The ascending and descending InSAR deformations derived from ALOS-2 and Sentinel-1 satellite SAR images and GPS displacements are used to estimate the fault model of the 2018 Mw 6.4 Hualien earthquake. The sinistral strike-slip fault dipping to the west with a high dip angle of 89.4° and a rake angle of 201.7° is considered as the seismogenic fault of this event. This seismogenic fault also triggered the ruptures of the Milun fault, which dips to the east with a dip angle of ~72°, and an unknown west-dipping fault with a dip angle of 85.2°. Two predicted faulting models indicate that the InSAR deformation fields include more postseismic slip than those of the GPS data. The north segment of the Milun fault and west-dipping fault have been triggered by the rupture of the seismogenic fault, but the postseismic slip occurred only in the south segment of the Milun fault. The InSAR-derived co-seismic and postseismic faulting model suggests that the significant slip concentrates at depths of 2.4–15.0 km of the main fault, 0.0–14.0 km of the Milun fault. Only minor slip is detected on the west-dipping fault. The maximum fault slip of ca. 2.1 m is located at the depth of ca. 2.4 km under the Meilun Tableland. The Coulomb failure stress (CFS) change calculated by the co-seismic and postseismic faulting model shows that there is a significant CFS increase in the east of the south segment of the Milun fault, but few of the aftershocks occur in this area, which indicates a high risk of future seismic hazard.

We found some obvious abnormal strain induced by heavy rainfall from borehole strainmeters deployed in Western Taiwan. The strain induced by rainfall can be divided into two parts, one is the quick response for extra loads of rainwater on the ground, and another one is the slow response for rainwater infiltrating into the strata. The quick and slow rainfall responses of areal strain data are analyzed using the technique of recursive digital filtering. Moreover, the rainfall impact functions of the studied stations are calculated using deconvolution. We found, in most cases, the response strain will reach the maximum in half an hour after heavy rainfall, and then show an exponential decay, it might persist more than 200 h depending on the hydrogeological condition around the station. Whereas the river flowing beside the station will help accelerating the runoff dispersion and reducing rainfall decay time in the hill or mountain region. We also compare the results after calibration in term of isotropic and vertical coupling individually. We found that the response strains are smaller in vertical coupling rather than isotropic coupling. The effects of debris avalanches caused by intensive rainfall in the mountain areas can be viewed as two types of rock deformation: generated only under the influence of rainfall and generated by the increased load in the river channels due to rainfall-induced landslides or debris flow. When the cumulative rainfall exceeds a certain threshold, the strain response curves show a noticeable anomaly likely due to the effects of the debris flow events in places prone to landslides.

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 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 CGPS stations, reducing RMSE from 6.03 to 4.85 mm/yr (R2 from 0.889 to 0.898) and virtually eliminating the systematic bias 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 exist in Changhua. The horizontal velocity field reveals 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.