Abrupt Changes of Hydrothermal Activity in a lava Dome Detected by Combined Seismic and Muon Monitoring

Abstract : The recent 2014 eruption of the Ontake volcano in Japan recalled that hydrothermal fields of moderately active volcanoes have an unpredictable and hazardous behavior that may endanger human beings. Steam blasts can generate devastating ejecta and create craters of several tens of meters. The management of such hydrothermal hazards in touristic places is problematic because of their very short time constants. At present no precursory signal is clearly identified as a potential warning of imminent danger. Here we show how the combination of seismic noise monitoring and muon density tomography allows to detect, with an unprecedented space and time resolution, the appearance of an active hydrothermal focus located 50 to 100 m below the summit of an active volcano, the La Soufrière of Guadeloupe, in the Lesser Antilles. We demonstrate in detail how the combination of those two methods improves the risk evaluation of short-term hazards and the localization of the involved volumes in the volcano. We anticipate that the deployment of networks of cosmic muon telescope around such volcanoes could valuably contribute to early warning decisions.