Analysis of hydrological and hydrogeological deep-seated landslides generation mechanisms in Mt Wanitsuka, Kyushu Island, Japan

In September 6^th of 2005 Kyushu Island, west of Japan, was hit by the typhoon No 14 causing about 124 landslides events in Miyazaki prefecture due to heavy rain conditions (Tanigushi, 2008). 30 deep-seated landslides were observed in Mt Wanitsuka (1,118 masl) most of them generated debris flows, expanding the damaged area. From that catastrophic event only few studies about the landslide generation mechanisms were carried out in that area. However, the hydrological conditions which generate and triggered deep-seated landslides are not well known, fact that motivates this research. For that purpose, the runoff of three small catchments (D, M and U) located in Mt Wanitsuka, next to a deep-seated landslide scarp of 2005, were controlled. Also the rainfall, groundwater levels (10m and 40m depth boreholes) and the O18/Deuterium concentration of all water sources were controlled. The geology in the study area is mainly shale interbedded sandstone partially fractured, part of the sedimentary complexes of the Shimanto Terrane (Late Early Oligocene - Early Miocene). The groundwater levels observed in the 40m borehole reflects a complex behavior of the groundwater flow. Those levels show a fast elevation associated to low intensity and relatively long duration precipitations. The relationship between the height of groundwater level peaks and the antecedent precipitation index (API, 6 hrs half-life) is discontinuous. For API₆ over 50mm, the height values define a gap of about 3 m with no peaks. Over that gap the height peaks values mark a maximum (about 11.5 m below surface) which remains stable even if the precipitations continue or increase their intensity. Also, under moderated high rainfall conditions it was observed springs flowing out near to the footslope, downhill to the boreholes position and next to the landslide scarp. The O18 isotopic signature of the spring water is correlated to the 40m borehole groundwater. In other hand, the hydrograph analysis and hydrograph separation shows different response in one of the controlled catchments (catchment D) with a strong influence of groundwater in the runoff. At the head of catchment D, also it was possible to identify extensional features in the soil cover, evidences of creep deformation. According to Akther et. al. (2011) those structures could be associated with a potential area of deep-seated landslide generation. The evidence presented suggests the existence of a structure in the bedrock which control de groundwater flow. That structure, associated with the creep deformation, acts as a conduct of meteoric water helping its infiltration into the bedrock and conditioning the groundwater flow, gathering groundwater from the different high conductivity layers, as shale layers, in the bedrock. Under relatively high intensity and long duration rainfall, as observed in 2005 (peak of intensity: 45 mm/h. accumulated rainfall in 70 hrs: 900 mm), the structure with high storability could define a potential slip surface to generate deep-seated landslides.