The Taipei Metropolis area, the political and economical capital of Taiwan with several million inhabitants and my hometown as well, situates in a complex geologic environment with numerous imposing geological hazard (Chan et al., 2010). One recognized but poorly understood, yet probably the most devastating kind of hazard is the potential earthquakes generated by the Shanchiao Fault (e.g. Shyu et al., 2005).
Through decades of devoted research this active normal fault is known to govern the development the Taipei Basin and is responsible for accommodating the extension in the northern tip of the island during post-collisional collapse of the Taiwan Orogen (Wang Lee et al., 1978; Teng et al., 2001; Rau et al., 2008). The details and recent activities of the Shanchiao Fault, in contrast, remain elusive owing to the lack of surface outcrops and related seismicity. Series of deep boreholes penetrating the entire basin deposits to reach the basement carried out by the Central Geological Survey shed light on the possible locations of the surface fault trace (Lin, 2001) and allowed ventures to investigate its paleoseismic history (Huang et al., 2007). Despite the efforts mentioned above along with other geodetic (e.g. Yu et al., 1999a), geophysical (e.g. Wang and Sun, 1999) and geochemical (e.g. Walia et al., 2005) inquiries, key parameters of this active fault including the exact location of the surface trace, geomorphic fingerprint of faulting, fault geometry in the near surface to the upper crust, and rate of deformation of contemporary to long-term time scales, went unconstrained or debated, although they bear critical clues to the seismic hazard evaluation and mitigation as well as the understanding of post-orogenic tectonics in northern Taiwan. A multi-discipline approach is therefore presented in this thesis aiming at characterizing the Shanchiao Fault, based mainly upon the extreme wealth of leveling and borehole records across the Taipei Basin thanks to the plentiful previous works and the efforts by the Central Geological Survey in collecting and making public the precious data.
The itinerary of the work generally follows the present-to-past and surface-to-deep tracks in unraveling the properties the fault. Leveling data of past four decades is first examined for signs present-day fault activities, and growth sediment accumulation from borehole correlations yield millennial to late-Quaternary fault vertical offset rates. Fault-related scarps are found to represent only part of the complex fault zone at shallow underground from borehole synthesis, which denotes a half-tulip structure inherited from the transtensional nature of fault movement. Growth faulting analysis revealed an intimate coupling between sea level rise and normal faulting since the Last Glacial Maximum (LGM) in the fault zone along much of the Shanchiao Fault, which permits fault slip rate estimations noted previously and identification of a key
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tectonic subsidence marker, the Jingmei Formation top horizon. The horizon depth distribution across the basin is converted to estimates of post-LGM vertical fault offset, which demonstrate the extent and magnitude of fault-induced deformation in the Taipei Basin. Finally the tectonic subsidence variation deduced from horizon depth normal to the fault strike is modeled to resolve the upper crustal geometry of the Shanchiao Fault. These results are presented in four chapters; each of the chapters is an independent manuscript published (information listed in the Appendix section) or under preparation for submission. Briefing of these chapters is outlined below:
Chapter 2: Leveling across the Taipei Basin during 1975 to 2003 is analyzed to
illuminate the modern vertical ground motion. Initially intended to elucidate and quantify contemporary tectonic subsidence rate, the subsidence and uplift of basin floor observed during the period was dominated by aquifer deformation as residual compaction caused by preceding severe groundwater pumping and elastic rebound due to pore-pressure recovery in natural recharge. Other than the intermediate depth (about 50 to 100 m deep) aquifer component, shallow soil compaction of the most recent sediments contributed significant subsidence in several areas and an attempt was made to quantify this natural subsidence. After tentative removing of soil compaction and discussion on extent of aquifer elastic rebound, enhanced subsidence was still found along the western margin of the Taipei Basin in the near-fault hanging-wall region, which serves to be a solid proof of ongoing modern fault creep although the exact amount of slip rate is not obtainable.Chapter 3: Basin sediments of three well-documented boreholes (SCF-1, SCF-2 and
WK-1) across the central Shanchiao Fault zone in Wuku area is analyzed in the light of growth faulting. The stacking pattern of sediments in the fault zone is controlled by the sea level change and modified by normal faulting. During the LGM the basin was in an eroding environment until the captured Tahan River laid down a massive alluvial fan across the basin as the Jingmei Formation which capped the entire fault zone. The hanging wall and extensional fault block within the fault zone were subsequently down thrown by the fault, and with the rising sea level growth sediments were first accumulated on the hanging wall and progressively onlapped to the extensional fault block and then the footwall. The latest Sungshan Formation as the sediments covering the Jingmei Formation contains horizons downward-displaced towards east, providing estimations on regional sedimentation rates on the footwall (SCF-1) and additional tectonic subsidence rates on the extensional fault block (SCF-2) and the hanging wall (WK-1) since 8.4 ka to present and 9 to 8.4 ka. From the offset pattern of the horizons in the Sungshan Formation and the top of the Jingmei Formation, the fault located between SCF-1 and 2 is a branch fault while the majority of slip occurred on the main3
fault between SCF-2 and WK-1, together they consist of a half-tulip structure.
Chapter 4: This chapter intends to summarize the available surface and shallow
underground data to give a thorough picture of the Shanchiao Fault zone and the deformation it exerted across the Taipei Basin. Fault zone topographic analysis is first presented delineating series of right-stepping en-echelon vague scarps. In examining fault zone structure a comparable but more detailed half tulip structure is revealed in the Luzhou Profile northwards of the Wuku Profile with similar growth strata stacking fashion, suggesting this kind of fault zone configuration is operating along its entire length and is probably the result of basement geometry and the strong rheological contrast between the basement rock and the loose basin sediments. The Shanchiao Fault is therefore better mapped as a several hundred-meter wide fault zone instead of a single fault line. The Jingmei Formation top horizon in the base of growth sediments deposited during the recent eustatic rise serves to be a clear and widespread tectonic subsidence marker since the LGM ~ 23 ka, and a basin-wide compilation of its depth and the calculated vertical offset demonstrates that the hanging wall is deformed into a prominent roll-over monocline with tectonic subsidence decreases systematically towards the southern tip of the fault.Chapter 5: Deformation in the Shanchiao Fault hanging wall normal to the strike is
most likely to reflect its underground fault plane geometry, which was unknown due to lack of constraints. Based on the Jingmei Formation top horizon offset data compiled in Chapter 4, different geometric configurations are tested against the geological constraints using simple half-space elastic dislocation modeling. Drastic listric geometry at shallow depth is needed to produce the pronounced roll-over folding immediately east of the fault. At further depth a steeper fault patch is likely to take over the extensional detachment inverted from the syn-convergence Hsinchuang Thrust considering the fitting of model outcomes with tectonic subsidence in the eastern half of the basin, the existence of pre-orogen normal fault underneath the fold-thrust pile, the few but well-resolved steep-dipping normal faulting focal mechanisms, and the debated seismogenic ability of extensional detachment. The Shanchiao Fault is hereby proposed to possess a ramp-flat-ramp (double-ramp) geometry reactivating both syn-convergence thrust and pre-orogen rift normal fault, and the seismic hazard this geometry may imply is discussed.4