Unraveling the Dynamics of a Creeping Slope in Northwestern Colombia: Hydrological Variables, and Geoelectrical and Seismic Signatures

Mass movements are quite common in the Northern Andes and constitute one of the major hazards in the region. In particular, along valley flanks where the city of Medellin (Colombia) is located, rainfall is the main trigger of these phenomena, but little is understood about how water in the soil and subsoil behaves. In this study, we show data from some basic soil hydrology measurements and conventional geophysical surveys within a ~4 ha experimental plot that is experiencing soil creep. The seasonally wet study site has an average slope gradient of 33%, and its surface geology consists of a series of older deposits of debris flows. Our measurements show a low surface runoff, which ranges from 4 to 11% of the rainfall; infiltration is 89–96% of the rainfall, and 15–33% corresponds to drainage water at shallow levels in the soil (20–50 cm); piezometric measurements reveal a mostly steady-state water table. About 14–54% of the rainfall becomes subsurface flow within the first ~1–2 m below the surface. Geoelectrical and seismic surveys suggest small temporal changes in the properties of materials shallower than 2 m, consistent with the steady-state water table and the permanent and high subsurface flow. These geophysical surveys also indicate the presence of a major discontinuity at ~4–6 m below the surface, which we interpret as the limit between different prior debris flows