著者

代田 寧 大場 武 谷口 無我 十河 孝夫 原田 昌武

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.130, no.6, pp.783-796, 2021-12-25 (Released:2022-01-28)

参考文献数

24

被引用文献数

2 2

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Earthquake swarms have occurred with volcanism repeatedly at Hakone volcano in Kanagawa prefecture, Japan. In 2015, a phreatic eruption took place about two months after the start of an earthquake swarm. Hakone volcano is a popular tourist destination. If it is possible to forecast at the early stages of an earthquake swarm whether or not an eruption will occur, the forecast could contribute to preventing disasters involving tourists. At Hakone volcano, increases in the ratio of components (CO2/H2S) contained in the volcanic gas from fumaroles were observed in synchronization with earthquake swarms and ground deformation in 2013 and 2015. Similar increases in CO2/H2S ratio were also observed in 2017 and 2019, although the increases in the CO2/H2S ratio in 2017 and 2019 were not as sharp as those in 2013 and 2015. Furthermore, the maximum values of the CO2/H2S ratio in 2017 and 2019 were lower than the values in 2013 and 2015. These differences in the CO2/H2S ratio may be related to the limited and smaller scale of volcanic activity in 2017 and 2019 relative to 2013 and 2015. Another explanation for the difference is a possible irreversible change in the underground structure of the Owakudani area, which is a geothermal area around Hakone volcano, because the phreatic eruption took place in the Owakudani area in 2015. During all four seismically active periods in 2013, 2015, 2017, and 2019, the CO2/H2S ratio decreased simultaneously with decreases in the number of volcanic earthquakes. The lower limit of CO2/H2S ratios after the peak of the CO2/H2S ratio time series was about 20 in all periods. This implies that subsequent unrest would not start until the CO2/H2S ratio drops to about 20. The CO2/H2S ratio might be an effective tool for forecasting activity at Hakone volcano. During the active periods in 2013, 2015, 2017, and 2019, extensions of the baseline across Hakone volcano were observed by GNSS with increases in the CO2/H2S ratio. A good correlation was found between the extensional velocity of the baseline length and the increasing rate of the CO2/H2S ratio. These variations could be examined at the early stage of unrest. The findings argue that the CO2/H2S ratio is a promising tool for predicting and evaluating volcanic activity at Hakone volcano.

著者

北川 隆洋 風早 竜之介 谷口 無我 篠原 宏志 福岡管区気象台 大分地方気象台

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.1, pp.113-123, 2022-03-31 (Released:2022-04-26)

参考文献数

21

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Volcanic gas composition provides us a crucial clue to investigate magma plumbing and geothermal systems. Sensor-based instruments named Multi-GAS have been used for monitoring the volcanic gas compositions at volcanoes. A sensitivity of sensors changes with time caused by deterioration, masking volcanic signals especially during long-term monitoring. Frequent calibration of the sensors is desirable for precise monitoring; however, that is pragmatically not easy because a location of a targeted volcano is remote and rural in general. Sophisticated evaluation of the long-term changes in the sensor sensitivity has not been made yet. In this study, we examined the sensitivity change of the chemical sensors within the Multi-GAS during long-term observations by comparing with other methods such as gas detector tubes and gas sampling. The volcanic gas compositions were monitored using Multi-GAS at Kusatsu-Shirane volcano and Kuju volcano, Japan. Intermittent gas composition measurements using gas detector tubes and gas sampling were conducted at fumaroles around where the Multi-GAS stations are installed. Some disagreements of the CO2/H2S ratios are observed between those measured using the Multi-GAS from those measured using other methods. In such cases, large decreases of the H2S sensor sensitivity were found by the sensor calibration after the monitoring. We found a roughly linear behavior of the H2S sensor sensitivity changes with time based on a long-term sensor sensitivity monitoring in a laboratory and propose a simple linear sensitivity correction of the H2S sensors using the calibration results obtained before and after the monitoring. The corrected Multi-GAS results agree well with the results of other methods. Our results open up a possibility for extraction of volcanic signals from the long-term volcanic gas data streams monitored using the Multi-GAS that are masked by the changes in the sensitivity of the sensors.

著者

大場 武 谷口 無我 代田 寧

出版者

一般社団法人日本地球化学会

雑誌

日本地球化学会年会要旨集 2017年度日本地球化学会第64回年会講演要旨集

巻号頁・発行日

pp.141, 2017 (Released:2017-11-09)

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箱根山,草津白根山,霧島硫黄山で,火山ガス組成と地震活動の間に調和した傾向が見られた.これは火山ガス観測が火山活動の評価に有用であることを証明している.観測頻度の高い箱根山では群発地震の前兆もとらえることができた.火山における地震は,地殻内の流体圧の上昇により起きると考えられる.流体圧の上昇を引き起こすのがマグマから脱ガスした揮発性成分であり,それを成分として含有する噴気を観測することは,火山活動を評価する近道である.一方で,火山ガスの観測はいまだ人手に頼っており,火山ガスを本格的に火山活動評価に用いるには観測の自動化は避けて通ることができない.

著者

谷口 無我 村松 容一 千葉 仁 大場 武 奥村 文章 山室 真澄

出版者

一般社団法人日本地球化学会

雑誌

日本地球化学会年会要旨集 2013年度日本地球化学会第60回年会講演要旨集

巻号頁・発行日

pp.69, 2013 (Released:2013-08-31)

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甲府盆地北東部の花崗岩体周辺では、近年、深部掘削による温泉開発が盛んに行われてきた。温泉の新規掘削や既存源泉の保護には、温泉水の起源や泉質形成機構を解明することが不可欠である。本研究では、当該地域に分布する非火山性温泉水を採取し分析するとともに、水-鉱物相互作用の化学平衡論による検証を実施し、温泉水の起源と泉質形成機構を考察した。温泉水のδD, δ18O値の特徴はいずれの温泉水も天水が起源であること示唆した。温泉水の泉質は、花崗岩地域では曹長石の風化に起因したNa-HCO3型であり、火山岩類の分布地域では曹長石の風化のほかに硬石膏の溶解、およびモンモリロナイトによるイオン交換作用が寄与し、Na-HCO3型、Na-SO4型、Na-HCO3・SO4・Cl型、Ca-SO4型と多様な泉質を示すと考えられた。