著者

風間 卓仁 栗原 剛志 山本 圭吾 井口 正人 福田 洋一

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.61, no.4, pp.593-604, 2016-12-31 (Released:2017-01-13)

参考文献数

24

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Continuous time variations in relative gravity and tilt were observed by a CG-3 M relative gravimeter at Arimura, Sakurajima Volcano (Southern Japan) during the rapid inflation event on August 15, 2015. The gravity/tilt signals were retrieved from the original data by correcting several disturbances such as instrumental drift and tidal effect. The retrieved gravity change is -5.86±0.27μGal;its amplitude is smaller than the typical uncertainty of relative gravimeters (∼10μGal), but the continuous measurement of relative gravity in a one-minute interval contributed to the detection of the small gravity change in the case of Sakurajima Volcano. The tilt change of 55.9μrad is also retrieved from the CG-3 M’s tilt data. The success in detection of the tilt change shows that the gravimeters can be utilized as portable tiltmeters as long as significant tilt variations are expected at volcanic areas. The observed gravity change is consistent with one of the dike intrusion models provided by Geospatial Information Authority of Japan, if the density value in the dike of 0.98±0.37g/cm3 is assumed.

著者

小幡 涼江 海野 進

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.44, no.4, pp.201-216, 1999-08-30

被引用文献数

1

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Aokigahara Lava Flow erupted in A.D. 864 covers the northwestern foot of Fuji volcano. It consists of two flows: Ishizuka Lava erupted from a parasitic vent at the foot of Ohmuroyama, and Nagaoyama Lava effused from a cinder cone of Nagaoyama. Ishizuka Lava flowed down to Motosuko Lake, spreading laterally like an alluvial fan in front of the lake. Approximately 36 flow lobes that protruded from the aa front, pinch and swell, occasionally bifurcating into several branches from swollen portions of the lobes. Most lobes have a few longitudinal clefts with similar features to crease structures of rhyolitic and andesitic lava domes and to be banded on cleft walls on pahoehoe tumuli. The flow lobes are classified into three types on the basis of surface structures. Type-I lobes are characterized by well-developed clefts, which exhibit two distinct features: Stripe-type has alternating stripes of red and grey on the cleft wall parallel to the elcngation of the lobes. The stripes exist only for a depth a cm from the surface of the walls. The groundmass of the grey stripes contains abundant dendritic crystallites, while that of the red stripes consists of palagonitized glass; Blister-type develops blisters with pipe vesicles on the cleft wall. It does not have stripes, however, they may be present at different levels on the same cleft wall. Type-II lobes have clinkers with stripes similar to those of Stripe-type clefts of Type-Ilobes, however, they do not develop clefts. Type-III lobes are thinner than Type-I obes and are covered with red platy clinkers, giving the whole lobe a reddish appearance. Clefts are poorly developed. Continuous supply of lava into a flow lobe, which ceased advancement, causes tensile stress on solidifying crusts. We suggest that stepwise opening of cracks resulted in the difference in crystal density and habit of stripes on the clefts. Occasional opening of the crack and quenching hot semi-molten lava exposed at the crack tip under water resulted in the fomation of red stripes, followed by solidification of the new crust beneath the crack tip. Subsequent fracturing of newly formed brittle crust resulted in the formation of the grey stripes. The Blister-type clefts were formed when the supply of lava temporaily stopped and reslarted again. Solidification of the crust and the formation of blisters proceeded when lava within the lobe was static. We conclude that Type-I was produced by intermittent supply of lava into a static flow lobe. Type-II lobes were moving during inflation of the lobe. As a result, cleft walls with stripes were disrupted to produce clinkers. Type-III was produced by breaking of crust immediately after its fomationd uring rapid emplacement of the lobe. Therefore, the shear rates during the fomation of flow lobes increase from Type-I, II, to III lobes.

著者

三村 弘二 金谷 弘

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.46, no.1, pp.17-20, 2001

参考文献数

11

被引用文献数

2

著者

後藤 芳彦 松塚 悟 亀山 聖二 檀原 徹

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.1, pp.17-33, 2015-03-31

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我々は,北海道洞爺カルデラ中島火山の火山地質を明らかにするため,ヘリコプター搭載型の高分解能レーザースキャナを用いたレーザーマッピングと,地表地質調査を行った.レーザーマッピングは中島の全域を含む3×3kmで行い,計測データから3次元のデジタル地形図を作成した.地表地質調査は3次元デジタル地形図を用いて中島の全域で行い,詳細な産状記載と岩石記載を行った.デジタル地形図と地表地質調査から,中島の詳細な火山地質と形成史が明らかになった.中島は,デイサイトおよび安山岩質マグマが噴出して形成した8個の溶岩ドーム(東山ドーム,西山ドーム,北西ドーム,北山ドーム,南西ドーム,観音島ドーム,弁天島ドーム,饅頭島ドーム),デイサイトマグマが湖底堆積物を押し上げて形成した潜在ドーム(北東岬ドーム),およびデイサイト質のマグマ水蒸気噴火により形成したタフコーン(東山火砕丘)からなる.中島の北東部と南西部には,泥岩と砂岩からなる湖底堆積物が分布しており,中島の火山活動がカルデラ底の隆起を伴ったことを示す.中島は,洞爺カルデラ中央部のリサージェントドームの形成と,それに伴うデイサイト〜安山岩質マグマの噴出により形成されたと考えられる.高分解能レーザースキャナによる地形計測と3次元デジタル地形図を用いた地質調査は,火山地質の解明に極めて有効である.

著者

後藤 芳彦

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.4, pp.137-145, 2011-09-30

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北海道東部知床半島の天頂山は,安山岩質溶岩からなる小型の火山で,山頂部には北東-南西方向に配列する爆裂火口列がある.本論では,天頂山の爆裂火口列を形成した降下テフラ(Ten-a)の分布と年代を明らかにした.Ten-aテフラは天頂山の山頂部から知床半島の東海岸に分布し,東北東方向に伸長する分布主軸を示す.テフラは,新鮮〜変質した安山岩質の石質岩片と新鮮な軽石からなり,マグマ水蒸気噴火の噴出物であると考えられる.テフラ直下の土壌層から得られた放射性炭素年代値は,1930±40 years BP(1960-1810 cal BP)である.天頂山は約1900年前に噴火し,山頂部に爆裂火口列を形成したと考えられる.

著者

宮地 直道 中川 光弘 吉田 真理夫

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.45, no.2, pp.75-85, 2000-05-10

被引用文献数

8

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Recent eruptive episodes since the last 2200 years of Rausudake volcano, east Hokkaido, are revealed by tephrochronology, geological survey of volcanic edifice and petrology of eruptive products. Eruptive ages of these episodes are estimated by ^<14>C age dating, presence of wide spread tephras, Ma-b from Mashu volcano (about 1 ka) and Ta-a from Tarumai volcano (AD 1739), and thickness of soil between tephra. We identify three major eruption episodes occurring in ca. 2200, ca. 1400 and 500-700 y. B. P. In each episode, plinian eruption associated with generation of pyroclastic flows and possibly with effusion of lava flows and domes had occurred from the summit area. Volcanic explosive index (VEI) of each eruption is 2-4. Tephra identified as the deposits of ca. 1400 y. B. P, eruption had spread widely and has been found in Kunashiri Island which locates about 60 km east of Rausudake volcano. In Shiretoko Peninsula, east Hokkaido, Mt. Shiretoko-Iouzan has been recognized to be an active volcano. We should note that Mt. Rausudake is also active volcano that repeated its magmatic eruptions at intervals of ca. 800 years.

著者

早川 由紀夫 小山 真人

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.37, no.4, pp.167-181, 1992-10-01

被引用文献数

4

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The eruptive history of the Higashi Izu monogenetic volcano field for the past 32,000 years is revealed by tephrochronology and loess-chronometry. Morphology, color, and size of basaltic tephra grains are widely variable depending on the mode of the eruption; e.g., spinose red scoria are fallout from a strombolian eruption column when a scoria cone is established around the crater; yellowish green lapilli and hard tuff (kata) are products from phreatomagmatic explosions, the former being more magmatic than the latter. In the field, these discriminations are useful not only for identification of each tephra bed, but also for understanding the transition of eruptive styles during one eruptive event. Because vents are closed or sealed at the end of an eruption, an absolutely quiescence occurs between eruptive events in a monogenetic volcano field. This proves the validity of loess-chronometry. Some of the NW-SE or NE-SW trending alignments of volcanoes proved to be created by eruptive fissures; i.e., they are erupted simultaneously. Among them, the 11 km-long Iwanoyama-Iyuzan volcanic chain is the most conspicuous, which was active about 2,000 years ago. Eruptive events more than 10^9 kg of magma discharge are recognized 13 times during the past 32,000 years, so that the average frequency of eruption in this field is calculated one every 2,500 years. The last is the Iwanoyama-Iyuzan eruption. The discharge rate of magma is 100×l0^9 kg/ky for the past 32,000 years or 330×10^9 kg/ky for the past 5,000 years. The rate seems to be accelerated recently, however, it is still an order of magnitude lower than that of a polygenetic volcano such as Izu Oshima. The Kawagodaira eruption of 3,000 years ago is remarkable for two reasons: the largest with 765×10^9 kg of magma and the first appearance of rhyolite in the field.

著者

宇井 忠英 吉本 充宏 佐藤 十一 橋本 勲 宮村 淳一

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.6, pp.429-431, 1997-12-26

被引用文献数

7

著者

早川 由紀夫 井村 隆介

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.36, no.1, pp.p25-35, 1991-04

被引用文献数

4

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The eruptive history of Aso volcano for the past 80,000 years is revealed by tephrochronology and loess-chronometry. Around the Aso caldera is a thick accumulation of loess, which is intercalated with numerous Aso tephra layers of limited dispersal as well as three widespread tephra layers of known age that are good marker horizons ; the Akahoya ash (6.3 ka), the Aira-Tn ash (22 ka), and the Aso-4 ignimbrite (70 ka). Loess-chronometry is based on the assumption that, in the Aso region, the accumulation rate of loess has been constant as 12 cm/ky from 80 ka to the present. Most of tephra layers after the caldera-forming Aso-4 eruption are composed of volcanic sand or scoria lapilli of basaltic andesite composition. However the 27 ka Kusasenri dacite (SiO_2 = 67%) pumice is a conspicuous exception. The large volume of 5.85 km^3 (bulk) and wide dispersal of this pumice suggests that it is a product of plinian eruption. From October 5 to the end of November 1989, the Nakadake crater of Aso volcano was in eruption. Ash was uninterruptedly emitted from a 500-1,000 m high eruption column coming out of the crater. The average discharge rate of ash was 5 × 10^7 kg/day. The total mass of ash discharged during the two months reached 3 × 10^9 kg. The penultimate eruption in recent history was June-August 1979, when 7.5 × 10^9 kg of ash was discharged. Outside the Aso caldera, the thickness of the 1989 ash is less than 1 cm. It is almost impossible to detect an old ash layer of thickness about 1 cm in a loess cross section, suggesting that sedimentary records 10 km away from a volcano are insufficient to reconstruct past eruptions smaller than 10^<10> kg. Eruptions smaller than 10^<10> kg can be determined only from proximal deposits. The history of eruptions of Aso volcano over the last few thousand years is tentatively determined from cross sections 2-4 km west of the Nakadake crater. After a 580-1,250 year dormant period, Aso volcano became active about 1,780 years ago. From then, small eruptions each with 10^9-10^<10> kg ash discharge have been repeated 48-88 times up to the present. The duration of each eruption was a few months, and the dormant interval between eruptions averaged 20-37 years.

著者

馬越 孝道 清水 洋 松尾 のり道

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.39, no.5, pp.223-235, 1994-11-20

被引用文献数

9

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Fugendake, the main peak of Unzen Volcano of Kyushu Island in southwest Japan, started to erupt on November 17, 1990, after 198 years of dormancy, and lava extrusion has continued over three years since May 1991. Hypocenters of earthquakes which occurred before and during that eruption were precisely determined using P-wave arrival time data from five selected seismic stations near the focal region. The hypocenters in Chijiwa Bay are distributed in Chijiwa Caldera. Two linear arrangements of epicenters directed nearly from west to east emerged clearly in the western part of the Shimabara Peninsula, whose hypocentral depths became shallower toward the summit of Fugendake. The distribution of hypocenters is restricted by the fault systems which have been formed by the crustal movements of Unzen Graben. The stress which generates these earthquakes is dominated mainly by the north-south extension ; consistent with the regional tectonic stress. The directions of pressure axes are controlled by the magmatic pressure beneath the focal region. It is inferred from the hypocentral distribution and the orientations of pressure axes that the magma involved in the 1990-94 eruption is situated below an inclined boundary between seismic and aseismic regions in the western part of the Shimabara Peninsula. The magma ascent path is located at 13±2 km in depth beneath the western shore of the Shimabara Peninsula, becoming shollower eastward with an angle of elevation of 40〜50°.

著者

宮縁 育夫 渡辺 一徳

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.45, no.1, pp.25-32, 2000-03-10

被引用文献数

3

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Jigoku spa is located west of Ikenokubo basin of Yomineyama volcano, southwestern part of the central cones of Aso volcano. In this basin, "Ikenokubo tuff ring" and two smaller maars were developed. Two layers of ejecta, about 10^5 m^3 in bulk volume, formed by phreatic explosions were found in this area. They are very poorly sorted and mainly composed of altered fine-grained ash including lithic fragments. Kikai Akahoya ash erupted about 6,300 years BP (^<14>C) is intercalated between two layers of phreatic ejecta in thick ash layers from Nakadake, the only active central cone. Based on ^<14>C ages of buried Andisols and the succession of the deposits, the eruption ages of the two ejecta are estimated at about 10,000 years BP for older deposit and at about 4,500 years BP for younger one. Distributions of thickness and maximum grain size of the lithic fragments suggest that the both of two phreatic ejecta were exploded not from "Ikenokubo tuff ring" and two maars but from the Jigoku explosion crater in the western area. Ejecta correlative to the "Ikenokubo tuff ring" Iies under the older of the phreatic ejecta layers. Consequently, the eruption age of "Ikenokubo tuff ring" is probably older than 10,000 years BP. Frequency of phreatic explosions, Iarger than 10^5 m^3 in volume, is considered to be once in about 5,000 years. This fact is noteworthy for volcanic hazards assessment in the surrounding area of Jigoku spa and adjacent Tarutama spa.

著者

安田 裕紀 佐藤 鋭一 和田 恵治 鈴木 桂子

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.4, pp.447-459, 2015

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Eruption interval between Hb- and Px-types pyroclastic-flows from the Ohachidaira caldera of Taisetsu volcano, central Hokkaido, Japan, was estimated from the paleomagnetic directions. Px-type pyroclastic-flow deposit rests on Hb-type one, and gravel beds are intercalated between them. Oriented 138 samples were collected from 13 sites for paleomagnetic analysis. The paleomagnetic direction of Hb-type pyroclastic-flow deposit shows a normal polarity with a westerly declination(overall mean is N=7, D=-27.1°, I=66.3°, α_<95>=2.7°, k=511.2), while that of Px-type pyroclastic-flow deposit shows a normal polarity with an easterly declination (overall mean is N=6, D=19.8°, I=67.5°, α_<95>=4.6°, k=213.8). The two paleomagnetic directions are significantly different, and the time interval between the two pyroclastic eruptions is estimated to be more than about 100±40 years based on the geomagnetic secular variation in China, Russia, Europe, North America, and Japan.

著者

長宗 留男 横山 博文 福留 篤男

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.37, no.1, pp.1-8, 1992-04-01

被引用文献数

2

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Earthquake swarms which have frequently occurred off the east coast of the Izu Peninsula since 1978 are investigated, and the relationships between the swarms and the submarine volcanic eruption at Teisi Knoll in 1989 are discussed. The results are summarized as follows: 1) Shapes of epicentral areas of the earthquake swarms can be classified into the following two types, Type I and Type II. The former is an ellipse elongated NE-SW; the latler, an eilipse elongated approximately NW-SE. The swarms in the early years are of Type I, and those in the later stage (in particular, since 1984), for the most part, are of Type II.2) The largest earthquakes in the respective swarms for Type I are located along a straight line in the NE-SW direction, and those for Type II, along a curve line in the E-W to NW-SE directions. These two lines are probably indicative of active tectonic lines. 3) The epicentral areas for Type I and Type II, migrate periodically with a recurrece time of 6-7 years along the tectonic line in the NE-SW direction, and with a recurrence time of about 3.7 years along the tectonic line in the E-W to NW-SE directions, respectively. 4) Teisi Knoll where the submarine volcanic eruption took place on July 13, 1989, is situated in the northwestern part of the tectonic line trending E-W to NW-SE. The eruption was triggerd by the largest shock in the swarm which occurred around the northwestern end of the line.

著者

宮町 宏樹 泊 知里 八木原 寛 井口 正人 為栗 健 山本 圭吾 大倉 敬宏 安藤 隆志 尾西 恭亮 清水 洋 山下 裕亮 中道 治久 山脇 輝夫 及川 純 植木 貞人 筒井 智樹 森 済 西田 誠 平松 秀行 小枝 智幸 増田 与志郎 加藤 幸司 畠山 謙吾 小林 哲夫

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.58, no.1, pp.227-237, 2013-03-29

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2008年に実施された屈折法地震探査によって得られたP波初動走時により,姶良カルデラおよび桜島火山の深さ3kmまでの速度構造を推定した.本研究地域の基盤層である四万十層群は4.6-5.0km/sのP波速度を持ち,姶良カルデラの中央部に向け傾斜している.姶良カルデラの中央部には,4.2-4.4km/sの低速度域が深さ1.5-3kmに存在している.そして,この低速度域はカルデラ下に存在する深部マグマ溜まりからのマグマ供給系が活発であることを示唆している.また,基盤層は鹿児島地溝帯の北西域の境界に沿って深さ1kmから2.5kmに急激に落ち込んでいることがわかった.桜島火山の速度構造は3.6-3.7km/sの領域が存在することで特徴づけられる.桜島火山の山頂直下で発生している火山性地震の震源域と速度構造の比較から,地下構造が種々の火山性地震の震源域の広がりに強い影響を与えていることを示した.

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

為栗 健 MARYANTO Sukir 井口 正人

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.5, pp.273-279, 2007-10-31

被引用文献数

2

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桜島火山において発生するハーモニック微動のモーメントテンソル解析を行った.B型地震群発後に発生する微動(HTB)と爆発的噴火直後に発生する微動(HTE)のモーメントテンソル成分に大きな違いはなく,等方成分は50%以上,CLVD成分は20〜30%,DC成分は20%以下であった.鉛直方向のダイポール成分が大きく,鉛直方向の力が優勢な震源が推定される.震源は火口直下の浅部であり,爆発的噴火発生前に火口底直下に形成されているガス溜まりが微動の発生に関与していると考えられる.

著者

吉田 明夫 瀬野 徹三

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.37, no.6, pp.297-301, 1992-12-25

被引用文献数

4

著者

上澤 真平

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.53, no.6, pp.171-191, 2008-12-29

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On May 24^<th> 1926, the eruption of Tokachidake volcano, in central Hokkaido, efficiently melted the snow pack on the hill slope, triggering the Taisho lahar which killed 144 people in the towns of Kamifurano and Biei. A geological survey and paleomagnetic and granumetric studies were conducted on the northwestern slope of Tokachidake volcano to reconstruct the sequence of the 1926 eruption and decipher the triggering mechanism for the Taisho lahar. The Taisho lahar deposits in the proximal area of the volcano are divided into five distinct units (unit L1, L2, and A through C, from oldest to youngest). Unit L1 is an older lahar deposit that underlies the 1926 deposits. The 1926 sequence consists of debris avalanche deposits (unit A and C), a laminated sandy debris flow deposit (unit B), and a lahar deposit including scoria clasts (unit L2). Each unit contains hydrothermally altered rocks and clay material with more than 5 wt.% fragments smaller than 2mm in diameter. The progressive thermal demagnetization experiments show that the natural remanent magnetization (NRM) of all samples in unit A, B and C have a stable single or multi-component magnetization. The emplacement temperatures are estimated to be normal temperatures to 620℃ for unit A, 300 to 450℃ for unit B, and normal temperature to 500℃ for unit C. On the basis of geological and paleomagnetic data and old documents, a sequence for the eruption and the mechanism of formation and emplacement of the Taisho lahar can be reconstructed. The first eruption at 12: 11 May 24th triggered a small lahar (unit L2). Collapse of central crater at 16:17 May 24th 1926 then resulted in a debris avalanche containing highly altered hydrothermal rocks with hot temperatures ranging from 300 to 620℃ (unit A). The debris avalanche flowed down the slope of the volcano, bulldozing and trapping snow. Immediately following the collapse, a hot (approximately 400℃) hydrothermal surge (unit B) melted snow and transformed into a lahar causing significant damage and deaths in the towns downstream. Just after the generation of the lahar, another collapse occurred at the crater causing another debris avalanche (unit C).

著者

田中 康裕

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.44, no.2, pp.109-110, 1999-04-30

著者

大野 希一 国方 まり 鈴木 正章 西村 裕一 長井 大輔 遠藤 邦彦 千葉 達朗 諸星 真帆

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.47, no.5, pp.619-643, 2002-11-29

被引用文献数

4

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The volcanic activities of the Usu 2000 eruption were monitored and reported by many scientists and the mass media. Summarizing these observation results, most of relatively large explosion events occurred during March 31 to April 7, 2000. Around the Nishiyama and Kompira crater groups, the pyroclastic deposits with multi units can be divided into 19 layers on the basis of their visible color, grain size and sedimentary structure; from Layer A to Layer S in ascending order. The eruptive dates of each layer inferred from the wind directions, the eruptive sequence, and the distribution of deposits are summarized as follows; the Layer A, characterized as the light gray color ash fall deposit including in some pumice layers, was generated by March 31 phreatomagmatic explosions occurred at Nishiyama crater group. The Layer B, composed poorly sorted breccia and ash layer with gray color, was generated on March 31 p.m. at Nishiyama craters. The Layer C to the Layer G, dark brown-gray aggregate ash, were derived from the volcanic eruptions occurred on April 1 to 2 in Nishiyama and Kompira crater group. The Layer H to Layer M and Layer O, mainly consist with gray and reddish brown aggregate ash including in lithic fragments, were generated during April 3 and 4 in Kompira crater group. The Layer N, which distributes around N19 crater, generated on April 4. The Layer P, massive ash with gray color, was generated on April 6 in Kompira crater group. After April 7, the Layer S, characterized as light brown aggregate ash, has been generated from the recent minor activities around limited craters. The amount of Layer A fallen in the range from the source to Toyako Onsencho Town is estimated at 1.2×10^8 kg, and total amount of Layer A including in the distal area is 2.4×10^8 kg. On the other hand, amount of other deposits generated during April 1 to 6 (e.g. Layer B, N, and Q) is an order of 10^6-10^7 kg. Total amount of the pyroclastic deposits erupted from the Usu 2000 eruption is more than 6.4×10^8 kg.