Fig. 4

Ratio of ³He/⁴He observed by Horiguchi and Matsuda (2013), which is plotted on the estimated \(Q_{\text{P}}\) structure at 17 km depth shown in Fig. 2. Circles are color coded according to ³He/⁴He ratios in R_atm. Gray stars indicate the largest foreshock, mainshock, and induced event. The small circles are the epicenters of the aftershocks. The black triangles denote the active volcanoes