Large earthquakes sometimes trigger local seismicity that is distal to their rupture zones. Various mechanisms for this triggered seismicity have been proposed, based on either the static stress change or ground shaking from seismic waves, but local geological structure is rarely studied to discern why this seismicity is remotely induced. We present the results of a joint three-dimensional resistivity and isotopic analysis of the groundwater system surrounding Mount Fuji, Japan, where increased seismicity was observed following the A.D. 2011 Tohoku-Oki megathrust earthquake. An electrically conductive zone and high concentrations of magmatic gases (He and CO2) correspond to the zone of triggered seismicity. In contrast, a contribution of magmatic water is not suggested from 2H (deuterium, D) and 18O isotope ratios. These results suggest that the earthquakes were triggered within a fractured zone through which magmatic gases preferentially migrated. We hypothesize that the upwelling of gas-rich hydrous fluids and/or gas bubbles occurred along this fracture pathway, causing an increase in the pore pressure and triggering the resultant earthquake sequence.