Revisiting Rapid Surface Deformation in Southwestern Taiwan Using GNSS and ALOS-2 InSAR Data: Case study in Chungliao Tunnel

Under the assumption that the plate convergence rate is distributed across faults along the plate boundary, in the Chungliao Tunnel area of southwest Taiwan, the total surface velocity change between the Chegualin fault (CGLF) to the west and the Chishan fault (CSNF) to the east exceeds 90 mm/yr, which is larger than the palte convergence rate of approximately 82 mm/yr in Taiwan. However, the physical processes driving these high-rate deformation is still debated. As the deformation is mainly aseismic, and to increase the spatial resolution of the large-scale surface deformation field, we used GNSS and ALOS-2 InSAR to understand tectonic processes. To examine the spatial continuity of the ultra-rapid deformation beyond the Chungliao Tunnel, InSAR processing was conducted using ALOS-2 ascending and descending datasets to improve the spatial extension and resolution of surface deformation. We introduced a priori phase discontinuity at mapped fault trace by setting the temporal coherence to correct the unwrapping errors. Then several Line-Of-Sight (LOS) velocity discontinuities are consistent with fault traces, indicating shallow creep along those faults. Furthermore, we demonstrated the continuity of few-hundred meters of high deformation between the CGLF and the CSNF with LOS velocity of 30-40 mm/yr, a LOS velocity gradient of 20-30 mm/yr across two faults. A 3D velocity reconstruction inverted by combining GNSS and ALOS-2 InSAR result reveals a local counter-clockwise rotation from NW to SW align north to south and the significant uplift (~80 mm/yr) in the narrow band between the Chishan fault and Chegualin fault near the Chungliao Tunnel. The local deformation implies the opposite lateral components of CSNF and CGLF in different segments of two faults as well, providing precise constraints to enhance the tectonic interpretation of this area. This rapid deformation identified in the narrow zone may be resulting from the interaction between the thrust faults and the surrounding mobile shale, in agreement with the hypothesis of a mud diapir of large mud diatreme that developed in the thick two thrusts.