Active fault zone as a barrier of hydraulic conduits in shallow aquifers: insights from on-site injection experiments at the Chihshang fault in eastern Taiwan

Hydraulic conductivity is the property to describe how water is moving through porous medium. It is related to the intrinsic permeability of the geological materials in two general groups: consolidated and unconsolidated materials. In general, groundwater is prone to flow through the porosity and fractures. In consolidated rocks, the flow channel is mostly defined by the fractures, such as fault zones, joints and etc. By contrast, the flow channel is defined by fractures as well as sediment porosity in the unconsolidated materials. In this study, we tested the hydraulic conductivity and its variations in the unconsolidated alluviums, which are cut through by thrusting of the Chihshang fault with a creeping rate of 2 cm/yr in eastern Taiwan.

We conducted injection tests with an array of six tiltmeters to measure the surface tilt during the experiments. The results of the hydraulic experiments exhibited a high heterogeneity of the aquifer hydro-mechanical responses and are in agreement with the existence of a lower permeable hanging wall and a higher permeable footwall. The surface tilt signals displayed a hydraulic gradient increasing toward the injection well particularly in the hanging wall. On other hand, no significant signal in the footwall, which means that the water flow slowly parallel through the fault zone and infiltrated quickly in the relative higher permeable footwall. We conclude that the fault plane, which dips at 40-50 degrees toward the hanging-wall side, plays a critical role as a barrier of water conduits.