温泉科学（J. Hot Spring Sci.），70，70-79（2020）

短報

Abnormal Discharge of High-temperature Hot Spring Water
Observed at the Abandoned Well in the Okuhida
Hot Spring Area during the Earthquake Swarm
Occurred at Gifu-Nagano District, Central Japan
Muga Yaguchi1）(Received Jun. 4, 2020, Accepted Jun. 29, 2020)岐阜-長野地方の群発地震に伴って観測された
奥飛騨温泉郷の枯渇源泉の異常湧出
谷

口

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要

我1)
旨

2020 年 4 月下旬から長野県中部-岐阜県飛騨地方を震源とする地震が頻発し，この地震活動に伴って奥飛騨温泉郷の枯渇源泉から高温泉が再湧出した．奥飛騨温泉郷は活火山の焼岳の西方約 4 km に位置するため，今回観測された高温泉の再湧出現象が焼岳の火山活動に起因するかどうかを調べるために，温泉水の主要化学組成および水素・酸素安定同位体組成を分析した．
その結果，再湧出した温泉水は天水を主体とする中性の Na-HCO3・Cl 型で，地化学温度計による推定では温泉貯留層の温度は 142-177℃と推定された．本研究で推定した温泉水の起源，水質および貯留層温度はいずれも既往研究による報告と同質・同等であり，今回観察された枯渇源泉からの高温泉の再湧出が最近の焼岳の火山活動の変化に起因する証拠は見出せなかった．
キーワード：焼岳，奥飛騨温泉郷，地震，異常湧出，化学・安定同位体（δD, δ18O）組成，地
化学温度計

Abstract
Since late April 2020, earthquakes in the central Nagano Prefecture and the Hida regionof Gifu Prefecture have increased. During this seismic activity, an abnormal discharge ofhigh-temperature hot spring water was observed at the abandoned well in the Okuhida hot
spring area. As the Okuhida hot spring area is located about 4 km west of Yakedake volcano,the major chemical and stable isotopic compositions of hydrogen and oxygen of the discharged
1)70

Department of Volcanology Research, Meteorological Research Institute, Japan Meteorological Agency,
1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. E-mail：myaguchi@mri-jma.go.jp 1）気象庁気象研究
所火山研究部 〒305-0052 茨城県つくば市長峰 1-1.

第 70 巻（2020）岐阜-長野地方の群発地震に伴って観測された奥飛騨温泉郷の枯渇源泉の異常湧出

water were analyzed to investigate whether this abnormal discharge phenomena is causedby the volcanic activity of Yakedake volcano.
The analysis revealed the water was predominantly meteoric in origin, having a neutral
pH and Na-HCO3・Cl type, and the reservoir temperature was estimated to be 142-177℃ by
the geochemical thermometer. The origin, water quality, and reservoir temperature estimatedin this study were comparable to those in previous studies ; no evidence was found that theabnormal discharge observed at the abandoned well was caused by recent changes in the
volcanic activity of Yakedake volcano.
Key words : Yakedake volcano, Okuhida hot spring area, earthquakes, abnormal discharge,
chemical and isotopic (δD and δ18O) compositions, geothermometer

1.

Introduction

Since approximately 2 : 00 am on April 22, 2020, frequent earthquakes have occurred in thecentral Nagano Prefecture and the Hida region of Gifu Prefecture (Fig. 1a and b). In this region,
seismicity was also active in 1998, and the maximum seismic intensity of 5-lower was observedon August 7, 1998 (Gifu Meteorological Office, 2020). In 2020 activity, the maximum seismic
intensities of 4 were observed on April 23 in Matsumoto City of Nagano Prefecture and on May
19 and 29 in Takayama City of Gifu Prefecture, and after that, earthquakes with seismic intensity
2-1 continued to occur (Fig. 1c and Gifu Meteorological Office, 2020 a and b). The recent
earthquake information is available on the web site of the seismic intensity database of Japan
Meteorological Agency.
During this seismic activity, an abnormal discharge of high-temperature hot spring waterfrom an abandoned hot spring source well in the Okuhida hot spring area of Takayama City wasobserved. The Okuhida hot spring area consists of five hot spring groups, Shin-Hodaka, Tochio,
Shin-Hirayu, Fukuji, and Hirayu Onsen (Fig. 2). There are many high-temperature hot springsin this area (maximum outlet temperature is about 98℃), and one of the assumed heat sources ofthese hot springs is an active volcano, such as Yakedake that is located 4 km east of the hot springarea (Furuta, 2020). Based on chemical and isotopic analyses, the origin of those hot springwaters is considered mostly meteoric, but it has been pointed out that the hot spring waters
may contain a small amount of volcanic fluid (Mizutani et al., 1980 ; Koji et al., 2016). Therefore,
in this study, the major chemical and stable isotopic compositions of hydrogen and oxygen of thewater sampled at the abandoned well were analyzed to investigate whether the abnormal
discharge is caused by the volcanic activity.

2.

Hot spring sample and Analyses

Abnormal discharge of high-temperature hot spring was observed at an abandoned well of
the Okuhida Garden Hotel YAKEDAKE in the Shin-Hirayu hot spring group (Fig. 2). According
to an interview with the hotel, detailed information on this abandoned well, such as a drilling
data and hot spring analysis data, has already been lost, but the bottom depth of this well was
50 m, the outlet temperature was 70℃ or more, and the discharged rate was about 500 L/min, at
the time when the source was in use. This well stopped about 30 years ago due to scale clogging
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温泉科学

Fig. 1 Seismic activity around the Gifu-Nagano district (modified after original
data from Japan Meteorological Agency).  (a) Index map. (b) Epicentral map.
(c) Magnitude ─ Time diagram of earthquakes.

and was abandoned. The well was once cleaned more than 10 years after it stopped, but the hotspring water was not discharged and was abandoned.
The abnormal discharge reported in this paper was a temporary phenomenon, observed
twice from 07 : 00 on May 10 to 15 : 00 on the following day and from 07 : 00 to 09 : 20 on May72

第 70 巻（2020）岐阜-長野地方の群発地震に伴って観測された奥飛騨温泉郷の枯渇源泉の異常湧出

Fig. 2 Index map of sampling site and Okuhida hot spring area.  Base map was created byoverlaying the standard and shaded relief map of the Geographical Survey Institute ofJapan and a seamless geological map of the Geological Survey of Japan.  The double
circle (◎) shows the abandoned well.   Cross marks (×) are source wells currently in
use near the abandoned well.  The solid lines show the fault and estimated fault.

19 (Fig. 3a and b). Afterward, the water discharge stopped again (Fig. 3c). The water wascollected at approximately 08 : 30 on May 19, and the temperature was 77℃.
Analyses were conducted at the Meteorological Research Institute. The pH and electricalconductivity were measured using a handheld pH/EC meter (Horiba, D-74) equipped with aglass electrode (Horiba, 9615S-10D) for the pH measurement and a platinum-platinum black
electrode (Horiba, 3552-10D) for the EC measurement. The concentrations of F, Cl, and SO4 were
measured by using ion chromatography (Thermo Scientific, Dionex Integrion HPIC), and Na, K,
Ca, Mg SiO2 were measured by using a microwave plasma atomic emission spectrometer(Agilent, 4210 MP-AES). The total alkalinity as HCO3－ was determined by titration with H2SO4
to the final pH of 4.8 (Hach, AL-DT). Stable isotope ratios (δD and δ18O) were analyzed by usinga cavity ring-down spectrometer (Picarro, L2140-i) and reported in per mil (‰) relative to Vienna
Standard Mean Ocean Water (V-SMOW). The analytical results are listed in Table 1.

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Fig. 3 Photograph of the abandoned well.  During the abnormal discharge (May 19,
2020 ; a and b).  After the discharge stops (May 27, 2020 ; c).

3.
3. 1

Results and Discussion

Isotopic composition
As the δD and δ18O values of hot spring waters in the Okuhida area are almost the same as

the value of river water in the area, the hot spring waters are considered basically meteoric.
However, it is suspected that the hot spring waters in this area either contain volcanic water or
are affected by evaporation because some hot spring waters have δD and δ18O values close to
the volcanic gases of Yakedake volcano (Sugiura and Mizutani, 1978 ; Mizutani et al., 1980 ; Koji
et al., 2016). Further investigation is needed to determine whether the reason for such a heavy
isotope ratio is due to the contribution of volcanic gas or evaporation, but in any case, the δDand δ18O values of the hot spring water collected in this study is in good agreement with themeteoric line; there is no effect of volcanic water or evaporative concentration (Fig. 4).
3. 2

Geochemical feature of the sampled water
In general, hot springs containing magmatic volatiles have low pH and are rich in SO42－ andCl (e.g., Iwasaki, 1976). However, the hot spring water collected in this study has a neutral pH of
7.23 with a Na-HCO3・Cl type and does not contain much SO42－ (Table 1). This quality is very
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Table 1

Analytical results of the discharged water sampled on May 19, 2020, with reference values for the Okuhida hot spring area.

第 70 巻（2020）岐阜-長野地方の群発地震に伴って観測された奥飛騨温泉郷の枯渇源泉の異常湧出

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Fig. 4 δD-δ18O diagram for sampled water and the Okuhida hot spring area.  
◎, water sampled on May 19, 2020 ; ●, volcanic gases of Yakedake(Sugiura and Mizutani, 1978) ; +, hot spring waters in the Okuhida hot
spring area (Sugiura and Mizutani, 1978 ; Koji et al., 2016) ; △, river waterin the Okuhida hot spring area (Sugiura and Mizutani, 1978 ; Koji et al.,2016) ; GMWL, global meteoric water line (δD＝8・δ18O＋10) from Craig(1961).

similar to the hot spring water currently in use at the Okuhida Garden Hotel YAKEDAKE (Table1). Furthermore, the neutral Na-HCO3・Cl type has been typical in the Okuhida hot spring area
for more than 40 years (Mizutani et al., 1980 ; Kawamoto et al., 1984 ; Koji et al., 2016 ; Furuta,2020). Therefore, the effect of abnormal volcanic activity cannot be detected based on thechemical composition of the hot spring water that discharged from the abandoned well.
3. 3

Estimation of reservoir temperature using geothermometer
If the volcanic activity of Yakedake increases, underground temperatures may rise. To

investigate this possibility, the reservoir temperature of the hot spring discharged from theabandoned well was estimated by applying a geochemical thermometer. There are various
methods and application conditions for geochemical thermometers, but here, I used a Na-K geothermometer (Truesdell, 1976 ; Equation 1), a Na-K-Ca geothermometer (Fournier and Truesdell,1973 ; Equation 2), and a SiO2 geothermometer (Fournier, 1977 ; Equation 3), which have been
applied in the analysis of geothermal resources in the Okuhida hot spring area (Koji et al., 2016).
Na-K (Truesdell, 1976) :T (℃ )＝

855.6
log (Na/K)＋0.857－273.15

*Na and K in mg/L

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第 70 巻（2020）岐阜-長野地方の群発地震に伴って観測された奥飛騨温泉郷の枯渇源泉の異常湧出

Na-K-Ca (Fournier and Truesdell, 1973) :T (℃ )＝

1647
log (Na/K)＋4/3 [log (Ca 1/2/Na)＋2.06]＋2.47－273.15

⑵*Na, K and Ca in mg/L
SiO2, no stream loss (Fournier, 1977) :T (℃ )＝

1309
5.19－log SiO2

－273.15

⑶*SiO2 in mg/L
As a result, the reservoir temperature of the hot spring water discharged from the abandoned well was estimated to be 142-177℃ (Table 1). Although we must be cautious that thesame source should be compared to investigate the temporal change of the reservoir temperature
because the reservoir temperature varies even in a narrow area, the estimated temperature of142-177℃ is within the range estimated from the analytical value of the source wells near theabandoned well (96-184℃ : Table 1) and is also reasonable as the previously estimated reservoir
temperature in the Okuhida hot spring area (approximately 150-200℃ : Mizutani et al., 1980 ; Koji
et al., 2016). Therefore, based on this estimation, there is no evidence that the abnormal discharge
observed at the abandoned well was caused by the underground temperature change due to thevolcanic activity of Yakedake volcano.

4.

Concluding remarks

During the seismic activity at the Gifu-Nagano district, abnormal discharge phenomena of
high-temperature hot spring water from the abandoned well in the Okuhida hot spring area wasobserved. To investigate whether the abnormal discharge was caused by the volcanic activity of
Yakedake volcano, the major chemical and stable isotopic compositions of hydrogen and oxygen
were analyzed, and the reservoir temperature was estimated by applying geothermometers. As
a result, no evidence was found that this abnormal discharge was caused by an increase of volcanic activity of Yakedake volcano.
Although the relationship between earthquakes and hot spring anomalies has not been
completely clarified, many abnormal phenomena associated with earthquakes have been reported
in Japan as well as overseas, such as changes in water level, discharge rate, outlet temperature,chemical composition, and color appearance (e.g., Nishimura, 1999 ; Okusawa and Tsukahara, 2002 ;Itaba and Koizumi, 2007 ; Itadera et al., 2011 ; Sasaki, 2012 ; Cox et al., 2015). As shown in Figure
2, the abandoned well, in which the abnormal discharge was observed this time, is located at theposition where many faults exist, and it is assumed that the groundwater flow, including the hotspring water, in this area is restricted by these faults (e.g., Akiyama, 1978). Therefore, it may bereasonable to interpret that the abnormal discharge of the high-temperature hot spring waterobserved this time is caused by the change in the underground flow path due to the recent frequent earthquakes in the Gifu-Nagano district.

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Acknowledgments
I wish to express my sincere gratitude to the staff at Okuhida Garden Hotel YAKEDAKE
for providing sample water and valuable information and to Genki I. Matsumoto, professor
emeritus of Otsuma Women’s University, for providing comments and suggestions on a draft ofthis paper. Also, the prompt handling of the review process by Dr. Hideyuki Nagashima, Editorin-chief of JHSS, and valuable comments by two anonymous reviewers are deeply acknowledged.
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