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Contention between supply of hydrothermal fluid and conduit obstruction: inferences from numerical simulations
© The Author(s) 2018
- Received: 29 December 2017
- Accepted: 14 April 2018
- Published: 4 May 2018
In recent years, precursors of phreatic eruptions have been captured by observations near the crater (Barberi et al. 1992; Rouwet et al. 2014) and are diverse, including ground deformation and changes in gravity, total magnetic field, gas composition, heat flux, and crater temperature (Barberi et al. 1992; Rouwet et al. 2014). Measurement of crater temperature is relatively easy and has been done for a long time. Changes in crater temperature preceding phreatic eruption have been documented for several eruptions (Minakami 1939; Ossaka et al. 1997; Ehara 2007; Christenson et al. 2010; Strehlow et al. 2017). Thermal observation using drones and remote sensing has become feasible in recent years (Harvey et al. 2016; Mori et al. 2016), and these methods make acquisition of crater temperature easier. In this research, we focus on change in crater temperature as a precursor of a phreatic eruption.
An increase in crater temperature (often the temperature of a crater lake) has been observed as a precursor of phreatic eruption in many cases, including almost all events at Ruapehu during 1992–2012 (Christenson et al. 2010; Strehlow et al. 2017), Meakan in 1988 and 2008 (Japan Meteorological Agency 2012a), Azuma in 1977 (Japan Meteorological Agency 2012b), and most events at Kusatsu-Shirane in 1938, 1982, and 1983 (Minakami 1939; Ossaka et al. 1997). In such cases, an increase in the supply of hydrothermal fluid (or heat) from depth is often proposed as the primary mechanism causing phreatic eruption (Rouwet et al. 2014). However, phreatic eruptions after decreases in crater temperature have also been reported in some cases, such as Ruapehu in 1988, 2006, and 2007 (Christenson et al. 2010), Meakan in 1996 (Japan Meteorological Agency 2012a), some events of Kusatsu-Shirane in 1938, 1982, and 1983 (Minakami 1939; Ossaka et al. 1997), Tokachidake in 2004 (Takahashi et al. 2017). 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. Candidate physical processes include mechanical reworking of surface deposits (e.g., D’Oriano et al. 2014) and collapse of the conduit wall (e.g., Calvari et al. 2016). Candidate chemical processes include deposition of native sulfur and hydrothermal minerals in the conduit.
Many previous studies have compared numerical modeling of the hydrothermal system to the volcanic monitoring record. 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.
The goal of the study is to examine how crater temperature and other observations at an active volcano can be used for prediction of a phreatic eruption.
2.3 × 103 kg m−3
1.5 W m−1 K−1
1.0 × 103 J kg−1 K−1
Permeability of host rock
5.0 × 10−13 m2
Permeability of conduit
1.0 × 10−10 m2
To support the fumarolic activity at the crater, hydrothermal fluid was injected at the bottom of the permeable central conduit at fixed enthalpy (~1345 kJ/kg corresponding to liquid H2O at ~300 °C and 10 MPa) and a rate of 1000 kg/s. This injection rate was chosen to reproduce the initial heat discharge rate through the block corresponding to the vent (radius of 50 m; the smallest cell size in the model) being roughly 100 MW, which is a representative value for active fumarolic activities at volcano summit. The rest of the convective heat input (~1250 MW) spreads to the edifice and discharges on the flanks of the volcano and through the open distal boundary.
Hydrothermal flux and permeability of PCB in each run
Hydrothermal flux (kg s−1)
Permeability of PCB (m2)
1.0 × 10−10
1.0 × 10−11
1.0 × 10−12
5.0 × 10−13
1.0 × 10−13
1.0 × 10−10
1.0 × 10−11
1.0 × 10−12
5.0 × 10−13
1.0 × 10−13
1.0 × 10−10
1.0 × 10−11
1.0 × 10−12
5.0 × 10−13
1.0 × 10−13
Both increases in hydrothermal flux from the deep part of the system and reduction in permeability in the shallow part of the conduit cause pressurization and heat accumulation in the shallow part of the edifice. Crater temperature shows complex behaviors depending on the amount of increase in hydrothermal flux and the amount of reduction in permeability.
Clues for observation and prediction of phreatic eruption
It is difficult to predict phreatic eruptions by only the crater temperature alone. One should be wary of the potential for large eruptions when crater temperature decreases. Indeed, eruptions of Scale 4 (Scott 2013) occurred frequently after crater temperature decreased at Ruapehu between 1940 and 2012 (Strehlow et al. 2017). Observations of ground deformation, which can indicate pressure change in the shallow part of the edifice, and crater temperature can be combined to predict phreatic eruption and understand the activity of the hydrothermal system.
Coupling of ground deformation and crater temperature could make it possible to quantitatively investigate hydrothermal system activity. The location of maximum pressure change (see Figs. 3, 5) could reveal whether or not conduit permeability has decreased. We note that the apparent depth of the shallow pressure source at Tokachidake, Japan, varies with time (Takahashi et al. 2017). 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. Numerical simulation coupled with a ground deformation model constrained by field observations would enable quantitative examination of this hypothesis.
Changes in hydrothermal fluid flux and permeability reduction can induce heating and pressurization beneath volcanic craters below the depth of conduit obstruction. The change in temperature at the crater itself is controlled by the amplitude of the increase in hydrothermal fluid flux from the deeper part of the system and the degree of permeability reduction.
In this study, simulations were carried out using a simple structure. However, the response of the edifice to changes in of the hydrothermal fluid flux will be influenced by the heterogeneous permeability structures (Todesco et al. 2010; Currenti et al. 2017). The distribution of permeability affects the timing and amplitude of changes in temperature and pressure through space and time (Todesco et al. 2010). Pressure- and temperature-dependent permeability will also influence the behavior of the system (Coulon et al. 2017), as will the porosity of the rock, the topography, the composition of the hydrothermal fluid, the presence or absence of a crater lake, and other factors.
We suggest that it is difficult and potentially misleading to predict eruptions from increases in crater temperature. One should be wary of the potential for large eruptions when crater temperature decreases. Coupling observations of crater temperature change with observations of ground deformation can clarify mechanisms and help predict phreatic eruption. We will couple hydrothermal simulation with a model of ground deformation in future research.
RT carried out the numerical simulations and drafted the manuscript. TH made substantial contributions to the conceptualization of the model. NM and TI supported the entire study through discussion, suggestions, and by guiding RT through the numerical modeling. All authors read and approved the final manuscript.
We are grateful to Steven Ingebritsen and an anonymous referee for helpful comments and suggestions that greatly improved the manuscript. We really appreciate Stephanie Prejean handling the manuscript. We used the Generic Mapping Tools (Wessel and Smith 1998) to draw most of the figures.
The authors declare that they have no competing interests.
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