|dc.description.abstract||The Chihshang Fault is one of the most active creeping faults in the world at a rate of 2 cm/yr, which is situated along a plate suture between the Philippine Sea and the Eurasian plates in eastern Taiwan. Near the surface, the Chihshang Fault developed in the Holocene unconsolidated gravel layers. This fault behaviour apparently is be influenced by the hydraulic characteristics around the fault zone and in the vicinity of aquifers.
In this study, we combined a variety of measurements and analyses at the Chihshang Active Fault Observatory (CAFO), including surface-rupture mapping, three shallow borehole core analyses and kinematic analysis of geodetic measurements, in order to decipher the near-surface fault geometry. We found that the Chihshang Fault has a three-branch fault system with a rather diffused fault zone in the Chinyuan alluvial fan at CAFO, which is composed of at least 100 m thick alluvial deposits. Outside of the Chinyuan River channel, the Chihshang Fault exhibits a single fault system. Combining the uplift rate and subsurface profiles from trench excavation, we interpret that the three fault branches locally developed the structure at the uppermost 30-40 m unconsolidated gravel layers during the last few thousand years. Based on the ratio between the levelling vertical displacements and the creep meters and GPS horizontal displacements, we obtained dip angles of 34-42°, 60-65° and 16° for two west-vergent thrusts and an east-vergent backthrust, respectively, for these three branches. By compiling the ages data in the boreholes, trenches and terraces, we estimated a long-term relative uplift rate of 2.3 ± 0.1 cm/yr in the hanging wall of the Chihshang Fault and an average alluvial sedimentation rate of about 1.1 ± 0.1 cm/yr during the past a few thousands years.
In order to better understand how the effects of pore-fluid pressure variations in the aquifer within the alluvial gravels influences the near-surface behavior of the Chihshang Fault, nine observation wells of groundwater were drilled at depths ranging from 30 to 100 m through the aquifer from the footwall to the hanging wall. Monitoring of natural pore pressure variations in piezometers, monthly slug experiments (few seconds), and long duration pumping/injection experiments (hours to days) were carried out during 2007-2011. Together with the subsurface electrical resistivity imaging, surface fracture investigations, and core geological analysis, we identified an aquifer zone that is deformed and fractured by the fault zone. The results showed that the permeability of the fault zone is smaller 1 order than that of the footwall. The variance of permeability caused a 10 meter step of groundwater level from the hanging wall to the footwall in the view of spatial domain. On the other hand, repeated hydraulic tests revealed that the permeability varied with time increased 20 times in the hanging wall from 2007 to 2011. A drastic increase of the permeability in the fault zone was observed from April to September 2008. Two possibilities are interpreted this phenomenon: (1) the increased cumulated earthquake events changed the stress field along the Chihshang Fault and caused the new fractures around the fault zone; (2) the vertical displacement revealed that the dilatation may be happened in the fault zone which increased the porosity to induce a drastic increase of permeability in-situ.