| dc.description.abstract | Taiwan is located at a convergent boundary where the Philippine Sea Plate is moving northeastward to the Eurasian Plate at the rate of 82 mm/year. About half of the convergence is accommodated in southwestern Taiwan through folding and thrusting, as well as tectonic extrusion. Ground-based geodetic observations revealed a remarkable deformation pattern within the Western Foothills of southern Taiwan. There, two sub-parallel southeast-dipping thrusts are separated by only 500 m. The Chegualin fault cuts through the Plio-Pleistocene Gutingkeng mudstone, characterized by a scenic low-relief badland landscape. To the east, the Chishan fault has brought Late Miocene sandstone on top of the Gutingkeng mudstone: the hanging wall consists of a significant ridge topography associated with vegetated and locally unstable slopes. According to GNSS, levelling and traverse measurements along a freeway and tunnel crossing both thrusts, there is limited net shortening across the two major structures at a rate of 15 mm/year, but 50 mm/year of compression across the Chegualin fault and 32 mm/year of extension across the Chishan fault. In-between the faults, uplift increases southeastward from 33 to 100 mm/year relative to Penghu, but drops to 20 mm/year east of the Chishan fault. Sharp deformation gradients indicate aseismic slip on both structures. Such significant aseismic ground deformation has raised the question of the driving mechanism.
To investigate this phenomenon, this study monitors ground-surface deformation using the image correlation technique to quantify horizontal displacements and using DSM (Digital Surface Model) time series to access vertical displacements. This approach aptly complements the existing ground-based transect and regional observations from GNSS and InSAR: it would provide high resolution observations covering a larger area along the strike of the geological structures. We used 8 sets of 8 aerial images acquired from 2008 to 2015 covering a 3 x 3 km area, centered on the freeway and ground-based geodetic network. Images were processed with Micmac, a free open-source photogrammetric software suite.
The estimated horizontal displacements along the freeway closely match ground-based observations. Across the Chegualin fault, we observe during 2008-2015 cumulated horizontal compression along the fault-perpendicular direction ranging from 17 to 44 cm, 13 to 42 cm of right-lateral offset south of the freeway, and slightly left-lateral offset north of the freeway. The compression extends at least 2000 meters along the Chegualin fault and a major branch fault. However, it remains challenging to precisely quantify and locate the displacement discontinuity across the Chishan fault due to poor correlation caused by vegetation. Our results also reveal a previously unknown rotation pattern in-between the two faults, with a clockwise rotation north
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of the freeway and a counter-clockwise rotation south of it. Preliminary results on DSM time series also support the previously observed uplift pattern.
Field geological surveys reveal several fault zones with kinematic indicators on the outcrop scale. Along the Chegualin fault, we manage to trace the main shear zones with reverse slip sense for at least 2 km, and geological evidence is consistent with the aerial image correlation results in terms of fault kinematics. Along the Chishan fault, we found steep SE-dipping en echelon pattern normal faults with a right-lateral component up to 1.4 km northwest of the freeway, and these dextral normal faults also present changes in rake, according to the slickenlines on the shear planes.
To conclude on the aerial image correlation and geological survey results, we provide a map of active structures and a 3D ground deformation model with a range of 2.5 x 2.5 km. We propose a mud-piercement-related half-doming system to explain the local ground deformation pattern. The center of the dome is located near the northwestern end of the Zhongliao tunnel and the northwest side of the dome is bounded by the pre-existing Chegualin fault. Further explaining the mechanism may require more geophysical or geochronological research. | en_US |