric Synthetic Aperture Radar (MT-InSAR), continuous and campaign Global Navigation Satellite System (GNSS) measurements, and precise leveling surveys to characterize both vertical and horizontal surface displacements along the THSR corridor. Sentinel-1 C-band SAR data from ascending (A69) and descending (D105) tracks were processed using the Small Baseline Subset (SBAS) technique over the period of 2015–2021 and decomposed into east–west (EW) and vertical components via 2.5D decomposition. The InSAR-derived EW velocity field was calibrated using GNSS Ordinary Kriging interpolation, improving R2 from 0.147 (RMSE = 4.24 mm/yr) to 0.992 (RMSE = 0.24 mm/yr). The vertical velocity field was corrected using a polynomial trend surface fitted to 922 leveling benchmarks and 38 continuous GNSS stations, reducing the RMSE from 6.03 to 4.85 mm/yr (increasing R2 from 0.889 to 0.898) and virtually eliminating the systematic bias (a decrease from +3.47 to −0.47 mm/yr). Maximum subsidence exceeding 60 mm/yr was identified in the Yunlin Tuku area, while three secondary subsidence centers were found in Changhua. The horizontal velocity field revealed a convergent pattern directed toward subsidence centers, with magnitudes of 2–10 mm/yr, confirming that aquifer compaction induces significant lateral deformation. Along the THSR corridor, differential EW velocities across the Xizhou and Tuku subsidence zones highlight potential risks to rail alignment and structural safety, with horizontal strain rates reaching approximately 10−6/yr. GNSS observations additionally provide the north–south velocity component that InSAR cannot detect, enabling a more complete three-dimensional deformation characterization.