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地学杂志
东京地学协会
地学杂志

东京地学协会

0022-135X

地学杂志/Journal 地学杂志
正式出版
收录年代

    Overview of the Special Issue "Mechanism of Phreatic Eruptions and Challenges for Eruption Forecasting: Latest Advances and Volcanic Disaster Prevention"

    Kazutaka MANNENYohei YUKUTAKERyosuke DOKEDaiji HIRATA...
    5页
    查看更多>>摘要：Owakudani, the eruption center of the 2015 eruption of Hakone Volcano, is one of the most popular tourist destinations in Japan, attracting more than three million visitors every year. However, the new crater created by the eruption is only 200 m from an area where tourists stroll (e.g., Mannen et al., 2018). Moreover, several craters that are assumed to have been the sources of ancient phreatic eruptions much larger than the 2015 eruption have been recognized near Owakudani using topographic and geological analyses (e.g., Yamaguchi et al., 2021). On the other hand, a slight inflation of the volcanic edifice and an increase in seismic activity, as observed before the 2015 eruption, are not rare in Hakone volcano. Indeed, they have been occurring once every few years since 2001. Because sightseeing at Owakudani is valuable for the local economy, it is not practical to designate the area as restricted every time seismic activity increases. Consequently, it is important to understand the processes of volcanic eruptions and to evaluate the imminence of an eruption in order to minimize economic losses, while ensuring the safety of tourists and residents. In addition to analyzing the 2015 eruption, the Hot Springs Research Institute of Kanagawa Prefecture, which has been engaged in research on Hakone volcano for many years, has been surveying the latest studies related to phreatic eruptions to consider the direction of future research on volcano monitoring. Activities include publishing a special issue in an international journal named Earth, Planets and Space (Mannen et al., 2019) and hosting domestic and international workshops on phreatic eruptions (Abe, 2019; Mannen and Kato, 2020). This special issue contains presentations given at the International Workshop on Phreatic Eruption Mechanisms, which was co-organized with the Kanagawa Prefectural Museum of Natural History in January 2020 with support from local companies and organizations, the Town of Hakone, and the Tok
      原文链接:
    • NSTL
    • Tokyo Geographical Soc

    特集号「水蒸気噴火のメ力ニズムと噴火予知への課題-最新の知見と火山防災-」巻頭言

    萬年一剛行竹洋平道家涼介平田大二...
    5页
    查看更多>>摘要：箱根火山2015年噴火の噴出中心となった大涌谷は,年間300万人の観光客が訪れるとされる国内有数の観光地であるが,この噴火で新たに生じた火口は,観光客が散策する領域からわずか200 mほどしか離れていない(例えば,Mannen et al., 2018)。また,大涌谷の近傍には,2015年噴火よりもはるかに大規模な水蒸気噴火の給源となったと考えられる火口地形が複数認められる(例えば,山ロほか,2021)。こうしたことから,噴火前に立入規制などの防災行動をとることは人命保護の観点からきわめて重要である。一方,箱根火山では山体のわずかな膨張や地震活動の活発化は必ずしも珍しい現象ではなく,2001年以降は数年に1度の頻度で発生している。したがって,活発化のたびに経済的な損失の大きい規制を行うのは難しい。観光客や住民の生命を守りつつ,経済的な損失を最小化する火山防災対応をとるためには,噴火にいたる過程を理解して,噴火の切迫度を評価することは重要である。箱根火山の研究に長年従事している神奈川県温泉地学研究所では2015年噴火の解析を行うことと並行して,上のような問題意識に基づいて,英文誌Earth,Planets and Spaceでの特集号編集や(Mannen et al., 2019),研究集会の実施を通じて,今後の研究や火山監視の方向性を検討するために必要な知見の集約に努めている(安部,2019;萬年?加藤,2020)。本特集号は,地元企業?団体,箱根町および東京地学協会の助成を受けて,2020年1月に神奈川県立生命の星?地球博物館と共催で実施した「水蒸気噴火のメ力ニズムに関する国際ワークショップ」で行われた講演や,その後の研究を基にした論文を収録する。
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    水蒸気噴火の発生場と噴火過程の観測-最近の進展と今後の展望-

    行竹洋平萬年一剛
    23页
    查看更多>>摘要：A phreatic eruption is a phenomenon in which water near the surface expands rapidly due to magma-supplied heat, ejecting the surrounding rocks. Recent studies of conceptual models, subsurface structures, pre-eruption processes, and eruption processes of phreatic eruptions are reviewed. These eruptions often occur in volcanoes with well-developed hydrothermal systems, where a low electrical resistivity layer is found near the surface using magnetotelluric surveys. The low resistivity layer indicates a low-permeability structure that acts as a pressure-confining cap on the hydrothermal system. In the brittle-ductile transition zone above deep magma, a sealing structure associated with quartz crystallization develops. Volcanoes with open conduits that connect magma reservoir and surface crater also have the potential for phreatic eruptions. A low-permeable sealing structure in the shallow part of the conduit plays an important role in eruptions of this type of volcano. Phreatic eruptions are prepared by an imbalance in the hydro-thermal system, which is caused by increases of heat, volcanic gases, and fluids from the deep magma reservoir, and are triggered by depressurization of the aquifer due to the breakdown of the cap/sealing structure. In recent years, eruptive processes have been modeled using data from broadband seismograms and tiltmeters near vents. At Ontake, Hakone, and Aso volcanoes, slow crustal movements or very low-frequency earthquakes were observed just prior to phreatic eruptions. These phenomena result from crack opening due to the rapid vaporization of liquid water. Incremental seismic activities, low-frequency earthquakes, and expansion of volcanic edifice, and geochemical changes in volcanic gases and hot springs are identified as long-term eruption precursors. These precursors reflect the supply of new magma, related changes in volcanic gases, and increased fluid pressure in shallow hydrothermal systems. Several new techniques for monitoring volca
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    2015年ロ永良部島噴火の火山活動推移と避難の意思決定

    井口 正人
    16页
    查看更多>>摘要：On August 3,2014 and May 29,2015,eruptions occurred at the Shindake summit crater of Kuchinoerabujima volcano in the Ryukyu Islands,southwestern Japan. The Japan Meteorological Agency (JMA) upgraded the Volcanic Alert Level (VAL) to 3 (warned zone within 2 ㎞ from the crater) after the onset of the 2014 eruption and to 5 (evacuation) after the onset of the 2015 eruption. The possibility of implementing early warnings for eruptions and forecasting the area most likely to suffer damage from volcanic eruptions are examined based on monitoring data and disaster-affected areas of historic eruptions. The onset of the 2014 eruption was preceded by a 15-year prolonged increase in volcanic activity that started in July 1999. Only a short-term tilt change was observed immediately before the eruption. The prolonged volcanic activity is characterized by: 1) repeated bursts of seismicity; 2) ground inflation events around the crater associated with increases in seismicity; 3) increases in geothermal activity and 4) appearance of fumarole. The short-term process consisted only of a tilt change of crater-side up one hour before the onset of the 2014 eruption. The phenomena prior to the 2015 eruption were more intense than those prior to the 2014 eruption, as demonstrated by seismicity, which included a felt earthquake six days before the eruption; larger ground deformation; higher rate of discharge of SO_2 gas; and, higher temperature. Despite more intense activity, VAL remained at 3. VAL was upgraded from 3 to 5 immediately after the 2015 eruption and then all of the residents were evacuated from the volcanic island by ferry boat. Decreases in seismicity, SO_2 gas discharge rate, and geothermal activity led to an initial reduction of the alert zone radius to less than 2.5 ㎞ in October 2015. A further reduction to less than 2 ㎞ (VAL 3) was later implemented based on deflation around the summit area in June 2016. Problems related to evacuation decision-making in May 2015 are as f
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    火山の活動不安定評価における熱消磁現象の活用と噴火予測の可能性

    橋本 武志
    11页
    查看更多>>摘要：Volcanoes with shallow hydrothermal systems are often accompanied by background volcanic activity such as fumarolic activity, microseismicity, and ground deformation even in the non-eruptive phase. When elevated, they are said to be in a state of "unrest." It is not difficult to imagine that such events of unrest reflect changes in the state of the shallow hydrothermal system beneath a volcano. However, there is currently no method by which these events can be used to quantitatively evaluate eruption imminency or predict eruption intensity based on physical and/or chemical models. A potentially useful application of such unrest events for probabilistically forecasting eruptions is discussed. First, the method proposed by Hashimoto et al. (2019) for compiling and evaluating the sources of unrest events, such as thermal demagnetization, is described. Then, the volcanic unrest index (VUI) of Potter et al. (2015a) is proposed as another key tool. Finally, a concept is proposed for integrating the VUI and the unrest data to make probabilistically forecasting eruptions feasible. Also described is a recent attempt to introduce the VUI for evaluating a volcano in Japan. Information on sources of unrest in the form of the scatter plot of Hashimoto et al. (2019) can be used as one of the rating criteria on the VUI worksheet. The key idea is to divide the source diagram into regions based on the probability of posterior eruptions given unrest events and to assign VUI scores to these regions. Such a procedure may augment the VUI's function, partially enabling probability-based eruption forecasting. Irrespective of whether the VUI is applied or not, it is essential to obtain temporally homogeneous monitoring data during both eruptive and non-eruptive periods for a quantitative evaluation of unrest events. Surveys and analyses carried out regularly over long time periods also play an equally important role. Therefore, to realize of probabilistic eruption forecasting, it is fundam
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    箱根山火山ガス組成による火山活動予測-火山防災への活用-

    代田 寧大場 武谷口無我十河孝夫...
    14页
    查看更多>>摘要：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 (CO_2/H_2S) contained in the volcanic gas from fumaroles were observed in synchronization with earthquake swarms and ground deformation in 2013 and 2015. Similar increases in CO_2/H_2S ratio were also observed in 2017 and 2019,although the increases in the CO_2/H_2S ratio in 2017 and 2019 were not as sharp as those in 2013 and 2015. Furthermore, the maximum values of the CO_2/H_2S ratio in 2017 and 2019 were lower than the values in 2013 and 2015. These differences in the CO_2/H_2S 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 CO_2/H_2S ratio decreased simultaneously with decreases in the number of volcanic earthquakes. The lower limit of CO_2/H_2S ratios after the peak of the CO_2/H_2S ratio time series was about 20 in all periods. This implies that subsequent unrest would not start until the CO_2/H_2S ratio drops to about 20. The CO_2/H_2S 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 CO_2/H_2S ratio. A good correlati
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    火山灰中の鉱物からみた水蒸気噴火のメ力ニズム

    大場 司南 裕介シャレザサイディナアン力サ井村 匠...
    13页
    查看更多>>摘要：Steam-blast eruptions are classified into three categories: (1)hydrothermal eruption caused solely by a phase change of hydrothermal water within a hydrothermal system; (2) phreatic eruption caused by a new thermal input derived from a magma body in a sub-volcanic aquifer; and, (3) ultravulcanian eruption (gas eruption), a type of vulcanian eruption, which is caused by gas degassed from magma accumulating under a lava plug. It is proposed that these can be classified from a petrological analysis of eruption products based mainly on the authors' previous contributions. Volcanic ash from hydrothermal eruptions is characterized by abundant altered lithics. At some composite volcanoes, altered lithics exhibit a wide variety of alteration types including siliceous, advanced argillic, phyllic, and potassic alterations, which are considered to originate from alteration zones of composite volcanoes. The association of alteration zones are correlated with those around porphyry copper deposits. The products of phreatic eruptions are composed mainly of strongly acid altered rocks, but may also contain fresh volcanic rock fragments. The rocks are derived from selectively/partially altered rocks under the crater. Ultravulcanian eruptions mainly release fresh lithic fragments and may also emit sulfur compound minerals (mainly sulfate), but the products contain no alteration minerals indicating hydrothermal acid leaching.
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    地表面変位から推定される箱根火山浅部熱水系の構造

    道家涼介萬年一剛板寺一洋
    20页
    查看更多>>摘要：Since a phreatic eruption is caused by ruptures in hydrothermal systems beneath volcanoes, detecting and monitoring a hydrothermal system can play an important role in predicting such an eruption. Interferometric Synthetic Aperture Radar (InSAR), which detects ground deformations over a large area, may be a key technology for use in various fields, as shown from the exponential growth of recent studies in terms of number and quality. The present contribution reviews surface deformations caused by the hydrothermal system of Hakone volcano, as detected by InSAR before, during, and after the 2015 eruption. The opening of the NW-SE-trending crack and localized uplift in the Owakudani fumarole area were captured by InSAR analyses during the 2015 unrest at Hakone volcano. Moreover, an InSAR time series analysis showed steady subsidence on the west side of the Owakudani fumarole area. Based on models explaining these surface displacements, the shallow hydrothermal system of Hakone volcano is characterized by NW-SE to WNW-ESE-trending crack-shaped fluid supply paths and pocket-shaped fluid reservoirs. During the 2015 and previous phreatic eruptions, it is probable that fluid was supplied using the same crack-like path, implying that fluid was repeatedly supplied using the same structure. Therefore, in order to predict the occurrence of phreatic eruptions at Hakone volcano, it is necessary to monitor volcanic activity by taking into account these structures. The activity of Hakone volcano, including formations of these NW-SE to WNW-ESE-trending cracks, is dominated by a regional stress field. This stress field is caused by shear deformation due to plate motion occurring in this region; that is, the subducting Philippine Sea Plate, and the colliding Izu Peninsula.
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc

    箱根火山のマグマ?熱水系モデル

    萬年一剛
    25页
    查看更多>>摘要：Hakone volcano has been in an active phase since 2001,as implied by frequent volcanic unrest every 2-5 years, with each accompanied by deep inflation (6-10 ㎞), increase of deep low-frequency events (DLFEs) at a depth of ~20 ㎞, increase of CO_2/H_2S ratio in fumarole gas, and surge of volcano tectonic earthquakes (VT; < 6 ㎞ deep). A series of episodes of volcanic unrest culminated in a small phreatic eruption (erupted volume;~100 m~(3)) in 2015; however, lesser unrest in terms of seismic activity occurred in 2017 and 2019. Recent studies on crustal structures based on seismic tomography indicate a magma chamber 10-20 ㎞ beneath the volcano, which might be connected to a large magma chamber beneath Fuji volcano, approximately 30 ㎞ NW of Hakone. Interestingly, the DLFEs beneath Hakone volcano seem to take place in a high attenuation zone that connects the magma chambers. Deep inflation beneath Hakone volcano, however, is clearly located at a shallower location than the magma chamber of Hakone. The increases of CO_2 and He within the fumarole of Hakone during its unrest may indicate degassing of magma at depth. The maximum fumarole temperature after the eruption and constraints on subsurface temperature (~200°C at 400 m deep indicated by the mineral assemblage and ~370°C at 4 ㎞ below sea level where is the lower depth limit of VT) imply a vapor-dominated hydrothermal system in the volcano from the bottom of the cap structure (~100 m deep) to a depth of possibly 2-4 ㎞. Such a vapor-dominated system may allow rapid transfers of magmatic gases and their emission from the fumarole area in the very early phase of volcanic unrest, as was observed. Hakone lacks long period events (LF) and volcanic tremors, which are common at many active volcanoes. Because such events are considered to be related to fluid migration, the vapor-dominated system can be attributed to their absence in Hakone. An estimation of the water mass balance implies that the amount and rate of inflat
      原文链接:
    • NETL
    • NSTL
    • Tokyo Geographical Soc