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Permeability-control on volcanic hydrothermal system: case study for Mt. Tokachidake, Japan, based on numerical simulation and field observation
© The Author(s) 2017
Received: 19 October 2016
Accepted: 28 February 2017
Published: 8 March 2017
Hydrothermal systems are relevant to a variety of hydrovolcanic phenomena, including hydrovolcanic eruptions (Barberi et al. 1992; Germanovich and Lowell 1995; Browne and Lawless 2001). In this study, we focus on phreatic eruptions, defined by Barberi et al. (1992), as the eruption style caused by underground aquifers, whether or not they are phreatic or geothermal, without direct involvement of a magma body. Although most phreatic eruptions are local phenomena affecting a limited area, they can be accompanied by hazards such as ballistic blocks, base surges, and debris avalanches, and precursors are often subtle (Barberi et al. 1992). Germanovich and Lowell (1995) suggested a typical sequence of processes for phreatic eruptions: (1) dike injection, (2) hydrothermal system heating above the dike, and (3) rapid crack propagation due to thermal stress. This implies that the hydrothermal system may experience some change in state prior to phreatic eruptions. However, changes in the hydrothermal system do not always lead to explosive surface manifestations. Thus, improved understanding of changes in state in the hydrothermal system contributes to the assessment of whether phreatic eruption is likely.
Instrumental monitoring provides insight into ongoing changes in hydrothermal systems beneath specific volcanoes, whereas numerical simulations can provide useful clues as to the physical and/or chemical implications of such changes. Such clues can be obtained by sensitivity analysis (varying boundary conditions and parameters) and by forward modeling of the system’s temporal evolution. Hydrothermal simulators have most often been used to assess the productivity of geothermal fields, but have also been applied to volcanic systems. Several previous studies compare volcano-monitoring records to numerical simulations to develop process understanding. For example, Christenson et al. (2010) discussed the eruption of Mt. Ruapehu, New Zealand, in 2007, by analyzing the effect of low-permeability seal formation on the CO2 flux and temperature of the crater lake. Fournier and Chardot (2012) concluded that ground deformation at White Island, New Zealand, during 2002–2006 and 2007–2009 was caused by a pore pressure increase at depth, rather than thermal expansion. However, few if any studies so far have used magnetic field monitoring as reference data for numerical simulations.
Many previous studies have reported magnetic field changes accompanying volcanic activity (e.g., Dzurisin et al. 1990; Sasai et al. 2002; Hurst et al. 2004). Such magnetic field changes may reflect temperature changes (thermomagnetic effect), stress changes (piezomagnetic effect), fluid-driven telluric current (the electrokinetic effect), or chemical processes such as hydrothermal alteration of magnetic minerals (Sigurdsson 2000). A large thermomagnetic effect implies a substantial difference between heat supply from depth and surface and/or near-surface heat discharge.
A brief summary of historical eruptions and recent activity of Mt. Tokachidake
The major historical eruptions of Mt. Tokachidake in the modern era took place in 1926, 1962, and 1988–1989 (Katsui et al. 1990). In the 1926 eruption, phreatic explosions preceded magmatic eruptions of the main phase, which brought about the collapse of the NW sector of the preexisting central cone followed by hydrothermal surges and snow melting, resulting in lahars (Uesawa 2010). This eruption left the collapsed topography now called Taisho crater. Intermittent small eruptions continued until 1928. In the 1962 eruption, hydrothermal eruptions also preceded the magmatic eruption of the main phase. The column of the sub-Plinian eruption reached 12 km above the crater. New craters named 62-0, 62-1, 62-2, and 62-3 were formed in the course of this eruption. The total erupted volume was estimated as 7.1 × 107 m3 (Katsui et al. 1963). The 1988–1989 eruption started with a hydrothermal eruption from crater 62-2 in December 1988. Phreatomagmatic eruptions with small-scale pyroclastic flows occurred intermittently until March 1989. Most recently a very small-scale eruption with a colored plume was witnessed in 2004 (JMA 2013).
Crater 62-2 continues to host fumarolic activity but, according to reports that are regularly released to the public by the Japan Meteorological Agency (JMA), both plume height and temperature have gradually decreased since 2000. Meanwhile, localized ground inflation around crater 62-2 has been recognized since 2006 (observations performed by Geological Survey of Hokkaido, Hokkaido University and JMA). In addition, annual magnetic total field measurement campaigns by a group including the authors began in 2008 and revealed continuous demagnetization, suggesting heating at a shallow depth beneath the crater. Based on results from the first 2 years, Hashimoto et al. (2010) proposed that the localized demagnetization and deformation occur where hot gas (primarily water vapor) from depth condenses, releasing its latent heat, and then flows downward as liquid. Under this hypothesis, the amount of heat required to bring about the observed demagnetization would match the gradual decrease in heat discharge from crater 62-2 observed since 2000. Demagnetization would not necessarily be a consequence of enhanced heat supply from depth. However, their model is a conceptual one and does not account for the localized release of latent heat at a certain depth, nor does it provide mechanism for reducing heat discharge from the crater. A fully viable explanation might be common to other volcanoes where remarkable magnetic field changes during quiescent periods have been reported (e.g., Kanda et al. 2010; Kuchi-no-Erabu-jima Volcano).
Magnetic total field
Mineralogical sealing seems to play a key role in phreatic eruption at some volcanoes (Raoul Island, New Zealand: Christenson et al. 2007; Ruapehu, New Zealand: Christenson et al. 2010). We explore this conceptual but broadly cited hypothesis in our numerical model of the hydrothermal system at Mt. Tokachidake, where we invoke conduit obstruction due to permeability reduction to explain the decrease in heat discharge from the crater and consequent heating beneath it. The goal of the numerical simulations are to investigate (1) whether conduit obstruction can cause heat accumulation beneath the active crater, gradual cooling of the crater fumaroles, and waning of the plume height; (2) the effects of a specific structure such as a caprock or sealing layers; and (3) whether such conduit obstruction is likely to cause other detectable changes (e.g., ground deformation).
We calculate the time-dependent physical states (temperature, pore pressure, mass flow rate of liquid/vapor, and heat flow) over four steps. First, assuming homogenous permeability for the entire space and boundary conditions of 15 °C and 1.013 × 105 Pa at the ground surface, the model is run for 105 years until it reaches a quasi-steady state. Second, we reduce pressure at the ground surface from 1.013 × 105 to 1.013 × 103 Pa and run the model until another quasi-steady state is achieved (105 years). This step is used to induce an unsaturated zone within a shallow part of the edifice, since HOTH2O equation of state cannot explicitly deal with air. The unsaturated zone is treated as a two-phase zone of H2O over a less-than-atmospheric pressure range. Third, the high-permeability conduit and the surrounding low-permeability caprock are introduced in the middle of the slope and at the same time hydrothermal fluid (350 °C, 75 kg s−1) is injected at the bottom of conduit to reproduce fumarolic heat discharge of about 100 MW, a value estimated by applying the plume-rise method (Kagiyama 1978) to time-lapse photographs of the plume in 2004 (picture data courtesy of JMA), after reaching a quasi-steady condition (103 years). Fourth and finally, an abrupt reduction of permeability is imposed at a particular depth in the conduit, and the system response is observed for about 10 years.
Horizontal permeability (m2) of the subregions for Cases A–D
Region 1 (host rock)
5.0 × 10−14
5.0 × 10−14
5.0 × 10−10
5.0 × 10−17
5.0 × 10−14
5.0 × 10−14
5.0 × 10−10
5.0 × 10−16
5.0 × 10−13
5.0 × 10−10
5.0 × 10−16
5.0 × 10−16
5.0 × 10−10
Rock properties used in all cases (A–D)
2.3 × 103 kg m−3
1.5 W m−1 K−1
1.0 × 103 J kg−1 K−1
Results of numerical simulations
Fluctuation of heat supply from depth
Ground inflation at the crater
Implications for the recent activity at Mt. Tokachidake
According to our numerical simulations, conduit obstruction decreases vent temperature and heat flux and results in heat accumulation below the level of obstruction. Conduit obstruction may be caused by either physical or chemical processes. Candidate physical processes include mechanical reworking of surface deposits (e.g., D’Oriano et al. 2014) and the collapse of the conduit wall (e.g., Calvari et al. 2016). Conduit-wall collapse and intensive surface reworking should have been detected and reported by means of geophysical monitoring such as seismometers and GNSS or by eyewitness accounts.
Candidate chemical processes include deposition of native sulfur and/or hydrothermal minerals in the conduit. Hurst et al. (1991) studied cyclic behavior of the crater lake at Ruapehu (New Zealand) and suggested that the temperature-dependent viscosity of native sulfur played a key role in the cyclic activity. This mechanism may also be effective at Mt. Tokachidake. In June 1923, a molten sulfur pond (locally called “Yunuma”) appeared south of the so-called central cone, and there was a sulfur mine at Mt. Tokachidake until the 1962 eruption. The discharge of sulfur dioxide from crater 62-2 was 210 t day−1 on July 7, 2007 (Mori et al. 2006) and 140 t day−1 in 2006 (Mori et al. 2013). At geothermal fields, buildup of solid silica often reduces the permeability of pipelines and wellbores. The solubility of amorphous silica decreases remarkably when it is cooled (Fournier and Rowe 1977), so cooling in the conduit may cause silica scaling that reduces permeability. This permeability reduction would cause further cooling above the obstruction level, generating positive feedback. However, silica scaling is inhibited in low pH environments (Gallup and Barcelon 2005), and the mid-slope hot spring at Mt. Tokachidake has pH between two and four (Takahashi et al. 2015). Many other types of hydrothermal alteration, such as clay formation, cause volume increases that tend to decrease permeability. It might be possible to clarify the dominant chemical processes under the specific conditions of Mt. Tokachidake by using other kinds of numerical simulation that incorporate fluid–rock interaction. Christenson et al. (2010) performed such simulations for a simple one-dimensional case. Based on the records of CO2 flux and temperature of the crater lake at Ruapehu, and numerical simulations, they discussed the 2007 eruption at Ruapehu. Their simulations, in which volcanic gas was injected into a porous zone saturated with lake water, revealed that significant deposition of quartz, sulfur, and clay minerals could occur over a short timescale of about 10 days. In their calculations, deposition of sulfur reduced permeability from 10−12 to 10−17 m2. Although the same mechanisms are not likely to translate completely to Mt. Tokachidake (there is no stable lake in crater 62-2), meteoric water inflow to the conduit resulting from decreased hydrothermal upflow from depth could cause deposition of sulfur.
Validity of the two-dimensional approach
We modeled a volcanic hydrothermal system by means of numerical simulation with three key observables as reference: the magnetic total field, vent temperature, and heat flux. Volcanic activity at Mt. Tokachidake since the early 2000s was considered as a case study. Field observations there have revealed continuous demagnetization, suggesting heat accumulation beneath the active crater, gradual cooling of the crater fumaroles, and waning of the plume height from the crater (a proxy for heat loss). Our numerical simulations of the hydrothermal system reveal that conduit obstruction (i.e., permeability reduction in a conduit) could cause these changes. We also confirmed that the changes could not be consistently explained by fluctuation of heat supply from depth. A caprock structure that confines hot gases and hydrothermal water at shallow depth may play a key role in controlling the location of heating and pressurization, consistent with the concept of fluid confinement prior to phreatic (or hydrothermal) eruptions. Without such a caprock structure, hot fluids are prone to percolate diffusely, so that localized and long-lasting pressurization and demagnetization cannot be achieved. Conduit obstruction may be caused either by physical or by chemical processes, although the latter seems more likely in the case of Mt. Tokachidake. Further investigation is necessary to understand the details of the chemical processes, perhaps by using numerical simulations in which fluid–rock interaction is incorporated. We expect that a numerical study coupling physics and chemistry would contribute to better understanding of the preparation processes of phreatic/hydrothermal eruptions.
RT carried out the numerical simulations, contributed to the magnetic field observation, and drafted the manuscript. TH organized and performed the magnetic field observation and made substantial contributions to conception of the model. NM and TI supported the entire study with discussion, suggestions, and guiding RT through the numerical modeling. All authors read and approved the final manuscript.
We sincerely thank JMA and Geological Survey of Hokkaido for providing data of crater temperature and plume height. We are grateful to Steven Ingebritsen and an anonymous referee for helpful comments and suggestions that greatly improved the manuscript. We really appreciate Shan de Silva handling the manuscript. We used the Generic Mapping Tools (Wessel and Smith 1998) to draw most figures.
The authors declare that they have no competing interests.
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