SVC49-01

Room:102B

Time:May 25 14:15-14:30

Geostatistical analysis of VLF-MT resistivity data at the Tatun Volcano Group, Taiwan
KOMORI, Shogo1∗ ; KAGIYAMA, Tsuneomi2 ; CHAO, Benjamin fong3
1

GSJ,AIST, 2 Graduate School of Science, Kyoto Univ., 3 IES, Academia Sinica

Tatun Volcano Group (TVG) is composed of over twenty volcanoes, which were formed within faults at the northern tip of
Taiwan. Its active heat discharge from fumaroles and springs also suggests a large amount of volcanic fluids released from apotentially-existing magma beneath Chishinshan volcano (e.g., Yang et al., 1999; Ohba et al., 2010; Konstantinou et al., 2007).
Komori et al. (2014) conducted AMT surveys for a better understanding of this magma degassing, and found low resistivity
anomalies associated with the hydrothermal activity of the TVG.
In the TVG, we also conducted VLF-MT, which uses an electromagnetic wave with a singular frequency such as 22.1 kHz.
Generally, its high simplicity of use enables a huge numbers of measurements at low cost, compared to AMT surveys. Therefore,
VLF-MT could easily and extensively trace a surface distribution of hydrothermal fluids discharged from a deeper part, although
its sounding depths are only a few tens to hundreds meters. However, it is quite often that there are many missing data points,
mainly due to inaccessibility and high-level noises. Consequently, a sparse distribution of the surface resistivity is obtained,
which might result in wrong spatial features of the resistivity at the survey areas.
Geostatistics is potentially a good tool to solve the above problem. It was theoretically developed by Krige (1951) to evaluatea quality of mines by estimating data values at unsampled points from a viewpoint of statistics. To date, it has been widely used
for explorations of oils and geothermal/groundwater resources, by applying to data such as temperature, crack density, porosity,
permeability, and chemical composition (e.g., Koike and Ichikawa, 2006; Anderson and Fairley, 2008). Because this method ismathematically based on statistics, it would enable a fair inference with regard to a spatial property of the obtained data.
The present study applied the Geostatistics to over 300 VLF-MT resistivity data of the TVG, which were conducted in 20092010 by Kagiyama et al. (2010, JPGU meeting) and in 2012-2013 by Komori et al. (2014, CA meeting). The numerical code
GSLIB (Deutsch and Journel, 1998) was used for our geostatistic works. The data were first processed to make an experimental
semivariogram. It was used to estimate a model variogram by assuming values of nugget, sill, and range, on the basis of
the criteria put by Kitanidis (1997). The modeled variogram was input to Simulated Annealing (SA) process to estimate astatistically-plausible spatial distribution of the surface resistivity. In the presentation, we will show preliminary results by theabove processes, and examine its relation to structural features and hydrothermal activity of the TVG.
Keywords: Tatun Volcano Group, hydrothermal activity, VLF-MT, geostatistics, simulated annealing

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SVC49-02

Room:102B

Time:May 25 14:30-14:45

Hydrothermal system beneath the Jigokudani valley, Tateyama volcano, inferred from
AMT surveys and hot spring chemistry
SEKI, Kaori1∗ ; KANDA, Wataru2 ; TANBO, Toshiya3 ; OGAWA, Yasuo2 ; TAKAKURA, Shinichi4 ;USHIODA, Masashi2 ; SUZUKI, Atsushi1 ; SAITO, Zenshiro1 ; MATSUNAGA, Yasuo1
1

Tokyo Institute of Technology, 2 Volcanic Fluid Research Center, 3 Tateyama Caldera and Sabo Museum, 4 National Institute ofAdvanced Industrial Science and Technology
We carried out AMT surveys and the analyses of hot spring water in the Jigokudani valley (JV), Tateyama volcano, Japan.
The JV was formed by repeated phreatic eruptions some 40,000 years ago, and has laminated lacustrine sediments that arelocated at the base of an extinct crater lake. Currently, the JV is an active solfatara field dotted with hot springs and fumaroles,
and has experienced several anomalous events indicating the increase in volcanic activity. The objective of this study is toclarify the characteristic of electrical resistivity structure where multiple phreatic eruption events occurred, and to investigate thespatial relationship between subsurface structure and recent volcanic activity. We collected the AMT data at 25 locations in andaround the JV, and estimated a 3D resistivity structure using the inversion code of Siripunvaraporn and Egbert (2009). Electricalconductivity, temperature and pH were measured in 50 hot springs, and chemical analyses were performed on representative 12
samples of those springs. Hot springs in the JV showed features of strong acid Cl-SO4-type, and turned out to be derived frommagmatic hydrothermal fluids because of the high ratio of Cl/SO4 concentration. A highly conductive region with thickness ofapproximately 50m was detected beneath the most active geothermal field, and was interpreted as representing clayey sediments.
A slightly resistive portion was present beneath this layer and hot spring water of this area showed high ionic concentrations,which suggests that there are high temperature gases of magmatic origin. A deep feature of 3D resistivity structure suggeststhat such magmatic gases are provided from east of the JV, which is consistent with the location of seismically inferred magma
reservoir.
Keywords: Resistivity structure, AMT, Tateyama volcano, Phreatic eruption, Hydrothermal system, Clay cap

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SVC49-03

Room:102B

Time:May 25 14:45-15:00

Hydrothermal system around the active crater of Aso volcano inferred from a threedimensional resistivity structure
KANDA, Wataru1∗ ; UTSUGI, Mitsuru2 ; TAKAKURA, Shinichi3 ; INOUE, Hiroyuki2 ; OGAWA, Yasuo1
1Volcanic Fluid Research Center, Tokyo Institute of Technology, 2 Aso Volcanological Laboratory, Grad. Sch. Sci., Kyoto University, 3 National Institute of Advanced Industrial Science and Technology (AIST)This paper presents a three-dimensional (3-D) electrical resistivity model around the active crater of Aso volcano using theaudio-frequency magnetotelluric (AMT) data obtained during 2004-2005. The AMT data were collected at 43 locations on an
approximately 300-m grid in and around the Nakadake crater, the summit of the central cone complex, and were already interpreted by two-dimensional models (Kanda et al., 2008).
Over the past 80 years, all of the volcanic events within Aso volcano have originated from the 1st crater of Nakadake. The
volcanic activity of Aso has showed a cyclic activity pattern: a crater lake is formed at a quiet period and a Strombolian eruption
occurs at an active period. From November 2014, Strombolian eruptions have been observed at the 1st crater, which is considered to be a part of a similar activity cycle. This cyclicity of the volcanic activity implies that a specific structure that acts
to accumulate the energy required to trigger eruptions develops at approximately the same location beneath the 1st crater. The
objective of this study is to reveal such specific subsurface structure.
As a result of 3-D inversion using the code of Siripunvaraporn and Egbert (2009), we have obtained the following featuresin the 3-D resistivity model. A highly conductive zone is present at depths between 100 and 300 m beneath the 1st crater, but
unexpectedly the resistivity of the shallow subsurface around the Nakadake crater is not low in general. The low resistivity zone
widely distributed at a depth around sea-level which was found in the previous studies is seen only beneath the northern half of
the Nakadke crater including the 1st crater. We will discuss these features in connection with the variation sources inferred fromother geophysical studies.

Keywords: Aso volcano, resistivity structure, active crater, audio-frequency magnetotellurics, hydrothermal system

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SVC49-04

Room:102B

Time:May 25 15:00-15:15

Time variation in the chemical and isotopic composition of fumarolic gases at Hakone
volcano, Japan
OHBA, Takeshi1∗ ; AOKI, Ryou1 ; SHIGEMURA, Taisei1 ; SAGO, Masakazu1 ; DAITA, Yasushi2 ; CHENG, Lin3 ;IKEYA, Kosuke3 ; TSUNOGAI, Urumu3
1

Dep. Chem. School Sci. Tokai Univ., 2 Hot Spring Res. Inst. Kanagawa Pref., 3 Grad. School Environ. Studies Nagoya Univ.

The driving force of eruption is the degassing of magma or the explosion of hydrothermal reservoir. The volcanic gas contains
the component originating in the degassing magma or the hydrothermal reservoir. Therefore, the volcanic gas is essentially
important object for the understanding of eruption and the eruptive prediction.
Although no historical eruption was recorded at Mt Hakone, the swarm of volcanic earthquakes has been observed several
times. For example, in 2001 the swarm of volcanic earthquakes was observed with the deformation of volcanic body suggestinga pressure source the depth of which was estimated to be 7km (Daita et al, 2009). In parallel to the swarm of earthquakes, thesteam pressure in the borehole located in the geothermal area of Owakudani significantly increased.
Sampling and analysis of fumarolic gas
The Owakudani geothermal area is developed on Mt Kamiyama, one of the central cones of Hakone caldera. The fumarolicgases have been sampled and analyzed at two outlets in Owakudani geothermal area almost every month since May 2013 to Jan
2015. One fumarolic gas (T) is located near the parking for sightseeing people of Owakudani geothermal area. Another fumarolicgas (S) is located on the north flank of Mt Kamiyama, 500m far from the fumarole T. The temperature of gas at the outlets was
about 96C, which is close to the boiling temperature of the altitude of the fumaroles. The fumarole T associates the dischargeof hot spring water. The fumarolic gas was sampled in the evacuated glass bottle containing 20ml of 5M KOH solution. For
the determination of SO2/H2S ratio, KIO3-KI solution was reacted with fumarolic gas at the sampling site. For the sampling ofcondensed water of gas, a double glass tube was used for cooler. The solution in the evacuated glass bottle was analyzed along
the method by Ozawa (1968) to determine the amount of H2O, CO2, total S (=H2S + SO2) and R-gas. The R-gas was analyzed
by GC with Ar and He carrier gases to determine the relative concentration of He, H2, O2, N2, CH4 and Ar. The isotopic ratioof condensed water was determined by use of an IR-laser cavity ring down analyzer (Picarro). The isotopic ratio of H2 in R-gas
was determined by use of the continuous flow system combining a mass spectrometer (Thermo Fischer Scientific Delta V).
Result and Discussion
The both fumarolic gases, T and S was composed by mainly H2O vapor the relative concentration is about 98%. The secondary
dominant component was CO2, the concentration was 1 to 2%. The T and S fumarolic gases contained H2S of 0.2 to 0.4% and0.04 to 0.05%, respectively. The CO2/H2O ratio of T decreased in May 2013 until Oct 2013, then stable until Sep 2014. In Oct
2014, a small increase was observed in the ratio. A similar change was observed in the S fumarole. In Oct 2014, about 50 ofearthquakes occurred at Mt Hakone, the number of which was higher than the preceding months. The increase of CO2/H2O ratioin Oct 2014 looks to be correlated with the occurrence of earthquakes. The change in CO2/H2S was almost similar the change
in CO2/H2O ratio for both T and S fumaroles. An interesting change was found in the He/H2O ratio of T fumarole. The ratio
increased in Sep 2014, which is one month before the increase in CO2/H2O ratio. The increased He/H2O ratio decreased in Oct
2014, which looks like a precursor of the change in CO2/H2O ratio and also the swarm of earthquakes. For the estimation ofhydrothermal reservoir temperature, the apparent equilibrium temperature (AET) was calculated between H2O and H2 assuming
the exchange of D between those species. The AET was high as 140C in May 2013 at T fumarole. The AET deceased quickly
down to 100C. A small increase was observed in Dec 2013 and Aug 2014. The AET of S fumarole was almost stable, showing
a gradual increase from 92C in May 2013 to 104C in Aug 2014. The AET seems to be useful for the estimation of reservoir
temperature.
Keywords: Hakone, Volcanic gas, Chemical composition, Volcanic earthquake, CO2, He

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SVC49-05

Room:102B

Time:May 25 15:15-15:30

Volcanic plume measurements of Ontake volcano by unmanned aerial vehicle (UAV)
MORI, Toshiya1∗ ; HASHIMOTO, Takeshi2 ; TERADA, Akihiko3 ; SHINOHARA, Hiroshi4 ;KAZAHAYA, Ryunosuke4 ; YOSHIMOTO, Mitsuhiro5 ; TANAKA, Ryo2
1

Laboratory for Earthquake Chemistry, Graduate School of Science, the Univesity of Tokyo, 2 Graduate School of Science,
Hokkaido University, 3 Volcanic Fluid Research Center, Tokyo Institute of Technology, 4 Geological Survey of Japan, AIST,
5
Mount Fuji Research Institute, Yamanashi Prefectural Government

On Sept. 27, 2014, a phreatic eruption occurred at Ontake volcano, central Japan, caused unprecedented disaster and took the
lives of many tourists. The explosion plume rose up to nearly ten thousand meters and ash fall was observed as far as about 100
km east in Yamanashi Prefecture (JMA, Sept., 2014). The plume activity gradually decreased but the volcano still vigorously
emits white plume as of Feb. 2015.
We made plume observation using a multicopter UAV on Nov. 20 and 21, 2014 to carry out following four kinds of measurements; sulfur dioxide flux, in-plume multiple gas concentrations, thermal images of the crater area and collection of fine particles
inside the plume. In Ontake volcano measurements, the UAV was expected to fly 4000 m roundtrip distance at the altitude of3000 m. Thus, the respective instruments were made compact and light to fit the payload of 1kg. The SO2 flux measurements
was carried out by traversing below the plume with a compact UV spectrometer operated by a single-board-computer. In themultiple gas concentration measurements, H2 S, SO2 , CO2 , H2 sensors and a thermohygrometer (H2 O)were loaded to the UAV
and flew inside the plume. In these in-plume flights, an adhesive film was used to collect fine particles inside the plume. An
infrared thermography camera mounted on the UAV was used to image Jigokudani crater area to reveal the thermal condition.
On Nov. 20, takeoff and landing of the UAV was located at Iimorikougen ropeway station about 3.5 km ENE due to NE plumedirection. On Nov. 21, the UAV was operated from Tanohara park about 3 km SE from the crater area to approach the plumeflowing in ESE direction.
Sulfur dioxide flux and SO2 /H2 S molar ratio obtained by the UAV plume measurements were 130-140 ton/day and about 0.09
for Nov. 20-21, 2014. No volcanic H2 , CO2 and H2 O were clearly detected due to high mixing ratio of atmosphere. The thermal
images of Jigokudani crater showed maximum temperature of 90.6 ℃ corresponding to the boiling point of the local altitude.
Although some fine particles including crystalline minerals were found on the adhesive films, they were probably dusts from theground.
Prior to the UAV measurements on Oct. 9, in-plume multiple gas concentration measurements were carried out using MultiGAS analyzer onboard a helicopter and we obtained SO2 /H2 S molar ratio of about 0.3, indicating a significant decrease of theratio by Nov. 20-21, 2014. Sulfur dioxide flux was over 1000 ton/day just after the eruption and was 400-500 ton/day (JMA, Oct.
2014) on Oct. 9 and decreased to 130-140 ton/day in two months after the onset of the eruption. In contrast, a total sulfur flux on
Oct. 9 was about 1000 ton/day and was about 800 ton/day on Nov. 20-21 showing no significant change. The plume activity of2014 Ontake volcano eruption is characterized by high total sulfur flux, however, low fumarolic temperature and low SO2 /H2 S
ratio suggest that volcanic gas at Ontake volcano is not directly emitted from magma.
After 1979 Ontake volcano eruption, SO2 /H2 S molar ratio showed significant decrease from 5 soon after the eruption to 0.1 anyear after the eruption (Ossaka et al., 1983). The decrease of SO2 /H2 S molar ratio after the eruption is the common characteristicof the two eruptions in 1979 and 2014. Detailed comparison of the two eruptions would be important to understand the future ofongoing Ontake volcano’s activity.
Keywords: Ontake volcano, volcanic gas, UAV

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SVC49-06

Room:102B

Time:May 25 15:30-15:45

Case study of the behavior of isotope in several hot-spring and geothermal field part2
YANAGISAWA, Norio1∗
1

Institute for Geo-resources and Environment, AISTThe Oxygen and hydrogen isotope data provide information about the origin and evolution of geothermal waters and relationship with hot spring reservoir and river water.
According to Enhanced Geothermal System (EGS), stable isotope data show the water source in reservoir or injected water.
Recently, several EGS project are carried out in USA, Australia and Europe. And after nuclear power plant accident at 2011, we
need new EGS project and the review of previous project at Hijiori. At Hijiori EGS site, the long term circulation test was carriedout from 2000 to 2002. The tracer response changed with circulation (Yanagisawa et al., 2002, 2003) and calcium carbonate andanhydrite scale precipitated in circulation system (Yanagisawa et al., 2008). This shows the geochemical condition change in
Hijiori system. And to develop EGS project, we have to survey the relationship between hot spring and EGS reservoir. Then,
this presentation shows the results of stable isotope change of production and injection well at Hijiori test site and discuss thereservoir condition during circulation test.
During this test, hydrogen and oxygen isotope composition of production wells (HDR-2 and HDR-3), injection well, riverwater and hot springs were analyzed.
The isotope ratio of the river water and hot springs plot along the meteoric water line. The isotope ratio of hydrogen andoxygen ratio of HDR-2 increased with circulation and reached value of -45 ‰ of δD and -4.6 ‰ of δ 18 O at end of May 2001,
respectively. However δD and δ 18 O rapidly decreased to -54.4 ‰ and -8.6 ‰ respectively from May to June and came close tometeoric line. The isotope ratio of HDR-3 plot at higher δ 18 O than that of HDR-2 and the change of the isotope ratio of HDR-3
is similar as that of HDR-2.
The change of δ 18 O value of HDR-2 is corresponds with Cl concentration change during the circulation progress.

Keywords: hot spring, geothermal, isotope, Hijiori, EGS

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SVC49-07

Room:102B

Time:May 25 15:45-16:00

Numerical modeling for a broad geothermal system of Kuju Volcano
FUJIMITSU, Yasuhiro1∗ ; ITO, Yohei2 ; NISHIJIMA, Jun1 ; OKA, Daisuke3
1

Faculty of Engineering, Kyushu University, 2 Graduate School of Engineering, Kyushu University, 3 Geological Survey of
Hokkaido, Hokkaido Research Organization
The Kuju volcanic field is located in the southwestern part of Oita prefecture, Japan, and consists of some andesitic volcanoes. There are many hot springs and several geothermal power stations (Hatchobaru, Otake, Takigami etc.) in this field. In
the previous studies on the Kuju volcanic field, the numerical models were mainly for the power station areas or the center ofKuju Volcano where fumarolic activity appears. Therefore, we constructed a new conceptual model that includes the center ofthe volcano and all of the geothermal power station areas in order to attempting to construct an integrative numerical model ofa broad geothermal system that has the scale of the volcanic field to explain the existence of the hydrothermal systems, which
are generated by a heat source like a magma chamber in the volcanic field. Based on this conceptual model, we constructed a
numerical model that replicates the hydrothermal systems of the geothermal power station areas roughly although the numericalmodel is relatively simple.
Keywords: Kuju Volcano, broad geothermal system, hydrothermal system, numerical model

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SVC49-08

Room:102B

Time:May 25 16:15-16:30

Re-evaluation of hydrothermal activity based on magnetic measurements at KusatsuShirane volcano, Japan
TAKAHASHI, Kosuke1 ; FUJII, Ikuko2∗
1

Kakioka Magnetic Observatory, Japan Meteorological Agency, 2 Meteorological College, Japan Meteorological Agency

Kakioka Magnetic Observatory, Japan Meteorological Agency commenced geomagnetic measurements around the summit
crater lakes of Kusatsu-Shirane volcano in 1976 to monitor the thermal activity of the volcano. We re-analyzed the geomagnetic
data over 36 years starting in 1978 and evaluated the long-term thermal activity beneath the crater lakes.
Changes in the geomagnetic total intensity due to thermal activity were observed in three periods: 1982-1985, 1988-1991,
and 1996-2012. A thermal-demagnetization source related to phreatic eruptions during 1982-1983 was estimated to be 400 m
below Mizugama crater lake during 1982-1985. The demagnetized body was also detected at a depth of 600 m below Mizugama
crater lake in 1988-1991 when substantial thermomagnetic signals and numerous volcanic earthquakes were observed without
an eruption. These demagnetization sources represent a hydrothermal fluid reservoir beneath the summit area. We suggest apossible mechanism of the phreatic eruptions during 1982-1983, based on our thermomagnetic model and previous geophysicaland geochemical studies.
In contrast, magnetization associated with cooling of rocks beneath the crater lakes was recorded from 1996 to 2012. According to our thermomagnetic modeling of this period, the source of the magnetization was 400 to 700 m below an area immediately
northeast of Yugama crater lake. In addition, we found that the cooling migrated gradually to shallower depths during this period.
These suggest that the decline phase of the volcanic activity was under way at this period.
Changes in the geomagnetic total intensity after 2013 show a different tendency from those before 2013. Therefore, thesesuggest that the decline of volcanic activity came to an end in 2013.
Keywords: Kusatsu-Shirane volcano, geomagnetic measurement, thermomagnetic effect, phreatic eruption, hydrothermal fluid

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SVC49-09

Room:102B

Time:May 25 16:30-16:45

Evolution of a hydrothermal system of Kusatsu-Shirane volcano inferred from aerial infrared surveys in the nighttime
SAKAMOTO, Yuri1∗ ; TERADA, Akihiko1
1

Tokyo Institute of Technology

Kusatsu-Shirane volcano is one of the most active volcano in terms of release of a large amount of heat as volcanic gas and hot
springs. In addition to steaming grounds and fumaroles, the Shirane pyroclastic cone contains a hot crater lake, locally referred
to as Yugama, which shows interesting variations of water temperature and chemical concentrations.
Intense earthquake swarms have occurred at shallow depth of the Shirane pyroclastic cone since March 2014, accompanied
by a ground deformation, changes in geomagnetic field and chemical concentration of volcanic gas. A location of the pressuresource is determined by network of our tilt meters at 550 m depth from Yugama crater lake. These unusual activities may becaused by an accumulation of hydrothermal hot water at shallow depth of Yugama crater lake, so we expect changes in thermal
activities around the Shirane pyroclastic cone.
To detect changes in thermal activities on Kusatsu-Shirane volcano, an aerial infrared survey was carried out on 25 October
2014. The observation was done in the nighttime because even slight anomalies in ground surface temperature can be detected.
Images of detailed ground temperature enable us to estimate precisely heat- and water-discharge rates from steaming ground as
well as Yugama crater lake.
Steaming grounds around Yugama crater lake show no significant changes compared with the record obtained in 2012, although changes in chemical compositions of volcanic gas emitted here were observed. On the other hand, we detect an increasein water surface temperature of Yugama crater lake. Water evaporation rate in 2014 is estimated approximately to be 5 kg/s,
which is 2.5 times higher than that of the value in 2012. Such an increase in water temperature occurred in May 2014 suggested
by monitoring of Yugama crater. Records of tilt meters indicate that an accumulation rate of the hydrothermal water decreased
after May 2014. We consider that the reservoir of hydrothermal water fractured gently in May 2014. Consequently, an additional
input of hydrothermal water started at the bottom of Yugama crater lake.

Keywords: Kusatsu-Shirane Volcano, airborne IR surveys, hot crater lake, steaming ground, hydrothermal system, heat-discharge
rate

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SVC49-P01

Room:Convention Hall

Time:May 25 18:15-19:30

Near surface structure of a Crater on mountain side of Mt. Shiretokoiozan and its mechanism of molten sulfur eruption
YAMAMOTO, Mutsunori1∗ ; GOTO, Tada-nori2
1

Earthscience.jp, 2 Graduate School of Engineering, Kyoto University

Mt. Shiretokoiozan, located in the middle of the Shiretoko Peninsula in Hokkaido Japan, is famous as molten sulfur eruption.
Since 1857, Mt. Shiretokoiozan has erupted with molten sulfur four times. At the last eruption from February through Octoberin 1936, approximately 200,000 tons of molten sulfur welled out of the Crater I, located on the northwestern mountain side, andthe brown liquid sulfur flowed into the Kamuiwakka Creek. The eruption was closely observed and documented for ten days in
September by Watanabe. He presumed the underground structure and possible existence of a molten sulfur reservoir under thecrater based on the periodic activity.
Since 2005 we have implemented further researches to find out the near-surface underground structure of the Crater I for
discussing the mechanism of molten sulfur eruption. The methods are various; geological survey, DC resistivity survey, selfpotential exploration, and chemical analysis of gas and hot spring.
As a result, we found that the crater had been created by depression due to hydrothermally altering of andesite lava sheet andthe following running-off of material. We suggest that there is a chamber under the crater and molten sulfur is supplied from theaquifer at the eruption where the sulfur had been generated during the inter-eruption period by chemical reactions of volcanic
gasses.
The geology of the Crater I and its vicinity is mostly composed of hydrothermally altered clay, gravel and onion structured
floats. Originally this area was composed of several-meters-thick sheet lava layers of andesite, which had flowed from the summitof mountain. The volcanic gasses, mostly hydrogen sulfide and carbon dioxide, come out through fumaroles and craters locateddirectionally along conjugate faults cutting through this area. Original andesite rocks suffered weathering by the reaction withthose acid gasses into onion structured boulders and seems to change to white gravels and clay. Because the small clay particles
and the gravel at ground surface have been drained, large boulders in several meters were left on the ground and they covered
most of this area.
In the cross section around the Crater I, we conclude that the crater is a depression hole opening in the hydrothermally alteredlava. An aquifer among sheet lava goes under the Crater I and hot spring wells in the crater. At the higher elevation than the
Crater I, there is a small creek called the Io Creek. And at the lower altitude, the Kamuiwakka Creek is located. We interpret thatthe underground water comes from the Io Creek and flows through lava-sheet aquifer, and upwells at the Crater I as well as hot
springs in the Kamuiwakka Creek.
Volcanic gasses, hydrogen sulfide and sulfur dioxide, dissolve into the underground water, and were involved in the chemical
reaction to generate the accumulation of sulfur in the aquifer. At the fumarole in the Crater I, water soluble sulfur dioxide is justbarely detected. At the same time, the gas temperature has never been higher than boiling point of water. These are the evidencesthat most of volcanic gas passed through underground water.
We suggest that the sulfur in the aquifer melts and flows into the chamber under the Crater I at the active term of volcano, andmay eject molten sulfur periodically. The amount of the molten sulfur erupted in 1936 was approximately 200,000 tons. If thechamber had reserved all amount of sulfur erupted in 1936, its volume might have been as much as 100,000 cubic meters. We
suppose the possible chamber size is much smaller than the estimation. It is concluded that the aquifer supplied the molten sulfur
continuously to the chamber, while the chamber made a periodic eruptions.

Keywords: Molten sulfur eruption, Hydrothermal alteration, Shiretokoiozan, The Crater I, DC resistivity survey, Hot springs inthe Kamuiwakka Creek

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SVC49-P02

Room:Convention Hall

Time:May 25 18:15-19:30

Mineralogical study of non-juvenile material in volcanic products at Tokachidake volcano, JapanIMURA, Takumi1∗ ; NAKAGAWA, Mitsuhiro2 ; MINAMI, Yusuke1 ; TAKAHASHI, Ryohei1 ; IMAI, Akira1 ;OHBA, Tsukasa1
1

Akita University, 2 Hokkaido University

Temperatures, depths, and fluid chemistry of sub-volcanic hydrothermal system were estimated based on mineralogical analysis of eruptive products of the 1926 and the 4.7-3.3ka eruptions at Tokachidake volcano, Japan. The deposit of the 1926 eruption
can be divided into three layers according to volcanic phenomena; the lower debris avalanche deposit, the middle hydrothermal
surge deposit and the upper debris avalanche deposit. The deposits of the 4.7-3.3 ka eruption can be divided into four pyroclastic
flow deposits layers; one from the 4.7 ka eruption and other three from the 3.3 ka eruption. Every deposit contains abundanthydrothermally-altered lithic fragments. Three layers of the 1926 eruption exclusively consist of altered lithic fragments without
any juvenile fragments. Minerals identified in the bulk sample of the 1926 eruption deposit are cristobalite, smectite, sericite,
kaolinite, alunite, gypsum and pyrite, and those in the deposits of the 4.7-3.3ka eruptions are cristbalite, tridymite, quartz, sericite,
pyrophyllite, alunite, plagioclase and hyperthene. Mineral assemblages of individual fragments were also determined with combination of SEM-EDS and XRD. The 1926 eruption product is characterized by the coexistence of cristobalite, alunite and/or
smectite in the fragments, whereas the 4.7-3.3 ka eruption product is characterized by the coexistence of pyrophyllite and quartz.
The mineralogical contrast implies difference in hydrothermal condition between the 4.7-3.3 ka and the 1926 eruptions. The former eruptions were derived from hotter (>230 C) and deep (1-2 km) hydrothermal systems and the latter from a colder (<100 C)
and shallow (near-surface) hydrothermal system, although both volcanic products are characterized by sulfuric acid fluid which
is typical in hydrothermal systems at volcanic centres.
Keywords: sub-volcanic hydrothermal system, hydrothermally-altered lithic fragment, Tokachidake volcano, eruption products
in 1926, pyoroclastic flow deposits in 4.7-3.3 ka

1/1

SVC49-P03

Room:Convention Hall

Time:May 25 18:15-19:30

Resistivity structure of geothermal area at south area of Yakedake Volcano
SUGANO, Kotaro1∗ ; YAMAYA, Yusuke2 ; MOGI, Toru3 ; SHIGEFUJI, Michiko3 ; OKUDA, Mao1
1

Hokkaido University Graduate School of Science, School of Science, 2 National Institute of Advanced Industrial Science andTechnology, 3 Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University
Resistivity structure of geothermal system at south area of Yakedake Volcano
Sugano, K., Y. Yamaya, T. Mogi, M. Shigefuji and M. Okuda
Some hot-springs and fumaroles are seen around the Yakedake Volcano. High temperature hot springs such as Nakanoyu andShirahone hot springs are located at the south area of the Yakedake volcano, but the relations between the volcano and geothermal
system have not been clarified yet. Geophysical studies concerning the structure of geothermal fluid reservoir and heat source ofthe hot springs have never been performed in this area.
Hokkaido University carried out a MT survey to clarify the subsurface structure at six sites between Sirahone hot spring and
Sawando area in 2013 and indicated distribution of geothermal fluid reservoir beneath Shirahone hot springs and Sawando area
(Yamaya et al., 2014). But they did not clarify the extent of geothermal fluid reservoir under these area. We installed two additional MT sites each at outside of the previous survey area in 2014 to investigate extension of these reservoirs.
We recorded MT signals for about 48 hours at each site, and obtained the apparent resistivity and phase at a frequency range of
0.03-100Hz. We applied the remote magnetic reference (Gamble et al., 1979) and manual data editing by MTEDITOR to remove
local electromagnetic noises.
The magnetotelluric phase tensors (Caldwell et al., 2004) and induction vectors were calculated to verify structural dimensionality and to determine the 2D strike direction for the 2D inversion. According to the phase tensor ellipse and induction vector at
the lower frequency range, the deeper layer have 2D structure and we decided that 2D strike direction is N60W in this area.
We performed two types of 2D inversion, which used the TM mode and TE+TM modes, respectively. We used the inversion
code proposed by Ogawa and Uchida (1996), which minimized ABIC as convergent criterion in the iteration process. The ABIC
criterion includes smoothness, least square mean error and static shift correction.
As a result, we indicate that geothermal fluid reservoir correspond with low resistivity is extending at directly under the Shirahone hot spring area, and it ranges in the limestone body. Dissolved limestone is origin of milky hot spring that characterizing
the Shirahone hot spring. The low resistivity zone was also found at the depths of 500m down in the Sawando area. Although
no geothermal manifestation is recognized at the surface of the Sawando area, but this low resistivity zone probably indicates ageothermal reservoir.
Furthermore, these two low resistivity structures corresponding each geothermal fluid reservoir join together at the depths of
2 km below. The columnar low resistivity zone extends to deep. Comparing the geology, the Sakaitouge fault runs through at
the columnar low resistivity zone. The resistivity structure suggests that geothermal fluid ascends from deeper zone along the
Sakaitouge fault. Based on this result, we can propose two possibilities of the heat source of geothermal fluid. One possibility isthat hot volcanic fluid flows out from the Yakedake volcano along the fault. The other is that heating water is ascending along
the fault from the hot rock area extending in the Japanese Northern Alps area.
Keywords: Geothermal area, Resistivity structure, Yakedake

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SVC49-P04

Room:Convention Hall

Time:May 25 18:15-19:30

Conductivity distribution of the surface layer around volcanic area in central Kyushu
KAGIYAMA, Tsuneomi1∗ ; UTSUGI, Mitsuru1 ; YOSHIKAWA, Shin1
1

Graduate School of Science, Kyoto University

Kagiyama and Morita (2008) indicated magma degassing is one of the important factors to control magma ascending. Discharge rate of volatiles from magmas through a crater has been estimated by direct observations of CO2 and SO2 gases, suchas COSPEC and DOAS, and by geochemical methods. However, discharge rate of volatiles through a volcanic aquifer has notbeen clarified because of difficulty of obtaining geochemical samples spatially from deeper part of volcanic aquifer. Electricalconductivity of ground strongly depends on the conductivity of pore water, and VLF-MT survey is a powerful tool to clarify the
distribution of hydrothermal water in the shallow depth. On this aspect, the authors carried out VLF-MT survey around volcanoes
in central Kyushu, Japan.
Aso Caldera: Aso Caldera has acid crater lake in Nakadake Volcano, which is one of the post caldera cones, and has many hot
springs within the caldera such as Uchinomaki, Jigoku & Tarutama. Conductivity distribution shows two typical features; caldera
floor has almost homogeneous and high conductivity (>10mS/m), while the post caldera cones show wide range. Most cones
have lower conductivity (<3mS/m), except active geothermal fields around Naka-dake Craters and western part of post caldera
cones (>30mS/m). Just north and south of Naka-dake Craters, high conductivity (3-10mS/m) was identified. This suggests down
flow of hydrothermal water from Naka-dake Craters to the flank of post caldera cones. Caldera floor has almost homogeneous
conductivity. This feature is explained by the fact that the caldera floor was under the lake until 9 ka and is covered by lake deposit. However, extremely high conductivity (>30mS/m) was found at several areas in the caldera floor. These high conductive
zones and Naka-dake are located along the NNW-SSE line. Hydrothermal water may be supplied along this tectonic line.
Kuju volcanic group: Central cones have lower conductivity (<3mS/m), except active geothermal fields around Iwo-yama(>30mS/m). Around volcanic cones, high conductivity zone was identified. This suggests down flow of hydrothermal waterfrom volcanic cones to the flank. On the other hand, another high conductivity zone is identified along the tectonic line; OitaKumamoto tectonic line, Yufuin Fault, etc. Hydrothermal water may be supplied along this tectonic line.
Tsurumi&Garan volcanoes (Beppu geothermal area): Many lava domes show low conductivity (<3mS/m), except active
geothermal spots in Tsurumi and Garan domes. High conductivity zones are identified along some tectonic faults; E-W trend
from Garan crater to Kan’nawa hot spring, along Asamigawa Fault, etc. The area size of high conductive (>30mS/m) zone
around northern Beppu hot springs (320MW) is estimated about 4 km2.
These results suggest Geothermal activity dominant volcanoes have wide high conductivity area related with degassing frommagma. And VLF-MT survey will be effective method to identify tectonic line around volcanic and geothermal field.
Keywords: Active volcano, Electrical conductivity, Central Kyushu, Geothermal activity

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