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

原田 昌武 細野 耕司 小林 昭夫 行竹 洋平 吉田 明夫

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.55, no.4, pp.193-199, 2010-08-31

被引用文献数

1

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Temporal changes in dilatational strain and the activity of low-frequency earthquakes around Mt. Fuji and Hakone volcano are investigated. It is shown that both cumulative strain and cumulative number of low-frequency earthquakes around Mt. Fuji have been increasing since the end of 2006. The tendency is more notable for relatively larger earthquakes rather than smaller earthquakes. The b value for earthquakes during the period after November 2006 is significantly smaller than the b value during the period from January 2004 through October 2006. These facts suggest that the crustal stress surrounding the source region of low-frequancy earthquakes has been increasing. We think this is the cause of the dilatational strain observed around Mt. Fuji. On the other hand, a clear relationship is not seen between extensional strain events observed three times since 2001 and the activity in low-frequancy earthquakes around Hakone volcano.

著者

萬年 一剛 原田 昌武 板寺 一洋 代田 寧 寺田 暁彦

出版者

特定非営利活動法人日本火山学会

雑誌

日本火山学会講演予稿集

巻号頁・発行日

vol.2013, 2013-09-29