著者

森 俊哉 野上 健治

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.3, pp.379-388, 2022-09-30 (Released:2022-10-27)

参考文献数

17

---

When volcanic ash is exposed to water after fell on the ground, various chemical substances will be eluted to water phase. Amounts of water soluble SO4 and Cl and their Cl/SO4 ratio of ash are useful for understanding eruptive activities of the volcano. For prompt evaluation of eruptive activities by water soluble components on ash, it would be useful if the analyses are made in-situ or near by the volcano instead of sending the samples to the laboratories far away. For this purpose, we utilized and established a method using a compact handheld absorptiometer for analyses of Cl and SO4 in ash leachate. The method uses a small digital scale, a handheld absorptiometer and other equipment (PP bottles, syringes filters and etc.). The scale is for ash leachates preparation and the absorptiometer is used for the turbidimetric measurements of SO4 and Cl. The calibration curves for SO4 and Cl were linear and parabola for the concentration range of the standard solutions up to 81.9 mg/L and 39.6 mg/L, respectively. Eleven ash samples from Kirishima Shinmoedake and Sakurajima volcanoes were analyzed by turbidimetry method of this study and by ion chromatography method, and were compared for validation of the method. The analyzed concentrations were basically within about 10 % compared to those of ion chromatography, except for samples whose absorbance were smaller than 0.1 unit. We also checked for the interfering components for turbidimetry analyses by checking the compiled ash leachate data of Airys and Delmelle (2012) and came to conclusion that effect of interference can be usually ignored. On the other hand, some of the ash samples with very low water soluble SO4 and Cl may be under detection limit with the proposed method. As a conclusion, ash leachate analyses for SO4 and Cl by turbidimetry using a handheld absorptiometer used in this study is an effective method and could be used for prompt evaluation of eruptive activities especially on remotes islands where the chemical laboratories are not available.

著者

栁澤 宏彰 及川 輝樹 川口 亮平 木村 一洋 伊藤 順一 越田 弘一 加藤 幸司 安藤 忍 池田 啓二 宇都宮 真吾 坂東 あいこ 奥山 哲 鎌田 林太郎 兒玉 篤郎 小森 次郎 奈良間 千之

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.3, pp.295-317, 2022-09-30 (Released:2022-10-27)

参考文献数

73

---

The 2016 eruptions of Niigata-Yakeyama volcano in central Japan consisted of several small eruptions that were accompanied by syneruptive-spouted type lahars. We have reviewed the sequence of the 2016 activity and modeled the eruptive processes based on observations of various volcanic phenomena, including ash fall and lahars, plumes, earthquakes and crustal deformation, and analysis of eruptive products. Eruptions of Niigata-Yakeyama volcano after the 20th century can be categorized into two types; 1) VEI=0-1 eruptions during which ash fall covered only the summit area and no ballistic blocks were ejected (e.g., 1997-1998 event) and 2) VEI=1-2 eruptions during which ash fall reached the foot of the mountain with ejected blocks (e.g., 1974 event). We also discuss the characteristics of the 2016 activity by comparing the sequence with those of other events of Niigata-Yakeyama volcano: the 1974 and 1997-1998 eruption events and the 2000-2001 intensified fumarolic event. The 2016 eruptions of Niigata-Yakeyama volcano are divided into the following six stages. Stage I was characterized by the onset of intensified steam plume emission activity (≥200 m). Stage II was characterized by the onset of crustal deformation, slight increase of high frequency earthquakes (approx.>3.3 Hz) and further activation of steam plume emission activity (≥500 m). The crustal deformation observed commenced at the beginning of Stage II and lasted until the end of Stage V. The total inflated volume was estimated to be approximately 7.2×106 m3. Several very small eruptions that provided only a small amount of ash to the summit area also occurred. Stage III was characterized by a rapid increase of high frequency earthquakes accompanied by tilt change, and the onset of low frequency earthquakes (approx.<3.3 Hz). A small eruption was accompanied by a syneruptive-spouted type lahar at this time. Stage IV was characterized by the occurrence of several small syneruptive-spouted type lahars. The occurrence of high and low frequency earthquakes continued, but with decreasing abundance. Stage V was characterized by the highest altitude of steam plume emission (≥1,200 m), while no ash emission nor syneruptive-spouted type lahars were observed. Stage VI was characterized by a gradual decrease in steam plume emission and earthquake activity. The aerial photographs indicate the ash fall distribution, and the maximum scale of the 2016 eruption, which is estimated to be VEI=1. The assemblage of altered minerals indicates that the volcanic ash originated from volcanic conduits affected by a high-sulfidation epithermal system and no magmatic components were detected. Judging from the depth of the crustal deformation source of magmatic eruptions at other volcanoes, the estimated source of crustal deformation during the 2016 eruption is considered to have been caused by a volume change of the magma chamber. The sequence of the 2016 event can be interpreted as follows: 1) magma supply to the magma chamber, 2) increase in seismicity and fumarolic activity triggered by volcanic fluid released from the new magma, 3) destruction of volcanic conduit by increased fumarolic activity and emission of volcanic ash, and 4) occurrence of syneruptive-spouted type lahars by the “airlift pump” effect. At Niigata-Yakeyama volcano, such small eruptions and fumarolic events have been frequently observed for the last 40 years. We thus consider that the accumulation of magma has progressed beneath the volcano, which is a potential preparatory process for a future magmatic eruption.

著者

三浦 大助

出版者

The Volcanological Society of Japan

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.273-294, 2022-09-30 (Released:2022-10-27)

参考文献数

44

---

火山災害の軽減において,破局的珪長質噴火の前兆期を認定し,その特徴を知ることは学術的に重要である.前兆現象が出現し,後に続く大規模噴火と関連する場合に,どのようなタイプの前兆現象が起こるのかは非常に興味深い.テフラ堆積物のシーケンスは,前兆期から大規模噴火までの貴重な記録と考えられることから,クッタラカルデラ火山のクッタラ-早来テフラ(Kt-Hy)を対象として,現地地質調査,粒度分析,XRF分析,EPMA分析により,その噴火推移を調べた.59-55 kaのKt-Hy噴火は,近傍相において初期のサブプリニー式噴火(LpfaおよびLpdc),その後のマグマ水蒸気噴火(MpdcおよびUpdc)までのシーケンスが記録されている.Lpdc-Mpdcユニットは,谷埋め型の火砕物密度流堆積物である.その後,発泡度の低い珪長質マグマが,希薄な火砕物密度流として広く拡がった(Updc).これらの噴火シーケンスの変化と堆積相の詳細な解釈に基づき,MpdcとUpdcの給源火口は,各々成層火山の南麓と現在の山頂カルデラ周辺と推定された.クッタラカルデラ火山では,成層火山体の形成時期について,議論があった.本研究による近傍-遠方堆積相と,移動する火口位置の証拠から,成層火山がKt-Hy期にも成長したことが示唆された.成層火山を含めたKt-Hy噴出物の推定総噴出量は,最大で7-8 km3 DREとなる.Kt-Hy噴火は,火口の移動とマグマ水蒸気噴火で特徴づけられ,円錐の成層火山体で,カルデラを伴うタイプにおける前兆期の特徴とよく一致している.Kt-Hy噴火に続く54 kaの大規模珪長質噴火(Kt-3)には,岩石学的な類似性がみられ,これらのことから,大規模珪長質マグマの早期貯留の可能性が示唆された.

著者

佐藤 鋭一 和田 恵治 野口 昌宏

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.3, pp.255-271, 2022-09-30 (Released:2022-10-27)

参考文献数

37

---

Kurodake volcano in the Taisetsu volcano group was formed approximately 0.2 Ma, producing andesitic lava flows and a dome. The lavas contain numerous mafic inclusions (<20 vol.%) ranging from approximately 1 cm to about 30 cm in diameter. The mafic inclusions exhibit typically rounded to ellipsoidal shapes and have smooth contacts with the host lavas. The mafic inclusions are classified into two types, fine and coarse, based on the size of the groundmass crystals. The groundmass crystals of the fine-type inclusions are composed of acicular minerals (0.1-0.3 mm in length). On the other hand, the groundmass of the coarse-type inclusions is primarily composed of tabular minerals (>85 vol.% and 0.2-0.5 mm in length). The plagioclase core compositions of the host lavas and two types of mafic inclusions vary substantially from An38 to An90. The plagioclase phenocrysts are classified into three groups based on their core compositions: An-rich (type A: An>80), An-poor (type B: An<60), and intermediate (type C: 60<An<80). Type A and type B plagioclases were derived from mafic and silicic magmas, respectively, and type C was derived from a hybrid magma formed by the mixing of the mafic and silicic magmas. The host lavas predominantly contain type B plagioclase phenocrysts, with infrequent types A and C, and most of the plagioclase microphenocrysts and groundmass crystals are type C. In the fine-type inclusions, type A and type B plagioclase phenocrysts coexist, and most of plagioclase microphenocrysts and groundmass crystals are classified into the type C, similar to the host lavas. In the coarse-type inclusions, most of the plagioclase phenocrysts, microphenocrysts, and groundmass crystals are classified as type B. These assemblages in the host lavas and fine-type inclusions can be explained by the mixing of the magmas, whereas the coarse-type inclusions were formed in the silicic end-member magma. Initially, mafic magma containing type A plagioclase was injected into bottom of the silicic magma chamber containing type B. A small amount of mafic magma was mixed with silicic magma to form the host magma. Subsequently, mixing occurred near the boundary between the mafic and silicic magmas, producing the fine-type inclusion forming magma. We presume that the margin of the silicic magma chamber was highly crystalline and the coarse-type inclusions were derived from the margin of the silicic magma chamber.

著者

高田 亮 大島 治 荒牧 重雄 小野 晃司 吉田 克史 梶間 和彦

出版者

The Volcanological Society of Japan

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.37, no.5, pp.233-250, 1992-11-15 (Released:2017-03-20)

参考文献数

34

被引用文献数

1

---

The subaerial history of Aogashima volcano (about 3 km3 in volume) was studied. The sequence of volcanic activity is summarized as follows: the growth of Kurosaki volcano (0.3 km3 in volume; basalt>andesite) in the northwestern area of Aogashima island; the construction of the main edifice of Main stratocone (basalt≫andesite) in the southeastern area; the fissure eruptions of Aphyric basalts (<0.1km3 in volume) on the northwestern flank; a surge activity (Ojiroike surge deposits) (basalt>ndesite) at about 3,000 y.B.P.; the eruptions of Kintagaura lavas (0.15 km3 in volume) filing the Southeasern basin, and airfalls (Yasundogo airfalls tephras) (0.4 km3 in volume; basalt>andesite) on the east and north flanks 3,000-2,400 y.B.P.; the occurrence of a debris avalanche (Nagashizaka debris avalanche deposits) associated with the formation of the Ikenosawa crater (1.7 km×1.5 km insize); the Tenmei (A.D. 1781-1785) eruption (0.08km3 in volume; andesite). Based on the historical records and the geological data obtained in this study, the sequence of the Tenmei eruption is restored as follows: according to historical records, a small ash eruption occurred in 1781; in 1783, the Tenmei eruption began with an explosive scoria effusion (Tenmei airfall tephras 1) associated with cone building (Maruyama pyroclastic cone); in 1785, ash fall continued intermittently for more than one month (Tenmei airfall tephras 2); finally, the Ikenosawa crater was filled with lava flows (Tenmei lavas 1 and 2). During the development of Aogashima volcano, magma paths were shifted over a distance of about 4 km. Some geological units derived from different magma paths have different, parallel trends of chemical composition in SiO2 vs. oxide diagrams. The chemical composition of magma changed with the shift in magma path. Each trend consists of a combination of plagioclase accumulation and crystallization. The magma-supply rate of Aogashima volcano was fluctuating in time and space with the growth of each geological unit, which may have led to the generation of andesite magma.

著者

山本 圭吾

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.2, pp.221-231, 2022-06-30 (Released:2022-07-28)

参考文献数

13

著者

青山 裕

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.2, pp.171-193, 2022-06-30 (Released:2022-07-28)

参考文献数

86

---

Volcanoes in Hokkaido had vigorous eruption histories in the last 400 years. Especially in the southern Hokkaido, Hokkaido-Komagatake, Usuzan, and Tarumaesan, reawakened in the 17th century after the long-dormant period and vigorous magmatic eruptions of VEI5 class have been recorded in the historical literature and also in geological layers. Contrary to these volcanoes, we have documented histories only after the 20th century for Tokachidake and Meakandake. Continuous volcano monitoring has been performed in major active volcanoes in Hokkaido since 1960s. In the 1970s, with the increase in seismic activity of Tokachidake, sponsored research from Hokkaido Government to Hokkaido University began to establish disaster management plans for future eruptions of active volcanoes in Hokkaido, and research reports was edited by Hokkaido University. Although 50 years have passed since the publication of the first report on Tokachidake, the reports of the research on active volcanoes are still one of the first documents to be referred when investigating the past activities of volcanoes in Hokkaido and the results of old scientific surveys. In addition, the reports include interdisciplinary contents for that time such as prediction of future eruptive activities and disaster prevention measures. The sponsored research by Hokkaido Government has continued to the present, and a new report on Tokachidake was published in 2014. The compilation of such research reports is very effective for volcano researchers and for relating field to share their awareness of the problems of volcanoes beyond their individual fields of expertise. Monitoring network around the active volcanoes in Hokkaido has been remarkably improved for these 20 years by Japan Meteorological Agency (JMA), Hokkaido University and other relating institutions. Recent data exchange in real-time among different organizations reduces duplication of monitoring resources and increases multi-parameter monitoring ability. The improved volcano monitoring network is expected to detect precursory activities of future magmatic eruptions concerned at the major volcanoes. Looking back on the eruption in the 20th century in Hokkaido, small phreatic eruptions preceded magmatic vigorous eruptions in many cases. Not only mountaineers but also tourists and citizens can easily approach the crater area without any special equipment at several active volcanoes, so even a small eruption can lead to severe volcanic disaster. (View PDF for the rest of the abstract.)

著者

下司 信夫

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.61, no.1, pp.101-118, 2016-03-31 (Released:2017-03-20)

被引用文献数

3

---

Large-scale pyroclastic eruption is one of the most awful natural disasters on the earth. Though their frequency is relatively low compare to the lifetime of human society, large-scale pyroclastic eruption can make serious impact on the global environment. Frequency of the volcanic eruptions shows a negative correlation against their scale: global frequency of the eruptions larger than VEI7 is approximately ten per 10,000 years, whereas more than 10 eruption of VEI4 occur every 10 years. The storage of voluminous magma within a shallow crust is a key process for the preparation for large-scale eruption. Inactive thermal convection in highly-crystallized magma bodies and visco-elastic behavior of the surrounding host rock can allow the stable storage of voluminous felsic magma at the neutral buoyancy level in the upper crust. Segregation of interstitial melt to form a melt pocket in highly-crystallized magma body can cause smaller scale of eruptions, whereas the remobilization of entire part of magma chamber will result a large-scale eruption with caldera collapse. Rupture and collapse of the roof rock of magma chamber induced by rapid decompression of magma chamber is the fundamental process of the eruption of voluminous magmas within short period. The decompression of magma chamber activates the slip of ring fault at the marginal portion of the roof and consequently the caldera starts subsidence. The collapse is controlled by the decompression inside the chamber and the strength of the roof rock. Ring fault turns to an open ring facture through which the voluminous magma can erupt to produce large ignimbrite. The volume of magma erupts during a caldera-forming eruption against the total magma chamber volume show negative correlation against the chamber size. This means that the large fraction of magma can remain even after caldera collapse particularly in large magma chamber. Evaluation of "precursory process" for catastrophic eruption is important to understand the driving mechanism of catastrophic eruption and also the hazard assessment. Accumulation of magma and building of a large-volume magma chamber within the earth’s crust is a long-term preparation process for catastrophic eruption. Short-term process for catastrophic eruption is the destabilization and rupturing process of the magma chamber.

著者

鈴木 毅彦 中山 俊雄

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.52, no.1, pp.23-38, 2007-02-28 (Released:2017-03-20)

参考文献数

47

被引用文献数

1

---

A widespread tephra referred to here as Tamagawa-R4 Tephra (Tmg-R4) is newly recognized. Tmg-R4, derived from the Pre-Yakeyama caldera located in the Sengan geothermal area, the Northeast Japan arc, covers the area from Tohoku to Kanto, northeast of Honshu Island. At the type locality in the proximal area, Tmg-R4 comprises a non-welded pyroclastic flow deposit (ignimbrite) and an immediately overlying welded pyroclastic flow deposit (Kurasawayama Welded Tuff). Absence of plinian fall deposits in the area of ca. 25 km south of the source and the fine vitric ash nature of the distal ash-fall deposits of Tmg-R4 suggest that they are co-ignimbrite ash-fall deposits. Tmg-R4 was identified using a combination of refractive indices and chemical compositions of major and rare earth elements of glass shards (n=1.498-1.501, SiO2: 78.3-78.6 wt%, K2O: 4.2-4.5 wt%, Ba: 830-911 ppm), mineral content, refractive indices of hornblende (n2=1.665-1.686). On the basis of these properties, Tmg-R4 was identified in Boso and Oga peninsulas, Choshi area, and in the core drilled on Musashino upland around 500 km south of the source. Calcareous nannofossil biostratigraphic (Calcareous nannofossil datum 13) and magneto-stratigraphic positions in Boso peninsula and Choshi, and paleomagnetic direction and many radiometric ages determined in the proximal area by previous studies indicate that the age of Tmg-R4 is ca. 2.0 Ma, positioned just below the base of the Olduvai Subchron. The distribution of Tmg-R4 showing emplacement of co-ignimbrite ash-fall deposit in the area 530 km south of the source, emphasizes the upwind transport direction relative to the prevailing westerly winds. This distribution shows similarity to those of a few co-ignimbrite ash-fall deposits derived from calderas in the Northeast Japan arc. As a key marker horizon in this age, the widespread occurrence of Tmg-R4 provides a tie line between many different sections over a distance of 530 km. Additionally, Kd44-Nk Tephra above Tmg-R4 is recognized in Boso peninsula, Choshi, Niigata and east Lake Biwa areas. Characteristic properties and stratigraphic positions indicate that Kd44-Nk possibly derived from the Sengan geothermal area occurred at 1.968-1.781 Ma.

著者

土出 昌一

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.42, no.1, pp.89-91, 1997-03-07 (Released:2017-03-20)

参考文献数

2

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

---

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

山里 平

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.50, no.Special, pp.S7-S18, 2005-12-20 (Released:2017-03-20)

---

Volcano observation is a basic to the fundamental research on volcanism and to the surveillance of volcanic activity for disaster mitigation. In this paper, the author reviews the history of volcano observation in Japan since 19th century, especially the history of volcano surveillance of the Japan Meteorological Agency (JMA) and the recent advancement of the monitoring technique of JMA. The volcano observation in Japan started by Prof. Sekiya on occasion of the eruption of Bandai volcano in 1888. Prof. Omori carried out pioneering observation at Usu volcano in 1910 and established the first volcanological observatory at Asama volcano in 1911. Since then, national universities established observatories at several active volcanoes. Their recent volcanological researches have been endorsed by the National Project of the Prediction of Volcanic Eruptions that started in 1974. JMA's continuous volcano observation started in 1888. JMA had started continuous observation at 10 volcanoes by 1950. In 1962-66, JMA divided active volcanoes into three classes (A, B and C) depending on the level of volcanic activity and the risk of disaster and installed seismographs at three and one stations for A class (4 volcanoes) and B class volcanoes (13 volcanoes), respectively, and organized mobile observation teams for B and C class volcanoes. Since late 1980’s, public concern for volcanic disaster mitigation has risen because of the eruptions at Izu-Oshima volcano in 1986, Unzen volcano in 1991, Usu volcano and Miyakejima volcano in 2000. To promote the disaster preparedness, JMA strengthened observation system and established Volcano Observation and Information Centers (VOIC) at Sapporo, Sendai, Tokyo and Fukuoka and centralized the volcano observation in 2001. VOIC installed TV cameras, seismometers, GPS stations, tiltmeters and infrasonic stations at each volcano. Most of the TV cameras are as sensitive as to detect visual phenomena even at night. These data obtained at each station are telemetered to VOICs and are monitored on a real-time in 24 hours. Each VOIC has Mobile Observation Team, which periodically collects basic observational data from active volcanoes. The observations consist not only of the installation of temporal observation stations but also of periodical thermal, geomagnetic and geodetic surveys. They carry out temporal observations to enhance monitoring capability whenever an unusual phenomenon is detected at volcanoes. VOIC issues Volcano Information to the disaster prevention authorities and to the public to initiate and take relevant disaster mitigation measures. There are three types of Volcano Information, Volcanic Alert, Volcanic Advisory, and Volcanic Observation Report. Since November 2003, JMA has introduced Volcanic Activity Levels as an additional index to Volcano Information for some volcanoes. To indicate the Level, JMA uses 6-level of numerical scheme to reflect an increasing order of unrest: 0 as dormant to 5 as large-scale eruption.

著者

横尾 亮彦 石井 杏佳

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.66, no.3, pp.229-240, 2021

著者

石峯 康浩

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.61, no.1, pp.183-198, 2016-03-31 (Released:2017-03-20)

---

This paper presents some basic concepts on possible cooperative framework for contributing to disaster mitigation during volcanic eruptions with the intention of enhancing discussion among members of the Volcanological Society of Japan. At first, this paper describes some examples of problems that have been argued during recent volcanic eruptions because of improper risk communication of volcanologists, and then, outlines the present state of a coordination system for effective disaster assistance by multiple stakeholders with a focus on recent efforts in public health and medical communities. Preliminary ideas on “Expert Assistance Team during Volcanic Crises” are also presented for further discussions.

著者

吉村 俊平

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.66, no.4, pp.375-384, 2021-12-31 (Released:2022-02-22)

参考文献数

24

著者

松本 亜希子 中川 光弘 小林 卓也 石塚 吉浩

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.66, no.4, pp.255-279, 2021-12-31 (Released:2022-02-22)

参考文献数

37

---

The Tokachidake volcano group, central Hokkaido, is one of the most active volcanoes in Japan; three magmatic eruptions occurred from the crater area on the northwestern flank of Tokachidake in the 20th century. The Sandan-yama, Kamihorokamettokuyama, and Sampōzan edifices are on the southern flank of the volcano, and the first two bound the west-facing Nukkakushi crater. Although fumarolic activity and hydrothermal alteration are ongoing at Nukkakushi crater, its eruptive history remains unknown. Therefore, we performed a geological investigation of the Nukkakushi crater area. Based on topographical features, we inferred the following eruptive history. Sampōzan and Kamihorokamettokuyama formed during ca. 70-60 ka, after which the northern flank of Sampōzan collapsed and a new edifice (Nukkakushi volcano) was built within the collapse scarp. Finally, the collapse of the western flank of Nukkakushi formed Nukkakushi crater—perhaps during the Holocene, according to previous work. We identified eight Holocene eruptive products generated from the Nukkakushi crater area, the most recent of which was generated from a crater on the western flank of Sandan-yama sometime since the early 18th century. We also recognized three debris avalanche/landslide deposits that were generated within the last 750 years. Comparing the eruptive products of the northwestern crater area of Tokachidake with those of the Nukkakushi crater area revealed that magmatic eruptions from the two craters alternated until 1.8 ka. Their distinct magmatic compositions suggest the simultaneous existence of two isolated magma systems beneath Tokachidake and Nukkakushi, at least until that time. Since 1.8 ka, magmatic eruptions at the northwestern crater area of Tokachidake and phreatic eruptions at the Nukkakushi crater area have occurred in parallel. Moreover, around Nukkakushi crater, small-scale collapses/landslides have occurred. Previous studies recognized hydrothermal changes at Nukkakushi crater area, originating from the northwestern crater area of Tokachidake around the last two magmatic eruptions; it is therefore presumed that the Nukkakushi crater area was hydrothermally altered, even during periods of little eruptive activity. Such continuous and pervasive hydrothermal alteration explains the frequent collapses of edifices. The parallel yet contrasting eruptive activities in these adjacent areas are important for forecasting future eruptive activities and mitigating volcanic hazards.

著者

中田 節也

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.60, no.2, pp.143-150, 2015-06-30 (Released:2017-03-20)

---

Statistical treatment of volcanic eruptions clearly shows the regularity of power law between the frequencies and the scales not only in the global scale but also in regional and individual volcano scales. However, ancient smaller eruption events tend to be not recorded, compared with recent data. In the log frequency-VEI plot, incompletely normalized frequency which ignores the time-dependent nature of the database, provides a gentler regression line than when the data time-dependence is considered; that is, the former reflects low numbers of small eruptions insufficiently recorded. The slopes of the regression lines are similar, irrespective of area scales. This regularity may help our understanding about the potential of future large eruptions in not only individual volcanoes but also caldera regions. Volcanic activity in Japan has been quiet recently; no VEI 4 eruptions occurred after the early 20th Century, and no VEI 5 eruptions did since the middle 18th Century. Considering the regularity of volcanic eruptions in the arc scale, it is likely that Japan will experience these large eruptions near future.

著者

石塚 治

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.61, no.1, pp.91-100, 2016-03-31 (Released:2017-03-20)

---

How subduction begins and its consequences for global tectonics remain one of the essential outstanding problems of plate tectonics. Two different endmember mechanisms for subduction initiation have been hypothesized: spontaneous, and induced (or forced). Numerical models suggest that subduction initiation is induced by externally forced compression along a preexisting discontinuity in an oceanic plate such as a fracture zone or transform faults. However, it has been pointed out that spontaneous subduction must have occurred at some points in Earth's history to initiate plate tectonics, and recent numerical models demonstrated that lateral thermal/compositional buoyancy contrast along plate discontinuity or within lithosphere can cause spontaneous subduction initiation. Recent geological and geophysical surveys in the Izu-Bonin-Mariana fore-arc have revealed igneous processes in the initial stages of subduction. The oldest magmatism after subduction initiation generated MORB-like fore-arc basalts, which was associated with seafloor spreading caused by onset of sinking of slab into mantle. Then boninitic magmatism followed by tholeiitic to calc-alkaline arc lavas collectively makes up the extrusive sequence of the fore-arc crust. This magmatic evolution from initial basaltic magmatism to establishment of normal arc magmatism took several million years. Fore-arc stratigraphy observed in the Izu-Bonin-Mariana arc shares some of the key geologic and petrologic characteristics with many supra-subduction zone ophiolite, which implies that fore-arc crustal section produced in the initial stage of oceanic island arc formation could correspond to in-situ section of supra-subduction zone ophiolite prior to obduction. Recent ocean drilling projects targeting initial stage of the Izu-Bonin-Mariana arc inception revealed that subduction initiation to form the Izu-Bonin-Mariana arc took place spontaneously. The drilling results also revealed that the whole arc was established on the ocean crust produced associated with subduction initiation.

著者

津久井 雅志

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.56, no.2-3, pp.89-94, 2011-06-30 (Released:2017-03-20)

参考文献数

9

---

Old historical documents on 1779 AD An’ei eruption of Sakurajima, southwest Japan were collected from distal places as well as those from neighboring area of the volcano. These records revealed that the ash-fall front traveled to the northeast at about 50-100km/h, reached as far as Tohoku district 1200km from Sakurajima, and covered area of ca. 2.33×105km2. Investigation of old documents helped to improve understanding of behavior of the volcano and environmental effects at the time of infrequent and great eruption. The wide distribution of ash-fall in 1779 Sakurajima eruption suggests that there is a high potential that ash discharged by future eruption of Sakurajima may cover down through the mainland of Japan. We should keep in mind both physical and economical effects of ash-fall in assessing the activity and making the scenario of an eruption.

著者

齋藤 和男 亀井 智紀

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.40, no.2, pp.99-102, 1995-04-20

被引用文献数

2