Measuring rapid aseismic ground deformation within the foothills of southwestern Taiwan using aerial image correlation and DSM time series

In southwestern Taiwan, about 45-50 mm/yr of westward shortening occurs across the 40-45 km wide fold-and-thrust belt, accompanied with tectonic extrusion towards the southwest. Within this broad framework, measurements from a local ground-based geodetic network revealed rapid ground deformation surrounding two sub-parallel geological thrust faults, located only 500 m apart. 50 mm/yr of shortening occurs on the western fault and 32 mm/yr of extension across the eastern one. In-between the faults, uplift relative to the east block increases eastward from 20 to 80 mm/yr. Sharp deformation gradients indicate aseismic slip on both structures. This remarkable deformation raises the question of the deep structure and mechanism at play: Is it driven by tectonic forces, possibly released as transient slip events? Or does it involve shale tectonics related to fluid overpressure within the mudstone formation that dominates the geology?

To investigate this phenomenon, we monitored ground deformation using image correlation for horizontal displacements and DSM time series for vertical displacements, aiming at high-resolution observations covering a wider area than the ground-based network. Eight sets of aerial images acquired from 2008 to 2015 were processed using the MicMac photogrammetric software. The resulting horizontal velocities are in good agreement with ground-based observations. The compressional gradient across the western fault (the Chegualin Fault) vanishes northward, but remains clearly visible towards the south, with an increasing right-lateral component. While we detect extension across the eastern fault (the Chishan Fault), precise location and quantification of the deformation gradient remains challenging due to poor correlation caused by dense vegetation. Elevation differences based on the DSMs derived from aerial images have a similar spatial pattern as ground-based observations, but the amplitudes are overestimated. On-going refinement in the processing and time series based on LiDAR datasets are expected to improve the results.

This work was complemented by the field survey of the numerous bedrock shear zones in the area to build a structural map of active structures. We confirm the Chegualin Fault as an active thrust fault, with an oblique component along its southern part. Extension across the Chishan Thrust is accommodated by SE-dipping en-echelon normal faults, found up to 1.4 km north of the ground-based network. The change in rake of the slickenlines indicates an increasing right-lateral component northward. While the pattern of horizontal velocities may fit with the regional tectonics, the hypothesis of a shale piercement so far best explains the ratio between uplift and shortening. Achieving a better imaging of the vertical deformation would help further discussing this assumption and eventually propose a structural model consistent with local and regional observations, which will also allow further assessing the associated natural hazards.