  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 expel
devastating ejecta and create craters of several tens of meters. The management
of such hydrothermal hazards in populated areas 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 increase of
activity of a hydrothermal focus located 50 to 100 m below the summit of an
active volcano, the La Soufri\`ere of Guadeloupe, in the Lesser Antilles. The
present study deals with hydrothermal activity events at timescales of few
hours to few days. We show 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
various sensors including temperature probes, seismic antennas and cosmic muon
telescopes around such volcanoes could valuably contribute to early warning
decisions.
