Earthquake geology of the active Shanchiao Fault in the Taipei Metropolis, Taiwan
Date Issued
2012
Date
2012
Author(s)
Chen, Chih-Tung
Abstract
The Taipei Metropolis, home to some 10 million people, is subject to seismic hazard from not only ground shaking in thick alluvial deposits due to distant faults or sources scattered throughout the Taiwan region, but also active faulting directly underneath. Northern Taiwan including the Taipei region is currently affected by post-orogenic (Plio-Pleistocene arc-continent collision) processes related to backarc extension of the Ryukyu subduction system. The Shanchiao Fault, an active normal fault outcropping along the western boundary of the Taipei Basin and dipping to the east, is investigated here for the areal extent and magnitude of its recent activity. Based on growth faulting analysis in the Wuku profile in the central portion of the fault, the Shanchiao Fault is found to be incessantly active since about 23 ka with an averaged tectonic subsidence rate about 3 mm/yr. A geologic profile across the north-central portion of the fault zone in the Luzhou area reveals similar main-branch fault half-negative flower structural pattern and slip rates observed in the Wuku profile, a phenomenon we interprete to originate from the geometry of the basin basement and the strong rheological contrast between unconsolidated basin sediments and basement rocks. One key horizon within the growth sediments – the top of the Jingmei Formation which was an alluvial fan formed rapidly when a major drainage reorganization occurred during the Last Glacial Maximum – is noted to serve as the marker of tectonic subsidence since its inception around 23 ka. A determination and compilation of the depths of the Jingmei Formation top horizon from nearly 500 borehole records within the Taipei Basin demonstrates that the hanging-wall of the Shanchiao Fault is deformed in a roll-over fashion with up to three branch faults sub-parallel to the main fault in the several-hundred-meter wide fault zone, and the offset is largest in the Wuku-Luzhou area in the central portion of the fault and decreases toward the southern tip of the fault. Along traces of the branch faults subtle fault-related geomorphic scarps can be mapped which exhibit a right-stepping en-echelon pattern, indicating recent sinistral transtensional faulting. Contemporary tectonic subsidence revealed by leveling data across the Taipei Basin during 1975 to 2003 was concentrated again in the Wuku-Luzhou near-fault hanging-wall area, probably representing an interseismic fault behavior. An attempt to resolve the poorly-known subsurface geometry of the Shanchiao Fault is carried out by simple elastic dislocation modeling of the surface deformation recorded by the Jingmei Formation top horizon compilation, which is representative of the latest Quaternary period as it spans probably more than 10 earthquake cycles. Preliminary results suggest that the Shanchiao Fault possesses shallow listric geometry where the low-dipping part may be inherited from the negative tectonic inversion of former thrusts, while deeper rift-related normal faults is also likely to be reactivated. Such constraints and knowledge are crucial in earthquake hazard evaluation and mitigation in the Taipei Metropolis, and in understanding the kinematics of transtensional tectonics in northern Taiwan.
Subjects
Taipei Metropolis
active fault
the Shanchiao Fault
growth fault analysis
leveling analysis
fault geometry
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-101-F94224120-1.pdf
Size
23.54 KB
Format
Adobe PDF
Checksum
(MD5):90f1e8607752c14b2dbe827dcb500646