Permeability-control on volcanic hydrothermal system : case study for Mt. Tokachidake, Japan, based on numerical simulation and field observation

摘要

We investigate a volcanic hydrothermal system by using numerical simulation with three key observables as reference:the magnetic total field, vent temperature, and heat flux. We model the shallow hydrothermal system ofMt. Tokachidake, central Hokkaido, Japan, as a case study. At this volcano, continuous demagnetization has beenobserved since at least 2008, suggesting heat accumulation beneath the active crater area. The surficial thermal manifestationhas been waning since 2000. We perform numerical simulations of heat and mass flow within a modelededifice at various conditions and calculate associated magnetic total field changes due to the thermomagnetic effect.We focus on the system’s response for up to a decade after permeability is reduced at a certain depth in the modeledconduit. Our numerical simulations reveal that (1) conduit obstruction (i.e., permeability reduction in the conduit)tends to bring about a decrease in vent temperature and heat flux, as well as heat accumulation below the level ofthe obstruction, (2) the recorded changes cannot be consistently explained by changing heat supply from depth,and (3) caprock structure plays a key role in controlling the location of heating and pressurization. Although conduitobstruction may be caused by either physical or chemical processes in general, the latter seems more likely in thecase of Mt. Tokachidake.