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

Abstract

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 of Mt. Tokachidake, central Hokkaido, Japan, as a case study. At this volcano, continuous demagnetization has been observed since at least 2008, suggesting heat accumulation beneath the active crater area. The surficial thermal manifestation has been waning since 2000. We perform numerical simulations of heat and mass flow within a modeled edifice 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 modeled conduit. 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 of the 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 conduit obstruction may be caused by either physical or chemical processes in general, the latter seems more likely in the case of Mt. Tokachidake.

Contention between supply of hydrothermal fluid and conduit obstruction: inferences from numerical simulations

“…At Tokachidake, ground deformation suggesting pressure increases in the shallow part of the edifice has been continuing for nearly 10 years and, since 2006, declining crater temperature has been recognized (Takahashi et al 2017). Decreasing permeability in the shallow part of the volcano may control volcanic activity (Tanaka et al 2017). The varying depth of the pressure source may reflect repeated occurrences of permeability reduction in the shallow part of the conduit and/or an increase in hydrothermal flux from the deep part of the system.…”

“…The mechanism leading to phreatic eruption after decreases in crater temperature has been proposed to be pressure increase in the shallow part of the edifice due to permeability decrease in the shallow part of the edifice, perhaps within a conduit that feeds hydrothermal discharge (Christenson et al 2010;Rouwet et al 2014;Strehlow et al 2017). A permeability decrease in the shallow part of such a conduit could restrict flow of heat and mass to the crater and cause a decrease in crater temperature (Christenson et al 2010;Tanaka et al 2017). Either physical or chemical processes may cause obstruction of the conduit.…”

“…Most of the numerical studies that have treated this particular problem invoked an increase in hydrothermal fluid (or heat) supply from the deep part of the system (Todesco et al 2010;Fournier and Chardot 2012;Currenti et al 2017). However, relatively few studies have examined the influence of permeability changes in the shallow part of the conduit (Tanaka et al 2017). No numerical modeling studies have discussed how both the crater temperature and the surrounding edifice react to a competition between increased hydrothermal flux from the depth and reduced permeability in the shallow part of the conduit.…”

Quantitative relationship between plume emission and multiple deflations after the 2014 phreatic eruption at Ontake volcano, Japan

“…While the present status of the hydrothermal system at Mount Ontake is an open system, it will likely shift to a closed system due to several factors, including the progression of mineralogical sealing. This will initiate the re-pressurization of the edifice (Christenson et al 2010;Hamling et al 2016;Tanaka et al 2017). High-frequency InSAR observations and continuous near-field monitoring will allow for detecting ground inflation preceding phreatic eruptions such as the 2015 Hakone eruption (Kobayashi et al 2018), which will provide important insights about the pressure conditions within active volcanoes.…”

Significance of Electromagnetic Surveys at Active Volcanoes: Toward Evaluating the Imminence of Wet Eruptions

“…(3) System behavior in terms of temperature and pressure is investigated in response to an altered heat/fluid supply and/or hydrological property, whether manually or spontaneously. There is a line of studies, such as [44,45,46], in which the response of a wet volcanic system is investigated based on versatile hydrothermal simulators under a simple topography and subsurface structure with modulated heat/fluid supply rate and/or vent permeability. We consider that such an approach has affinity to the evaluation of eruption imminence, since it deals with the temporal variation of the system itself, although the reality of the model configuration can be an intrinsic matter.…”