Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics

The imaging of hydrothermal systems within volcanoes is critical in evaluating the nature and likelihood of future volcanic activity and hazard assessment. In this study, we present a conceptual model of the hydrothermal system in a volcanic edifice, as deduced from the relationship between electric self-potential (SP) and high-resolution resistivity structures. In order to develop a comprehensive model of water flow in volcanoes, we conducted the audiofrequency (10,000-0.3 Hz) magnetotelluric surveys in five large stratovolcanoes (Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane) in Japan and found that the obtained 2-D resistivity profiles have a close relationship to the previously reported SP data: good extensive conductors occur beneath areas without SP anomalies, whereas good localized conductors only occur beneath large spatial wavelength SP anomalies on the volcano side of the SP minimum. Also taking into account the locations of surface geothermal activity, the good conductors roughly correspond to the hydrothermal zone, whose upper limit is sealed by a low-permeability clay layer. The sealing layer separates an upper groundwater flow from a lower hydrothermal flow in the subsurface and controls the geothermal manifestations and river locations on the surface. We confirmed the feasibility of the proposed model based on numerical simulations of a hydrothermal system. The horizontal extent of the hydrothermal zone is highly heterogeneous even in a volcanic edifice. This heterogeneity can reflect the geological age of flanks that may be related to the occurrence of a previous large sector collapse.