Volcanic Unrest at Hakone Volcano after the 2015 phreatic eruption — Reactivation of a Ruptured Hydrothermal System?

Since the beginning of the 21st century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed signicantly in terms of seismicity and geochemistry. In this paper, characteristics of the post-eruptive volcanic unrest that occurred in 2017 and 2019 are described, and changes in the hydrothermal system of the volcano caused by the eruption are discussed. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep ination below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reecting supply of magma or magmatic uid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was signicantly lower, especially in the shallow region (~2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~3 km away from the center. The 2015 eruption established routes for steam from the hydrothermal system ( ≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC), which was absent before the eruption. This steam showed an increase in magmatic/hydrothermal gas ratios (SO2/H2S and HCl/H2S) in the 2019 unrest, which may be interpreted as magmatic intrusion at shallow depth; however, no indicative seismic and geodetic signals were observed. Net SO2 emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic uid from depth; however, the breached cap-rock was unable to allow subsequent pressurization of the hydrothermal system beneath the volcano center and suppressed seismic activity signicantly. The heat released from the newly derived magma or uid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–plastic transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.


Introduction
Phreatic eruptions are eruption that have no direct involvement of magma and are instead driven by thermal energy of hydrothermal water. However, thermal energy itself is mainly provided by heat from magma, and recent geophysical observations revealed that deep intrusion of magma or magmatic uid precedes phreatic eruptions. The time lag between such an intrusion and the eruption makes forecasting The 2007 phreatic eruption of Ontake volcano was preceded by a magma intrusion approximately 3 km deep beneath the eruption center, which occurred approximately 2 months before the eruption (Nakamichi et al. 2009). In contrast, the 2014 phreatic eruption of the volcano, which killed more than 60 trekkers in the summit area, was only preceded by a volcano tectonic earthquake swarm that started 17 days before the eruption and no magmatic intrusion seems to have preceded it (Takagi and Onizawa 2016). However, surprisingly, the ejecta of the 2014 eruption contained a trace amount of juvenile (magmatic material newly emplaced beneath the surface) fragments, and a series of geological investigations implied that the magmatic body intruded to 3 km deep beneath the volcano just before the 2007 eruption was released by the 2014 eruption (Miyagi et al. 2020). This sequence of eruptions implies that an eruption can be triggered by an intrusion event years prior. We thus cannot estimate eruption probability only based on intensity of volcanic unrest or source depth of deformation especially when a series of eruptions and/or unrest episodes have occurred at the volcano recently. For such a volcano, evaluation of volcanic unrest based on the model of a magma-hydrothermal system is critical to avoid underestimation of eruption probability.
Hakone volcano, located near the nation's capital Tokyo, is one of the largest tourist destinations in Japan and attracts more than three million tourists annually to the center of phreatic eruption named Owakudani (Owakidani) steaming area (Fig. 1). In this volcano, volcanic unrest episodes have repeated every few years since the beginning of the 21st century, and eventually in 2015, a small phreatic eruption occurred at Owakudani steaming area ). Even after this eruption, volcanic unrest continued to take place, and evaluation of these events has yet to be done. Here we summarize the recent volcanic unrest and discussion the possibility of future phreatic eruptions.

Background
Hakone Volcano and its activity in the 21st century Here we review Hakone volcano and its latest activity based on Mannen et al. (2018). Hakone is a caldera volcano located at approximately 80 km SW of Tokyo (Fig. 1a). Its eruption history started at least 400 ka and after two caldera-forming stages. Andesitic effusive eruptions since 40 ka have formed a complex of lava ows and domes named the Younger Central Cones (YCC) in the center of the caldera (Fig. 1b). The latest magmatic eruption occurred near the northernmost part of YCC and formed a lava dome named Kanmurigatake, which erupted within an amphitheater that was created by a sector collapse just before the dome formation. The most active steaming area and the center of the latest phreatic eruptions named Owakudani is located at the eastern ank of Kanmurigatake.
Hakone is not very active in terms of magmatic eruptions; however, it is notable for its high seismicity, with at least 7 intensive earthquake swarms observed in the 20th century. Most of the swarms did not accompany clear intensi cation of steaming activity, while the volcanic unrest from 1933 to 1935 culminated with a formation of a new steam vent 1 km south of Owakudani, although the exact location of the vent is not known.
The continuous instrumental monitoring of Hakone volcano started after the volcanic unrest of 1959-60; however, the volcano monitoring network detected no major seismic swarm until 2001. The 2001 unrest accompanied an earthquake swarm, and deep and shallow in ation as observed by Global Navigation Satellite System (GNSS) and a tiltmeter network, and culminated with a blowout of a steam production well (SPW) in Owakudani (500 m deep). Since the 2001 unrest, major volcanic unrest episodes comprising earthquake swarms, deep in ation detected by a GNSS network, and deep low frequency events (DLF) were observed in 2006, 2008-2009and 2013Yukutake et al. 2019). In terms of seismicity, these events can be intensive as historical unrest episodes before 1960. These volcanic unrest episodes were not accompanied by signi cant increases of steaming activity in the steaming areas of the volcano; however, in March 2015, a new volcanic unrest episode started with a deep in ation and increase of both volcano-tectonic and DLF seismicity. This volcanic unrest was followed by a blowout of SPW in early May, and eventually, on June 29, a small phreatic eruption started and lasted until the early morning of July 1. The 2015 volcanic unrest after the eruption seems to have continued until late August, which is evident from crustal in ation monitoring by GNSS ).

Subsurface structure of Hakone volcano
Various geophysical and geochemical investigations over the last decade have modeled the subsurface structure of Hakone volcano. They are summarized in Fig. 2 and as follows. At approximately 20 km beneath the northern caldera rim of the volcano, DLFs occur sporadically. Since many of the DLF swarm events were followed by in ation of the edi ce and shallow volcano-tectonic earthquake swarms, DLFs are interpreted as a signal indicating migration of magmatic uid (Yukutake et al. 2015(Yukutake et al. , 2019. A seismic tomography study revealed velocity structure beneath Hakone volcano and showed that the volcano has an active magma-hydrothermal system (Yukutake et al. 2015). Yukutake et al. (2015) identi ed a high-Vp/Vs and low Vs body (Region 1), which was considered to represent a magma chamber located at approximately 10 km depth. Above Region 1, a low-Vp/Vs and low-Vs body (Region 2) was identi ed and interpreted as a uid-rich zone. The upper boundary of the Region 2 is shallower than 5 km, and interestingly, the boundary seems to reach near the surface just beneath Owakudani. Above Region 2 is the fracture zone, where most of the volcano tectonic earthquakes occur. Some fraction of the earthquakes in the fracture zone of Hakone volcano can be attributed to re-activation of pre-existing fractures caused by uid migrations (Yukutake et al. 2010(Yukutake et al. , 2011. Signi cant anisotropy in the shallow crust beneath Hakone volcano also indicates pre-existing fractures that are controlled by the regional stress eld (Honda et al. 2014). The fracture zone and Region 2 overlap slightly and both are considered to form the hydrothermal system.
A magnetotelluric study in and around Hakone volcano revealed a bell-shaped conductive body beneath the volcano, the top of which reaches the surface near Owakudani (Yoshimura et al. 2018). Since the bellshaped conductive body nests a resistive body beneath it, they are considered represent the hydrothermal system of the volcano. The bell-shaped body is interpreted as a smectite-rich zone, which was formed by a prolonged hydrothermal activity of the volcano. In Owakudani and the surrounding area (Fig. 1), a series of local high-resolution magnetotelluric surveys was conducted and revealed that the bell-shaped conductive body is The 2017 unrest of Hakone volcano was subtle to detect based on seismicity. Seismicity rates in 2017 were generally low and only 242 earthquakes were detected in the Hakone area by the routine analysis of Hot Springs Research Institute. This annual number is within the range of that in an ordinary year without volcanic unrest after 2000 (Fig. 5). However, slight increases of seismicity were observed in mid-April and early May at sea level beneath Mt. Kintoki at the northern rim of the caldera (Fig. 4b). Concurrently, in early May, the baseline length crossing Hakone volcano began to increase slowly and continued to increase until early November (Fig. 3). Daita et al. (2020) reported an increase in the CO 2 /H 2 S (C/S) ratio of fumarole gas in Kamiyu, a steaming area north of Owakudani. The increases in C/S ratio have been observed accompanying the volcanic unrest; however, this increase in C/S ratio was not sharp and did not attenuate swiftly, unlike the increases in C/S ratio accompanying the 2013 and 2015 unrest episodes  (Fig. 3). An increase in DLF events was also observed in early April (Fig. 3).

The 2019 unrest episode
The 2019 unrest episode at Hakone volcano appears to have begun with a slight increase in seismicity in March, which lasted until the end of October. A sudden seismic swarm occurred on May 18 beneath the western caldera rim (Fig. 4c). Although the location of swarm events was remote from Owakudani (3 km west and outside of the latest eruption centers), the number of earthquakes exceeded a set criterion and the Japan Meteorological Agency (JMA), which is in charge of volcano monitoring and alerting, announced a rise in Volcano Alert Level (VAL) from 1 to 2 for the volcano in the early morning of May 19, and the VAL2 continued until October 7. The baseline length crossing Hakone volcano began to increase in mid-March and continued until the beginning of August. The C/S ratio of Kamiyu also began to increase after the end of April; however, the increase in C/S ratio was not sharp and again did not attenuate quickly, similar to the 2017 unrest episode (Fig. 3). During the volcanic unrest, ratios of magmatic gases such as SO 2 and HCl relative to H 2 S, which is a hydrothermal gas, increased signi cantly in Owakudani, although a signi cant net increase in magmatic gas was not observed by a Differential Optical Absorption Spectroscopy (DOAS) campaign ( Fumarole temperature New fumaroles, which emit superheated steam (> 100 ºC) were created in the eruption center area in 2015. Most of them were formed during the eruption but some formed during the unrest phase before the eruption or even long after the eruption. Until the present, steam temperatures have been routinely measured for at least 20 fumaroles, ve of which are relatively intensive and long-lived and are shown in

Fumarole gas
An accurate chemical analysis of volcanic gas requires meticulous sampling and complicated lab procedures (Ozawa 1968), limiting the monitoring frequency. We thus launched a long-term test of simple gas measurements using a detector tube named Passive Dosi-tube (GASTEC Co. Ltd. (GASTEC 2018)). For this study, two sets of dositubes composed of H 2 S, SO 2 and HCl sensors (GASTEC No. 5D, 4D and 14D respectively) were installed near (2-4 m) the 15 − 2 fumarole vent (Fig. 1c). A set of dositubes is directly exposed to the air while another set is installed in a 500 ml ventilated container lled with silicagel granules (150 g) to prevent condensation of water in and around the dositubes. The dositubes were expected to measure ratios of volcanic gas in the atmosphere near the fumarole rather than a direct measurement of steam emitting from the volcano; thus, the observed ratio may be altered by processes in the atmosphere such as gas absorption by water droplets in the steam. However, we aimed to monitor obvious sequential changes in gas ratios with high frequency measurements. Since the dositube measures the volume fraction of the target gas in the atmosphere, the gas ratio is volumetric ratio (i.e., molar ratio) assuming an ideal gas. The sequential change of SO 2 /H 2 S and HCl/H 2 S ratios, both of which indicate the ratio of magmatic gas to hydrothermal gas, are shown in Fig. 8. Since the start of monitoring (March in 2018), SO 2 /H 2 S ratio show a constant decrease, and HCl remained nearly undetected until March 2019. However, both SO 2 /H 2 S and HCl/H 2 S ratios started to increase after March decline; however, both ratios are still higher than those before March 2019 at the time of writing (mid 2020).

Soil gas
Near the Owakudani steaming area, volcanic gas is seeping out from the soil under a building oor (Loc. Fig. 1c). We made weekly measurements of CO 2 and H 2 S in the ventilated air from under the building oor using detector tubes since the end the eruption (Fig. 9). Since the volcanic gas emitted from soil is not affected by nearby rainfall, and the building ventilation system enables almost constant ux of air from the sub oor, we can expect stable measurements of emitted gas. The soil gas shows a constant increasing in H 2 S while CO 2 remains almost stable. Interestingly, both H 2 S and CO 2

River water from the eruption center
The eruption center area forms the headstream of the Owakuzawa river. Thus, water from Owakuzawa is presumably affected by volcanic gas and natural hot springs within the area, and its chemical components can re ect hydrothermal activity. Indeed, just after the 2015 eruption, water from the river showed a signi cant increase in Cl and SO 4 ( Fig. 10; Mannen et al. 2018). After the eruption, the Cl and SO 4 contents showed constant decline; however, they apparently rose slightly at the beginning of the 2019 unrest. The Cl and SO 4 changes related to the 2017 unrest were ambiguous (Fig. 10).

Seismicity related to volcanic unrest after the 2015 eruption
We examined the depth variation of seismic events within the hydrothermal system to detect any changes related to the eruption. Figure 11 shows the cumulative ratio of earthquakes within the hydrothermal system beneath Owakudani during volcanic unrest episodes in this century. Interestingly, the seismicity depth change from before and after the eruption seems to be signi cant. Before the 2015 eruption, epicenters of more than 60% of earthquakes in and around the Owakudani steaming area were located shallower than 2 km depth, while such earthquakes comprise less than 40% of the total after the eruption. This observation indicates that the fraction of shallower earthquakes declined signi cantly after the 2015 eruption.

Discussion
Lowered VT activity and depressurization of the hydrothermal system  (Fig. 12a and b). Similar deep in ation was also recognized since the earliest phase of the post-eruptive unrest episodes in 2017 and 2019 by GNSS monitoring. DLFs were also detected in the earliest phase of the pre-, co-, and post-eruptive unrest. However, subsequent seismicity seems to be different for the post-eruptive unrest episodes. VT earthquakes beneath the central cone were not as prevalent during the post-eruptive unrest episodes, especially in the shallow region (Fig. 11), although signi cant seismic activity took place beneath the caldera rim, remote from the active fumarole (A and B in Fig. 4). Assuming pore-pressure increase is the trigger for VT seismicity (Yukutake et al. 2011;Mannen et al. 2018), such a signi cant difference can be interpreted as lower pore pressure rise in the hydrothermal system beneath the central cone, especially in the shallower part of the hydrothermal system (< 4 km deep). The insu cient pressurization of the hydrothermal system comparing to the preeruptive unrest episodes can be explained by the destruction of the cap-rock of the volcano by the 2015 eruption ( Fig. 12c and d). ). This line of evidence implies that the 2015 eruption tapped vapor from the uppermost part of the hydrothermal system and some vapor routes through the cap-rock created by the eruption are still alive as indicated by surviving high temperature fumaroles (≥ 160 ºC). Such degassing routes after the eruption can inhibit pressurization of the hydrothermal system during an unrest episode. The higher resistivity of the enlarged vapor pocket after the eruption (Mannen et al. 2019) indicates an increase in the vapor phase, presumably due to depressurization caused by the breach of the cap-rock (Fig. 12c). The intensive seismic and hydrothermal activities during the 2015 activity may have increased the permeability of the hydrothermal system beneath Owakudani and contributed to the inhibition of pressure increase in the region during the post-eruptive unrest (Sibson et al. 1975).
Even though the eruption breached the cap-rock beneath Owakudani, pore pressure seems to have increased in other hydrothermal systems in the adjacent area during the post-eruptive unrest episodes. The seismic swarm beneath the northern (2017; A in Fig. 4b) and western (2019; B in Fig. 4c) caldera rim during the unrest episodes could be the manifestion of uid injection from depth to the separated hydrothermal systems that caused a pore pressure rise and uid migration as observed in the previous unrest (Yukutake et al. 2011), although a detailed analysis remains yet to be done (Fig. 12d).

Increase of magmatic gas during the post-eruptive unrest
During the post-eruptive unrest episodes, magmatic gas species, such as SO 2 , HCl and CO 2 were observed to increase compared to H 2 S, which is a representative hydrothermal gas. In particular, since HCl tends to be degassed from shallow magma (Rouwet et al. 2017), this suggest intrusion and degassing of shallow magma during the post-eruptive unrest. However, no indication of shallow intrusion, such as low frequency earthquakes and harmonic tremor, was observed in our geophysical monitoring. Also, our DOAS survey observed no signi cant gross increase in SO 2 emissions. The emissions of SO 2 during the post-eruptive unrest episodes were less than the standard criterion to indicate magma involvement (> 100 t/d) (Symonds et al. 2001). SO 2 and HCl are magmatic, but they are highly soluble in liquid water. We thus attribute the increase of these gases to a change in liquid-vapor ratio in the uppermost part of the hydrothermal system (~ 150 m deep). In the environment where liquid water exists, SO 2 and HCl can be scrubbed from the coexisting volcanic gas; thus the increase of these gases can imply drying-out of the hydrothermal system (Symonds et al. 2001). Since the fumarole temperatures showed no signi cant increase during the posteruptive unrest episodes (Fig. 6), signi cant heating of the hydrothermal system that vaporized liquid water in the shallowest hydrothermal system is highly improbable. Instead, a slight increase of heat-ux changed the liquid-vapor ratio in the uppermost part of the hydrothermal system (~ 150 m deep) without any change of temperature in the system. The increase of heat-ux may be attributed to the injection of hot and pressurized magmatic uid into the hydrothermal system by the rupture of the sealing zone at depth, possibly at the brittle-ductile transition (Fig. 12c and d).
Previous model of volcanic unrest Ohba et al. (2019) proposed another unrest mechanism for Hakone volcano, which is possible even without newly derived magmatic uid from depth but instead steady degassing of magma. They emphasized the increase of atmospheric gases such as Ar and N 2 in the fumarole emissions a few months before the onset of the 2015 unrest and interpreted this as the development of a sealing zone between the hydrothermal and magma-peripheral systems. During the pre-unrest phase, permeability of the sealing zone decreased and the pressure of magmatic gas represented by CO 2 beneath the sealing zone increased. Meanwhile, pressure in the hydrothermal system dropped due to the lack of in ux of magmatic gas from the magma-peripheral system, allowing atmospheric gas to seep into the hydrothermal system, as detected by their survey of the fumarole gas.
This is an interesting model that interprets the results of gas monitoring; however, the model neglects geophysical observations such as deep in ation and DLFs in the early phase of the volcanic unrest episodes. If the deep in ation was caused by pressurization of volatile species beneath the sealing zone, de ation should have occurred after the breach of the sealing zone. Also, the DLF events in the very early phase of the unrest episodes indicates injection of magma or magmatic uid from depth. We acknowledge that it is hard to explain the increase of atmospheric species in the fumarole during the preunrest phase; however, we point out that this was observed only in fumarole N, which is located near the eruption center of the 2015 eruption. This means that the injection of atmospheric species seems to have occurred only in a very limited part of the hydrothermal system, probably near the surface of the steaming area. In the steaming area, reactivation of an almost extinct steam production well (SPW39) was recognized on April 17   Degassing of CO 2 during the unrest episodes and its implication The increase in C/S ratio from the very early phase of the unrest episodes at Hakone volcano can be explained by a breach of the sealing zone that leads to a signi cant pressure contrast between the hydrothermal system and a CO 2 reservoir beneath it. The sealing zone may not be necessarily be identical to the brittle-ductile transition as implied by Ohba et al. (2018). However, it is noteworthy that no signi cant change in SO 2 and H 2 S emission had been recognized in the peripheral fumaroles around the eruption center (Ohba et al., 2018). This means that only CO 2 was injected into the hydrothermal system, implying magma degassing deeper than the levels of SO 2 and H 2 S exsolution but shallower than that of CO 2 . The breach of the sealing zone and the migration of the hydrothermal uid from the magma-peripheral to the hydrothermal systems might have accompanied bubbling of CO 2 (Lowenstern 2001), although no corresponding geophysical signal was observed.
Degassing from newly-derived magmatic uid may have been the source of the CO 2 and the reason for the increase in the C/S ratio; however, the 2017 and the 2019 unrest episodes did not show sharp increases in the C/S ratio even though these events were accompanied by DLFs. Such a difference in the temporal change in C/S ratio during the volcanic unrest episodes indicates a change in physical structure caused by the 2015 eruption. We previously proposed a breach of the shallow cap-rock in. In addition to that, we propose a breach of the deep sealing zone during the 2015 eruption, with its annealing process after the eruption still incomplete. Assuming the sealing zone remained breached, volatiles steadily degassed from the magma chamber cannot be stored beneath the sealing zone in large quantity (Fig. 12c). Thus, only newly derived magmatic uid may contribute to the CO 2 rise during the volcanic unrest, and the observed CO 2 increase at the surface became subtle as observed during the unrest episodes in 2017 and 2019 (Fig. 12d). Based on this model, we expect that the sequential change of C/S ratio characterized by a strong increase just after the beginning of an unrest and a strong decrease after the climax as observed in 2013 and 2015 (Fig. 3) will resume after a restoration of the sealing zone in the future.

Model caveats
Deep in ation sources during Hakone unrest episodes have been located using inversion analysis; however their optimum depths were not unique; ranging from 6.5 km for the 2015 unrest and eruption  4.8 km deep) and considered the in ation source to represent a magma chamber. However, these source depths are shallower than Region 1, which is considered as representing a magma chamber from seismic tomography, and rather located in the hydrothermal system (Yukutake et al. 2015). We may thus need to assume magma intrusion or accumulation of magmatic uid in the deeper part of the hydrothermal system. As discussed above, intrusion of magma to the hydrothermal system is possible but relatively The volcanic unrest episodes of Hakone volcano observed after its latest phreatic eruption in 2015 were reviewed. Like the pre-and co-eruptive unrest, the post-eruptive unrest episodes that occurred in 2017 and 2019 were accompanied by in ation of the volcano and deep low frequency events, both of which can be interpreted as a magma replenishment to the deep magma chamber (~ 10 km). Seismicity during the post-eruptive unrest episodes was, however, signi cantly lower than that of pre-and co-eruptive unrest episodes of the volcano, especially in the shallower portion of the hydrothermal system. Such minimal seismicity can result from a breach of the cap-rock during the 2015 eruption, which inhibited pore pressure accumulation within the hydrothermal system beneath it. Accompanying the post eruptive unrest episodes, increases of magmatic components such as SO 2 , HCl and CO 2 relative to a hydrothermal component H 2 S of fumarole gas were observed. Amounts of Cl − in an arti cial hot spring generated by steam from deep steam production wells and river water were found to have increased. Intrusion of magma to shallow depth is, however, improbable given the stable fumarole temperature, the lack of nontectonic earthquakes (low frequency event and harmonic tremor), and limited SO 2 emission (< 100 t/d).
Instead, the increased proportion of magmatic gas (SO 2 and HCl) relative to the hydrothermal gas (H 2 S) during the 2019 unrest implies depletion of the liquid phase in the shallowest portion of the hydrothermal system due to the increased heat ux from depth.
The sealing zone of the volcano, presumably located near the brittle-ductile boundary, also seems to have been breached by the 2015 eruption as no sharp rise in the C/S ratio of the fumarole gas was observed during the post-eruptive unrest episodes. The sharp rise of the C/S ratio can be interpreted as a release of CO 2 that had accumulated beneath the sealing zone. We thus anticipate re-establishment of the temporal C/S change as seen during the pre-eruptive unrest episodes after the complete re-establishment of the sealing zone. The rupture of cap-rock by the 2015 eruption may lower the possibility of future phreatic eruptions originating from the shallow hydrothermal system; however, accumulation of magmatic uid in the deeper part of the hydrothermal system cannot be ruled out, and the possibility of a future large phreatic eruption cannot be eliminated.

Figure 5
Annual number of volcano tectonic earthquakes in Hakone volcano detected by routine analysis of HSRI.
Open circles indicate years that underwent volcanic unrest while dots indicate years without unrest.   The spikes in HCl/H2S ratio of the exposed set (HCl/H2S > 2) are considered to be errors caused by color changes due to steam precipitation within the tube.  Sequential changes in the chemistry of arti cial hot springs created by the steams from SPW52 (red circle) and SPW39 (orange triangle), and water from the Owakuzawa river (green diamond).

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