Indicators of the Activity Associated with Concealed Feeding Volcanic Fluids in the Tatun Volcano Group, Northern Taiwan

We analyzed 3,330 earthquake focal mechanisms and the fumarolic gases in the Tatun Volcano Group (TVG) during 2018–2021. Between June/2020 and June/2021, we found a concealed inflation beneath a depth of 2 km. We indicate this inflating mechanism was associated with the feeding volcanic fluids, which induced the past inflating cases in the TVG before 2018. We deliberated about the feeding features regarding this and the past cases and purpose three indicators to monitor such concealed activities, including the inflating indicator associated with the behaviors of earthquake faulting, heating indicator determined by the systematically high HCl/CO2 ratios, and discharging indicator displayed by the lasting high St/CO2 ratios. Using these indicators, we concluded that it was not rare during the last one decade that the concealed activities whose volcanic fluids were discharged occasionally.