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Shallow Hydrothermal Reservoir Inferred from Post-Eruptive Deflation

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Shallow Hydrothermal Reservoir Inferred from Post-Eruptive Deflation  Narita and Murakami Earth, Planets and Space (2018) 70:191 https://doi.org/10.1186/s40623-018-0966-6 FULL PAPER Open Access Shallow hydrothermal reservoir inferred from post‑eruptive defation at Ontake Volcano as revealed by PALSAR‑2 InSAR Shohei Narita1* and Makoto Murakami2 Abstract In 2014, a phreatic eruption occurred at the Ontake Volcano in Central Japan causing multiple deaths and missing persons. Interferometric Synthetic Aperture Radar data showed local-scale subsidence around the newly created eruptive vents after the eruption. Source modeling resulted in a nearly spherical defation source emplaced at a depth of 500 m below the vents refecting post-eruptive depressurization in a shallow hydrothermal reservoir. The cumula- tive defation volume reached 7 105 ­m3 3 years after the eruption. Comparison between our source model and GNSS data indicates that this shallow× reservoir could have formed by 2007 and remained stable until the 2014 erup- tion. The absence of signifcant syn-eruptive subsidence indicates that the shallow reservoir was not the main water source driving the phreatic eruption. Under simple assumptions, mass balance between the shallow reservoir and the discharge plume from the vents indicates most of the water contained in the plume comes from greater depth than the shallow reservoir. To constrain the post-eruptive process, it is necessary to track not only deformation but also plume discharge for 3 years after the eruption. Keywords: Phreatic eruption, Post-eruptive subsidence, Ground deformation, Mass balance, SAR, Volcanic– hydrothermal system, Ontake Volcano Introduction potential calamities. Tere are examples where ground Phreatic eruptions sometimes cause human fatalities deformation due to mass discharge or the subsurface when they occur in close proximity to populated areas. migration of hot fuids has been detected at volcanic– Such disasters might be mitigated by advanced predic- hydrothermal systems that can cause phreatic eruptions tions of eruption. However, in contrast to magmatic (e.g., Nakaboh et al. 2003; Maeda et al. 2017; Doke et al. eruptions, the precursors to phreatic eruptions are min- 2018; Miller et al. 2018; Kobayashi et al. 2018). A number ute and only appear in small proximal areas (Rouwet of authors have also discussed the behavior of volcanic– et al. 2014). Tis makes prediction challenging. It is also hydrothermal systems during non-eruptive unrest events difcult to clarify the structure of the subsurface hydro- using ground deformation data, gravity changes, geo- thermal system, which is a prerequisite for develop- magnetic changes, and heat fux changes (e.g., Fournier ing prediction strategies. On the other hand, some very and Chardot 2012; Ingebritsen et al. 2015; Currenti et al. active hydrothermal systems occasionally reveal their 2017; Tanaka et al. 2017). activity through detectable geophysical signals. Such Fewer studies specifcally focus on the sub-decadal manifestations present a valuable opportunity to gain depressurization process following a phreatic erup- insights into the hydrothermal systems at the root of tion. Lu et al. (2002) detected a subsidence source at a shallow depth in Kiska Island using Synthetic Aperture Radar interferometry (InSAR) data. Tey speculated that *Correspondence: [email protected] vigorous steam discharge caused a decrease in the pore 1 Department of Natural History Sciences, Graduate School of Science, Hokkaido University, N10W8, Kita‑ku, Sapporo, Hokkaido 060‑0810, Japan fuid pressure within a shallow hydrothermal system and Full list of author information is available at the end of the article subsidence of the ground surface. Hamling et al. (2016) © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Narita and Murakami Earth, Planets and Space (2018) 70:191 Page 2 of 12 presented InSAR results that showed 3-year-long local (Fig. 1). Likewise, multi-directional InSAR analysis using subsidence after a phreatic eruption at Tongariro Vol- data from the Advanced Land Observing Satellite-2/ cano in 2012. Tey concluded that the subsidence source Phased Array type L-band Synthetic Aperture Radar-2 was a hydrothermal reservoir emplaced just beneath low- (ALOS-2/PALSAR-2) indicated evolving defation on the conductivity zones that were regarded as the low per- summit region of the volcano. Te purpose of this study meability sealing layer. Likewise, Nakaboh et al. (2003) is to clarify the relationship between the ongoing defa- discussed the relationship between defation and steam tion indicated by the InSAR data and the 2014 eruption. discharge after a phreatic eruption in 1995 at Kuju Vol- cano. Tey showed a clear temporal correlation between the temporal vapor mass fux and defation rate, indicat- Ontake Volcano eruptive history ing that the post-eruptive defation was caused by steam Ontake Volcano, Central Japan, is the second highest discharge from a defating reservoir. Tey concluded (3067 m above sea level) stratovolcano in the country. that a large portion of the discharged steam originated Historically, four phreatic eruptions (1979, 1991, 2007, deeper than the defation source by comparing the total and 2014) have been identifed. All of the eruptions are discharge mass with the mass loss calculated from the presumably linked to an underlying volcanic hydrother- decreased volume. Tus, geophysical data obtained dur- mal system beneath the Jigokudani Valley (Fig. 1) that has ing and after eruptive events give us the opportunity to a nest of eruptive vents of recent explosions and persis- derive important insights about hydrothermal systems tent (at least 300 years) hydrothermal activity (Oikawa that would be otherwise unobtainable. 2008). Te magnitude of the eruptions has been uneven; Evolving post-eruptive ground deformation has been the 1979 and 2014 eruptions were signifcantly larger also observed around the eruptive crater region of than the other two events. Te total mass of lithic mate- Ontake Volcano in Central Japan following the 2014 rial extruded during the 2014 eruption was 0.89–1.2 mil- phreatic explosion (Yamaoka et al. 2016). A manifesta- lion tons, an amount on the same order of magnitude tion of the defation appeared as a decrease in the dis- as the 1979 eruption (1.9 million tons) (Takarada et al. tance between the GNSS stations spanning the volcano 2016). Fig. 1 Map of the study area. The inset is a close view of the summit area and corresponds to the area of interferograms (Figs. 3, 4) and modeling (Fig. 5). Most of the hydrothermal activity at the Ontake Volcano occurs around the Jigokudani Valley (indicated with the dashed line). The black diamonds indicate GNSS stations deployed around the edifce of the Ontake Volcano. Eruptive vents are also marked Narita and Murakami Earth, Planets and Space (2018) 70:191 Page 3 of 12 Te 2007 and 2014 eruptions were observed by mod- source at a depth of 6 km was also identifed from lev- ern monitoring networks to detect ground deformation eling surveys spanning 2009–2014 (Murase et al. 2016). and seismicity. Several authors discussed the 2007 erup- Te ongoing localized subsidence around the summit tion using the data of these observation networks (e.g., region during the post-eruptive period has not previously Nakamichi et al. 2009). GNSS data indicated that a mag- been discussed in the context of the overall the 2014 matic dike intruded at a depth of 8–13 km below the sur- eruption processes. Tis study attempts to clarify the ori- face (Takagi and Onizawa 2016). Seismic data suggested gin of the subsidence following the eruption and its role that heat supplied from the magmatic intrusion may have in the overall eruption process. reached the shallow hydrothermal system at depth of 1–3 km. Te stimulated fuid in the shallow hydrother- InSAR analysis mal system was considered to be the direct cause of the We applied interferometry to L-band SAR images unrest events, including the Long-Period (LP) swarms, acquired by ALOS-2 over Ontake Volcano between 2014 Very Long-Period (VLP) earthquakes (Nakamichi et al. and 2017 (Fig. 2). For all the InSAR processing proce- 2009) and infation rooted at a depth of 1 km below the dures, we used the RINC software package (Ozawa et al. surface (Takagi and Onizawa 2016). 2016). Te spatial averaging window (multilook) size Quiescence followed, until eruptive activity started was selected to ensure that the ground spacing of the on September 27, 2014, with a few precursory phenom- neighboring pixels was about 20 m after rearrangement. ena (Kato et al. 2015). A rapid increase in the number Topographic corrections were conducted using a 0.4-arc- of volcano tectonic (VT) earthquakes appeared 2 weeks sec digital ellipsoidal height model (DEHM) generated prior to the eruption and reached 100 counts/day. Rapid from a Digital Elevation Model (DEM) provided by the upward migration of VT hypocenters toward the sur- Geospatial Information Authority of Japan (GSI). Te face started only 10 min before the eruption. Distinct orbital phase ramp was removed by subtraction of the tilt change was also observed 7 min prior to the eruption phase simulated from the precision orbital information. at a station 3 km away from the eruption center (Maeda Atmospheric errors, which are caused by variable distri- et al. 2017). Around the Jigokudani Valley, several vents butions of water–vapor contents in the troposphere, were were newly created in close vicinity to the 1979 vents but reduced by subtracting the elevation-correlated com- slightly shifted to the southwest. ponent. For phase unwrapping, we used the SNAPHU Studies using data acquired during the 2014 event algorithm (Chen and Zebker 2002).
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