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Shallow hydrothermal reservoir inferred from post-eruptive deflation at Ontake Volcano as revealed by PALSAR-2 InSAR
© The Author(s) 2018
- Received: 8 June 2018
- Accepted: 29 November 2018
- Published: 11 December 2018
- Phreatic eruption
- Post-eruptive subsidence
- Ground deformation
- Mass balance
- Volcanic–hydrothermal system
- Ontake Volcano
Phreatic eruptions sometimes cause human fatalities when they occur in close proximity to populated areas. Such disasters might be mitigated by advanced predictions of eruption. However, in contrast to magmatic eruptions, the precursors to phreatic eruptions are minute and only appear in small proximal areas (Rouwet et al. 2014). This makes prediction challenging. It is also difficult to clarify the structure of the subsurface hydrothermal system, which is a prerequisite for developing prediction strategies. On the other hand, some very active hydrothermal systems occasionally reveal their activity through detectable geophysical signals. Such manifestations present a valuable opportunity to gain insights into the hydrothermal systems at the root of potential calamities. There are examples where ground deformation due to mass discharge or the subsurface migration of hot fluids has been detected at volcanic–hydrothermal systems that can cause phreatic eruptions (e.g., Nakaboh et al. 2003; Maeda et al. 2017; Doke et al. 2018; Miller et al. 2018; Kobayashi et al. 2018). A number of authors have also discussed the behavior of volcanic–hydrothermal systems during non-eruptive unrest events using ground deformation data, gravity changes, geomagnetic changes, and heat flux changes (e.g., Fournier and Chardot 2012; Ingebritsen et al. 2015; Currenti et al. 2017; Tanaka et al. 2017).
Fewer studies specifically focus on the sub-decadal depressurization process following a phreatic eruption. Lu et al. (2002) detected a subsidence source at a shallow depth in Kiska Island using Synthetic Aperture Radar interferometry (InSAR) data. They speculated that vigorous steam discharge caused a decrease in the pore fluid pressure within a shallow hydrothermal system and subsidence of the ground surface. Hamling et al. (2016) presented InSAR results that showed 3-year-long local subsidence after a phreatic eruption at Tongariro Volcano in 2012. They concluded that the subsidence source was a hydrothermal reservoir emplaced just beneath low-conductivity zones that were regarded as the low permeability sealing layer. Likewise, Nakaboh et al. (2003) discussed the relationship between deflation and steam discharge after a phreatic eruption in 1995 at Kuju Volcano. They showed a clear temporal correlation between the temporal vapor mass flux and deflation rate, indicating that the post-eruptive deflation was caused by steam discharge from a deflating reservoir. They concluded that a large portion of the discharged steam originated deeper than the deflation source by comparing the total discharge mass with the mass loss calculated from the decreased volume. Thus, geophysical data obtained during and after eruptive events give us the opportunity to derive important insights about hydrothermal systems that would be otherwise unobtainable.
Ontake Volcano, Central Japan, is the second highest (3067 m above sea level) stratovolcano in the country. Historically, four phreatic eruptions (1979, 1991, 2007, and 2014) have been identified. All of the eruptions are presumably linked to an underlying volcanic hydrothermal system beneath the Jigokudani Valley (Fig. 1) that has a nest of eruptive vents of recent explosions and persistent (at least 300 years) hydrothermal activity (Oikawa 2008). The magnitude of the eruptions has been uneven; the 1979 and 2014 eruptions were significantly larger than the other two events. The total mass of lithic material extruded during the 2014 eruption was 0.89–1.2 million tons, an amount on the same order of magnitude as the 1979 eruption (1.9 million tons) (Takarada et al. 2016).
The 2007 and 2014 eruptions were observed by modern monitoring networks to detect ground deformation and seismicity. Several authors discussed the 2007 eruption using the data of these observation networks (e.g., Nakamichi et al. 2009). GNSS data indicated that a magmatic dike intruded at a depth of 8–13 km below the surface (Takagi and Onizawa 2016). Seismic data suggested that heat supplied from the magmatic intrusion may have reached the shallow hydrothermal system at depth of 1–3 km. The stimulated fluid in the shallow hydrothermal system was considered to be the direct cause of the unrest events, including the Long-Period (LP) swarms, Very Long-Period (VLP) earthquakes (Nakamichi et al. 2009) and inflation rooted at a depth of 1 km below the surface (Takagi and Onizawa 2016).
Quiescence followed, until eruptive activity started on September 27, 2014, with a few precursory phenomena (Kato et al. 2015). A rapid increase in the number of volcano tectonic (VT) earthquakes appeared 2 weeks prior to the eruption and reached 100 counts/day. Rapid upward migration of VT hypocenters toward the surface started only 10 min before the eruption. Distinct tilt change was also observed 7 min prior to the eruption at a station 3 km away from the eruption center (Maeda et al. 2017). Around the Jigokudani Valley, several vents were newly created in close vicinity to the 1979 vents but slightly shifted to the southwest.
Studies using data acquired during the 2014 event revealed important information regarding the hydrothermal systems at different depths (1–6 km). Mineralogical analysis suggested that the 2014 eruption was phreatic, because no juvenile materials were found (Minami et al. 2016). Analysis of hydrothermally altered minerals in the deposit suggested that the minerals came from a shallow depth (~ 1 to 2 km) and had been formed at temperatures below 250 °C (Minami et al. 2016). Volcanic gas observation after the eruption indicated that discharge of magmatic fluids continued during at least 2 months after the eruption with the SO2 flux rapidly decreasing from 2500 to 130 ton/day between September 28, 2014, and November 21, 2014. After this period, the gas composition reflected degassing from an underlying volcanic–hydrothermal system (Mori et al. 2016).
The geophysical observations also provided significant data. The tilt change 7 min prior to the eruption was explained as the opening of a vertical crack source at 1.1 km depth beneath the Jigokudani Valley, and this opening was interpreted as inflation due to rapid boiling of liquid water (Maeda et al. 2017). The rapid upward migration of the VT hypocenters that began 10 min prior to the eruption at 3 km depth was interpreted as the swift ascent of fluid, causing propagation of crack failures along the flow path (Kato et al. 2015). A subsidence source at a depth of 6 km was also identified from leveling surveys spanning 2009–2014 (Murase et al. 2016).
The ongoing localized subsidence around the summit region during the post-eruptive period has not previously been discussed in the context of the overall the 2014 eruption processes. This study attempts to clarify the origin of the subsidence following the eruption and its role in the overall eruption process.
Since we could use the SAR data acquired from four independent directions, it is possible to resolve the 3-D displacement field. We estimated the three components of the displacement at each pixel using four interferograms (Additional file 1: Fig. S1, Table S1). The time span of the interferograms acquired during different periods was rearranged by temporal interpolation so that they represented cumulative deformation during the same period, i.e., from early October 2014 to end of July 2015. During this period, we assumed the displacement velocity was constant. Finally, we obtained the 3-D displacement field using an inversion method developed by Wright et al. (2004). We carried out least square process on a pixel by pixel basis. In this scheme, the observations are corresponding phase values of all the 4 independent interferograms for the same pixel. The sum of atmospheric noise component and decorrelation component was used for the diagonal elements of the covariance matrix as suggested in Morishita et al. (2016), whereas non-diagonal components were zero. A coherence value computed over a small spatial window around the considered pixel was used in the variance estimation process.
To unveil the geometrical structure of the deflation source, we modeled the deformation using both analytical and numerical methods. We began with the analytical modeling of an elastic half-space medium and then built the numerical model, taking into consideration the steep topography of the Ontake Volcano to get a more realistic assessment.
Deflation volume (m3)
3.6 × 105
2.9 × 105
3.5 × 105
In this section, we discuss behavior of the deflation source, which may be a depressurizing hydrothermal reservoir beneath Jigokudani Valley, by dividing the entire period into three sub-periods, the pre-, syn-, and post-eruptive periods.
Pre-eruptive period (2002–2014)
Here, we discuss when the shallow reservoir developed. Our modeling suggests that a shallow reservoir with a volume of at least 7 × 105 m3 existed before the 2014 eruption. During the development of a reservoir of this size, there should be pressurization-induced inflation due to fluid accumulation. The reservoir might have formed either slowly over a relatively long time period or might have formed instantaneously in association with certain episodic events. We can suggest four candidates for the inflation period. The first candidate is a small inflation event 1 month prior to the 2014 eruption (Takagi and Onizawa 2016). The second one is syn-eruptive deformation during the 2007 eruption. The third one is slow extensional change from 2002 to 2007. The final one is unknown deformation before 2002 when there are no GNSS data for the TO baseline. The first candidate is an inflation event small enough that it can only be detected by stacking several GNSS baselines data around Ontake Volcano (Miyaoka and Takagi 2016). The absence of extensional change of 1.5 cm in the TO baseline, which is the simulated value from the deflation source model, rules out this possibility (Fig. 7). The second candidate has already been modeled as a spherical inflation source at 3 km depth below Jigokudani Valley (Takagi and Onizawa 2016) and can also be ruled out. From 2002 to 2007 (third candidate), although there were relatively large seasonal fluctuations, we can recognize a slight baseline extension of about 1 cm. Finally, the fourth possibility cannot be discarded owing to lack of data before 2002. Thus, the shallow reservoir may have formed before 2002 or between 2002 and 2007 and remained stable until the 2014 eruption.
Syn-eruptive period (08/18/2014–09/29/2014)
Here, we discuss whether the deflated reservoir was the main source of water and energy for the phreatic eruption on September 27, 2014. Since the deflation began after the eruption, it is natural to suspect that the shallow reservoir was a major source of water and energy. However, this is not the case as suggested below.
First, there was no significant syn-eruptive deflation despite violent fluid discharge such that the eruption plume height reached 10.8 km a.s.l. (Sato et al. 2015). InSAR pair spanning the eruption period (8/18/2014–09/29/2014) showed no net deflation, only a small deformation that was interpreted as opening of a dike-like vertical crack corresponding to vent formation (Yamada et al. 2015). If most of the discharged water on the first day of the eruption had been provided from the shallow reservoir, syn-eruptive deflation should have manifested in the InSAR data. Unlike the 2014 eruption at Ontake Volcano, phreatic eruptions in 1995 at Kuju Volcano and in 2012 at Tongariro Volcano showed syn-eruptive deflation (Nakaboh et al. 2003; Hamling et al. 2016), which may reflect the rapid depressurization due to fluid discharge. However, the main water source of the 2014 eruption at Ontake Volcano was not the shallow deflated reservoir.
Another consideration is the spatial relationship among the deformation sources, including the crack-like eruptive vent and source of the tilt change immediately before the eruption. This crack-like vent reaches 1 km depth below the Jigokudani Valley (Fig. 6), and this depth is consistent with the source depth of the tilt change, which may have arisen from crack opening due to rapid boiling of liquid water and its subsequent rising to the surface (Maeda et al. 2017). Water and energy for the eruption appeared to be rooted below the shallow reservoir. The shallow reservoir began to depressurize after the eruption and was heated by hot fluid ascending from greater depth. The shallow source may not have been involved in the eruption.
These inferences are also consistent with volcanic gas observations. The SO2 flux observed on September 28, 2014, was 2500 ton/day, and this amount is too large to be explained by supply from a shallow and isolated hydrothermal reservoir (Mori et al. 2016). It is more reasonable to infer that the discharged fluid at the beginning of the eruption came from a deeper, magmatic region as inferred by Mori et al. (2016).
Post-eruptive period (09/30/2014–07/21/2017)
Post-eruptive deflation began after the 2014 eruption, and plume discharge has also continued (Yamaoka et al. 2016). Deflation caused by fluid discharge is typical at volcanoes and geothermal fields (e.g., Lu et al. 2002; Nakaboh et al. 2003; Hamling et al. 2016; Barbour et al. 2016; Juncu et al. 2017).
Plume mass flux data
Mass flux (kg/day)
2.0 × 108
5.5 × 107
6.0 × 107
Terada, personal communication
6.1 × 107
Terada, personal communication
7.2 × 107
Hashimoto and Tanaka, personal communication
5.1 × 107
Hashimoto and Tanaka, personal communication
However, the depressurizing hydrothermal reservoir is likely in a two-phase liquid–vapor state. Two-phase water can be no longer approximated as incompressible fluid but rather is extremely compressible (Grant and Sorey 1979). Furthermore, Segall (2010) points out that fluid compressibility is a key parameter in calculating mass change corresponding to volume changes estimated by geodetic modeling. More realistic estimation of the mass balance needs to take the two-phase water compressibility into account.
ALOS-2 InSAR data map deflation following the phreatic eruption in 2014 at Ontake Volcano. The deflation source appears to be a shallow depressurizing hydrothermal reservoir. This reservoir might not be the main source of water and energy for the phreatic eruption that caused violent ejections of rock. GNSS data for pre-eruptive period suggest that the reservoir may have been developed between 2002 and 2007 or even before 2002. The quantitative relationship between ongoing post-eruptive deflation and plume discharge remains unclear due to the complexity of pressure transmission in an evolving two-phase system.
SN performed InSAR analysis and source modeling and wrote the manuscript. SN and MM interpreted the InSAR results and drew the conclusions. Both authors read and approved the final manuscript.
We thank editor Tomokazu Kobayashi, Dr. Steve Ingebritsen, and an anonymous reviewer for their valuable comments to improve the quality of our manuscript. We are grateful to Dr. Tomofumi Kozono and Dr. Ryo Tanaka for valuable discussion with them. We appreciate Dr. Akihiko Terada, Prof. Takeshi Hashimoto, and Dr. Ryo Tanaka allowing us to use unpublished data of the plume mass flux. We thank Dr. Taku Ozawa for providing RINC to conduct InSAR analysis. We analyzed ALOS-2/PALSAR-2 data which are shared within the PALSAR Interferometry Consortium to Study our Evolving Land Surface (PIXEL). ALOS-2/PALSAR-2 data were provided by the Japan Aerospace Exploration Agency (JAXA) under a cooperative research contract with the Earthquake Research Institute (ERI) at the University of Tokyo. The ownership of the original PALSAR-2 data belongs to JAXA. The data were provided through PIXEL and working group of Coordinating Committee for Prediction of Volcanic Eruption. We used 10-m-mesh DEM published by Geospatial Information Authority of Japan. We used the Generic Mapping Tools (Wessel and Smith 1998) and matplotlib (Hunter 2007) for drawing the figures.
The authors declare that they have no competing interests.
Availability of data and materials
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program (Grant No. 1008).
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