Characterization of the groundwater response to rainfall on a hillslope with fractured bedrock by creep deformation and its implication for the generation of deep-seated landslides on Mt. Wanitsuka, Kyushu Island

In this study, the hydrogeological response of a hillslope affected by gravitational deformation was analyzed based on the data obtained for two observation boreholes: a 10-m borehole representing a relatively superficial section of the hillslope dominated by soil and old landslide deposits and a 40-m borehole representing the bedrock aquifer. For this analysis, the precipitation events in the area, groundwater levels of the two boreholes, electrical conductivity and isotopic concentration (oxygen and hydrogen) of the groundwater in the boreholes and rainfall samples were measured. Derived from the rainfall data, the antecedent precipitation index (API) with a 6-h half-life offers a good correlation with the peaks in the groundwater levels in the bedrock aquifer. The characteristics of the groundwater response suggest the existence of a single structure in the bedrock that controls the response of the hillslope. The structure serves as a conduit, which rapidly drives the rainfall water (recharge) into the bedrock. Evidently, this process is unrelated to the superficial section represented by the observations in the 10-m borehole. The structure is associated with an area of high hydraulic conductivity in the bedrock caused by the gravitational deformation in the hillslope. The strong control of this structure in the hillslopeʹs hydrogeological response makes it responsible for the hillslopeʹs stability during high-precipitation events. This information is highly relevant to areas featuring the generation of several deep-seated landslides under heavy-rain conditions.