After the 2011 Off the Pacific Coast of Tohoku Earthquake on 11 March, 2011, volcanic activity in Japan was observed to increase. In this study, we use the PIXE method to analyze elemental concentrations in stream water flowing directly from the fumarolic area on Mount Hakone, which erupted on 30 June 2015. Throughout a two-year monitoring period following the eruption, concentrations of 12 dissolved elements were measured. We found that the stream water was strongly acidic. For one year after the eruption, the concentrations of magma-derived elements (S, Cl) and rock-derived elements (Al, Ca, Fe, K, Mg, Mn, and Na), except for Si, showed a clear decreasing trend. These measurements suggest that magmatic activity and fumarolic activity reduced gradually over one year. After this year-long period, elemental concentrations stopped decreasing and instead showed broadly flat levels with sporadic increases and decreases. However, throughout the year immediately following the eruption, the number of volcanic earthquakes was consistently at zero or close to zero. These different indications of volcanic activity suggest that the elemental concentrations in stream water may be more sensitive indicators of small changes in subterranean behavior than of the number of volcanic earthquakes.
Point Process Models for Quasi-Periodic Volcanic Earthquakes