Magmatic and seismic activities are typically monitored using geophysical techniques, although geochemical analyses of groundwater can also be employed. Given their inert nature and high diffusivity, helium isotopes in volcanic gases, hydrothermal fluids, and groundwaters have been used to investigate magmatic and seismic activities. Helium isotopic ratios (3He/4He) are known to change before and after volcanic eruptions and large earthquakes. The migration and flux of crustal fluids are also important in this regard, because fluids are considered to play a major role in triggering earthquakes by changing fault strength through variations in pore pressure. In this contribution, the current state of helium-based monitoring of magmatic and seismic activities is reviewed; how to estimate the flux of helium and other volatile species is described; and, a method for estimating water flux from groundwater isotopic data is provided.
