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.65-87, 2011-06-30 (Released:2017-03-20)

参考文献数

27

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The 1783 activity of Asama volcano was reviewed from May to the end of the year based on 166 old documents, including those recorded at a distance. 1. Prior to the 1783 Asama eruption, the level of magma head ascended at Kama-yama crater-pit. Moderately explosive eruptions commenced on May 9, and repeatedly blew off the plugged magma. 2. Depending on the wind direction, ash fell N, NNE, and NE of the crater including Sado Island, Tohoku and Kanto districts. From August 3 to 5, climactic plinian eruption dispersed pyroclastic materials. Distributions of 8 tephra- fall units were presented. 3. The timing of rumbling and quakes at distant places farther than 100km from the crater well correspond with explosive events witnessed by neighbors of the volcano. 4. Duration of a single eruptive event rarely exceed 6 hours. It was true even during the culminating plinian stage from Aug. 3 to 5, 1783. The eruption was so violent in this stage that huge blocks larger than 10m were thrown from Kama-yama crater. 5. Documents concerning with Kambara pyroclastic flow and subsequent debris avalanche occurred on August 5, suggested that an explosion on the northern flank triggered collapse of northern sector. The event occurred at about 08:00 to 08:30am, which is 90 to 120 minutes earlier than estimations appear in previous work. 6. Small and less frequent eruptions continued until January 15, 1784.

著者

及川 輝樹 筒井 正明 大學 康宏 伊藤 順一

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.57, no.4, pp.199-218, 2012-12-28 (Released:2017-03-20)

参考文献数

65

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Shinmoedake (Kyushu, Japan), which is one of the Kirishima Volcanoes, experienced several small eruptions in 2010, finally culminating in a sub-plinian eruption on January 26-27, 2011. After this sub-plinian phase, the eruption style shifts to the phase of vulcanian eruption or ash emission. This volcanic activity is still occurring. We here summarize the eruption history of Shinmoedake during the Edo period on the basis of historical records. The eruptions of Shinmoedake during the Edo period occurred in AD 1716-1717 (Kyoho eruption) and AD 1822 (the 4th year of Bunsei eruption). The Kyoho eruption, which was a large-scale (total amount of tephra: 2×1011 kg) eruption, is divided into the following seven stages. Stage 1 (Apr. 10, 1716 to May 7, 1716): small eruptions occurred over two months; Stage 2 (Sep. 26, 1716): falling ash first observed at the foot of Shinmoedake; Stage 3 (Nov. 9 to 10, 1716): the first large eruption was observed, with pumice falling over a wide area; Stage 4 (Dec. 4 to 6, 1716): small eruptions; Stage 5 (Feb. 9 to 20, 1717): the second pumice fall eruption, with an intermittent ash fall eruption thereafter; Stage 6 (Mar. 3, Mar. 8, Mar 13, Apr. 8, 1717): ash fall eruptions; Stage 7 (Sep. 9, 1717): the last ash fall eruption. These eruptions, which continued intermittently over 17 months, were characterized by multiple repetitions of a large eruption. Based on the results of a comparison between the Kyoho eruption and the 2011 eruption, the eruptions from March 30, 2010 to January 26, 2011, were similar to Stages 1 to 3 of the Kyoho eruption; the eruptions after January 26, 2011, were similar to Stages 5 to 6 of the Kyoho eruption. In addition, the relatively large eruption events of Stages 3 and 5 of the Kyoho eruption and the January 26-27, 2011, eruption began without any noticeable precursors. The eruption in the 4th year of Bunsei (AD 1822) was a small eruption that lasted less than a day. The recent eruption sequences, which were also similar to the Edo period eruptions, are divided into a small-scale eruption (the 1959 eruption) and a large-scale eruption (the 2011 eruption). The eruption duration time of the small-scale (total amount of tephra: < 1010 kg) eruption was less than a day. The eruption duration time of the large-scale (total amount of tephra: > 1010 kg) eruption could be a few months or years. Both eruption sequences began with a small eruption. A large-scale eruption can occur a few months after the start of the eruption sequence. This is an important turning point in the eruption sequence of Shinmoedake.

著者

高橋 正樹

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.1, pp.33-42, 1995-03-10 (Released:2017-03-20)

参考文献数

22

被引用文献数

5

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There is no positive correlation between the long-term eruption rate of large-scale felsic volcanism and its discharge volume of a single eruptive episode. This means that the storage of voluminous felsic magma at high-level in the crust is caused not by high magma production rate but by continuous accumulation of magma during a long repose time, if the long-term eruption rate reflects the averaged magma production rate. If the cruslal defomation is weak, the magma chamber could be stable in the crust; it is favorable for efficient accumulation of voluminous magma. In fact, the large-volume felsic volcanism occurs exclusively in the region of low crustal strain rate. The low crustal strain rate is considered to be essential for the formation of large-scale felsic volcanism. The large-volume felsic volcanic activity is present in the compressional tectonic stress field as well as in the extensional one; the difference in arrangement of principal stress axes is not related to the occurrence of voluminous felsic volcanism.

著者

上野 龍之

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.61, no.3, pp.533-544, 2016-09-30 (Released:2016-11-08)

参考文献数

26

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The Tsumaya pyroclastic flow deposit is one of the main units of the Aira pyroclastic eruption, which produced the Aira caldera in Southern Kyushu, Japan, 30,000 years ago. The Tsumaya deposit overlies the main plinian unit, the Osumi pumice fall deposit, and is covered by the large-volume pyroclastic flow unit, the Ito pyroclastic flow deposit. The Tsumaya deposit consists of massive facies associated with smaller volume of stratified facies. The total eruption mass is 2.8×1013 kg (estimated by the crystal method), of which approximately 48 % was elutriated to a co-ignimbrite ash fall. The upper part of the Osumi pumice fall deposit is intercalated with the stratified facies of the Tsumaya pyroclastic flow deposit, indicating that the Tsumaya eruption began during the final phase of the Osumi eruption. The Tarumizu pyroclastic flow and the Osumi pumice fall were produced from the same vent in the southern part of the caldera. The Tsumaya pyroclastic flow deposit has been considered to be the same stratigraphic unit as the Tarumizu deposit;however, the two deposits have contrasting origins and different contents of lithic fragments, indicating they were erupted from different vents. Lateral variations in the altitudes of the depositional surface of the Tsumaya deposit indicate that the Tsumaya pyroclastic flow was erupted from the northeastern part of the Aira caldera.

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.61, no.1, pp.199-209, 2016-03-31

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Deterministic eruption scenarios may mislead taking countermeasures for coming hazards. Preparing the event tree covering all phenomena which may happen in future eruptions even with low probability for the volcano is important not only for forecasting eruptions but also for disaster prevention. Eruption event trees can be prepared in various concepts, for example, eruption type, scale, hazard type, impact direction or area and so on. The probability tree is the event tree equipped with probabilities for the branches. Probability trees by USGS and European scientists include the cumulative trees, trees based on scientists' elicitation and Bayesian trees. Introduction of the eruption event trees into the Japanese volcanologist community began around 2009. Then, event trees were prepared for Izu-Oshima, Miyakejima, Sakurajima, Usu, and Izu-Tobu volcanoes. Reasons for branching and time scales of events were also discussed and shown on the event trees together with probabilities. The event tree for Sinabung volcano, Indonesia, as an example of lava dome-forming eruptions was drawn in 2011, based on the geological study. On-going lava dome/flow eruption at this volcano just followed the most probable scenario. For Sakurajima volcano, a conceptual event tree was drawn for understanding the anomalies controlling the eruption scale.

著者

萬年 一剛

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.48, no.6, pp.425-443, 2003-12-25

被引用文献数

3

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Hakone volcano is situated at the northern tip of the Izu-Mariana volcanic arc in eastern Japan, and area that is both tectonically and volcanically active. Fumarolic activity is observed a.t post-caldera cone volcanoes within the caldera, and the northern extension of the Kita-Izu fault, the source of a M7.3 earthquake in 1930 (Kita-Izu earthquake), traverses the southern part of the caldera. Although there is no historical record of eruptive activity, many intense earthquake swarms have been reported since 1786 within the caldera. In this study, literature on earthquake swarms in 1917, 1920, 1933-35, 1943, 1944, 1953, 1959-60 are re-examined to reveal detailed development of the activity, seismic intensity and the epicentral region of these events. Two epicentral regions are recognized; the central cones region (1), and the southern part of the caldera (2). Earthquake swarms in (1) are often accompanied by rumblings and the main shock is not distinct; successive earthquakes are felt almost continuously during the peak of activity. On the other hand, earthquake swarms in (2) are rarely accompanied by rumblings and have obvious sequence of foreshocks, a mainshock and aftershocks. The largest earthquakes in the swarms in (2) are larger than those in (1). The two epicentral regions are both on the northern extension of the Kita-Izu fault system. Differences in the style of earthquake swarm activity in regions (1) and (2) may be due to differences of the geology and the source depth. Correspondence between fumarolic activity in the solfataras at central cone volcanoes and seismic activity was not observed except for the 1933-35 swarm. Most of the earthquake swarms at Hakone volcano are therefore probably tectonic earthquakes on the Kita-Izu fault system rather than being related to hydrothermal or magmatic activity within the caldera. Earthquake swarms at Hakone appear to have been rare before 1917, and except for 1786, no historical records exist even though one of the most important highways in Japanese history passed across the volcano. An interpretation that attributes the earthquake swarms since 1917 to foreshocks and aftershocks of the 1930 Kita-Izu earthquake would broadly explain the frequency of earthquake swarms at Hakone volcano since the early twentieth century.

著者

宮縁 育夫

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.5, pp.219-225, 2010

参考文献数

17

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Komezuka Volcano, located in the northwestern part of the post-caldera central cones of Aso Volcano, SW Japan, is a basaltic monogenetic volcano comprising a scoria cone (370-380m in basal diameter; 80m in height) and lava flows (10.5km²; 5×10⁷m³). We obtained ¹⁴C ages of 3,070±40 years BP from a buried soil below silty ash underlying Komezuka lava, which corresponds to 3,370-3,210cal years BP, and 2,760±40 years BP (2,950-2,770cal years BP) from a soil above silty ash overlying Komezuka lava. The age of soil below the lava suggests that the eruption age of Komezuka Volcano is about 3,300cal years BP. The eruption age is consistent with the age of Ojodake Volcano (3,600cal years BP) whose lava underlies Komezuka lava. In the northwestern part of the post-caldera central cones, Late Holocene monogenetic volcanic activity commenced with sub-plinian eruptions and lava extrusion from Kishimadake Volcano at approximately 4,000cal years BP, followed by sub-plinian eruptions and lava extrusion from Ojodake Volcano at 3,600cal years BP, and ceased with strombolian eruptions and lava extrusion from Komezuka Volcano at 3,300cal years BP.

著者

安井 真也 富樫 茂子 下村 泰裕 坂本 晋介 宮地 直道 遠藤 邦彦

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.43, no.2, pp.43-59, 1998-04-30

被引用文献数

4

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A large amount of pyroclastic materials (ca. 1.7 km^3) was erupted during the 1707 eruption of Fuji Volcano. Variety of lithic fragments has been recognized in the pyroclastic fall deposits, namely, accessory and accidental lava fragments, gabbros, and granitoids. A great variety of petrologic features is observed in gabbroic fragments consisting of olivine gabbro norite, gabbro norite, troctolite and anorthosite. The gabbros are divided into O, P and F groups on the basis of modal ratios of olivine, plagioclase and Fe-Ti oxide. O group mainly consists of plagioclase and olivine with minor amounts of pyroxenes and Fe-Ti oxide. O group is considered to have been adcumulated in the lower part of magma chamber because of their high depletion in incompatible elements, their well-sorted grain size and sedimentary structure. P group is composed of plagioclase, pyroxenes and minor amounts of olivine and Fe-Ti oxide. F group is similar to P group, but is enriched in Fe-Ti oxide. P and F groups are orthocumulates and may be solidified in the upper part and margin of magma chamber or dike because of their porphyritic texture. Such a variety of gabbros may correspond to the difference in location of the single gabbroic body beneath Fuji Volcano. The estimated source magma of the gabbros is similar to the basalt of Fuji Volcano in chemical and mineralogical compositions indicating that they are cognate origin. Chemical compositions of olivine and pyroxenes become magnesian and those of plagioclase become calcic with the decreasing of bulk-rock FeO^*/MgO ratio. It suggests that they are the products of continuous fractional crystallization. The magma of the 1707 eruption could have come up from under the gabbroic body, which was the solidified basaltic magma chamber, and have caught and brought the rocks from the gabbroic body up to the surface as cognate xenoliths during the eruption.

著者

山元 孝広 高田 亮 石塚 吉浩 中野 俊

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.2, pp.53-70, 2005-05-20

被引用文献数

5

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The previous eruption history of Fuji volcano has been re-examined by new 100 radiometric carbon ages. The major unconformity between Ko-Fuji and Shin-Fuji volcanoes of Tsuya (1968, 1971) was caused by the edifice collapse resulting in the Tanukiko debris avalanche at about Cal BC 18,000. Voluminous effusion of basalt lava flows in the older ejecta of Shin-Fuji volcano (Tsuya, 1968, 1971) had started at about Cal BC 15,000 and continued until about Cal BC 6,000. Deposition of black soil layer between the Older and Younger Fuji tephra layers of Machida (1964, 1977) started at Cal BC 8,000. After several thousands years quiescent time, basaltic eruptions in the middle ejecta of Shin-Fuji volcano (Tsuya, 1968, 1971) had restarted at about Cal BC 3,600 and thin lava flows had piled up as the central volcanic cone, until about Cal BC 1,700. The eruption style of the volcano changed into explosive basaltic eruptions from the summit and the flank at about Cal BC 1,500; the S-10 to S-22 scoria fall deposits were generated in this first half period of the younger ejecta of Shin-Fuji volcano (Tsuya, 1968, 1971). Also, basaltic pyroclastic flows cascaded down the western flank at about Cal BC 1,500, Cal BC 1,300, Cal BC 1,000 and Cal BC 770. The last summit explosive eruption (S-22) occurred at about Cal BC 300. Immediately after the S-22 eruption, basaltic fissure eruptions had repeated at the flanks until the 1707 Hoei eruption. New data suggest that the Fudosawa, Nissawa and Suyama-tainai lava flows in the southern flank are historical products at about Cal AD 1,000.

著者

中村 一明

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.20, pp.229-240, 1975

被引用文献数

14

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Volcanoes are generally classified into monogenetic and polygenetic types. Monogenetic volcanoes erupt only once to form smaller volcanoes, such as maars, pyroclastic cones and lava domes. Polygenetic volcanoes erupt repeatedly from the same general vents (summit or main crater) for up to 10⁵ years to form larger volcanoes such as strato-volcanoes (composite volcanoes of Macdonald, 1972) and shield volcanoes of Hawaiian type. Monogenetic volcanoes tend to occur in clusters as flank and post-caldera cones. Some of the clusters are however, independent of polygenetic volcanoes and appear to be equivalent to them. The essential part of the conduit of a monogenetic volcano is inferred to be a simple dike, intruded into a newly formed crack, whereas a long endured pipe-shaped conduit may exist under a polygenetic volcano. The common occurrence of xenoliths in the eruptive products of monogenetic volcanoes may be related to this difference. Various lines of evidence, indicating the existence, depth, shape, volume and internal structure, of magma reservoirs are tabulated. A shallow magma reservoir appears to exist beneath polygenetic volcanoes with one to one correspondence, which is not the case for monogenetic volcanoes. Most flank volcanoes are monogenetic, thus indicating dikes within the polygenetic volcanic edifice. Dike formation is understood as a magma version of hydraulic fracturing. For the dike to intrude and propagate, would require either the increase of differential stress due to a decrease of minimum compression or increase of pore pressure over the sum of the minimum compression and the tensile strength of the rocks. Earthquakes are understood as the generation of elastic waves associated with an acute release of tectonic stress due to faulting. Accumulation of tectonic stress and strain prior to earthquakes is, then, a necessary part of earthquake phenomena in a broad sense, as well as their release after the event. Based on the above-stated understanding, possible mechanical correlations between volcanic eruptions and earthquake occurrences have been studied. Contractional strain around the magma reservoir can cause the squeezing up of magma within an open conduit causing a summit eruption on the one hand, and dike formation resulting in a flank eruption through the increase of pore pressure, on the other. Second boiling triggered by both the magmatic pressure decrease caused by dilatational strain and the dynamic excitation due to seismic waves might have the same effect as contraction. Decrease of minimum compression causing the increase of differential stress leading to dike formation will also contribute to the liklihood of flank eruptions. Both volcanic eruptions and earthquake occurrences can precede each other depending on geographical location in terms of faulting-related stress-strain changes which are calculated by the fault model of earthquakes. Actual possible examples of volcanic eruptions and earthquakes which are allegedly mechanically related are given. In order to demonstrate which mechanism is responsible for the correlation of the two phenomena, continuous strain measurement on and around volcanoes is necessary together with the observation of changes in the level of magma in crater bottoms.

著者

伊藤 英之 脇山 勘治 三宅 康幸 林 信太郎 古川 治郎 井上 昭二

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.6, pp.427-440, 2005

参考文献数

27

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The Yakedake volcano is located in the southern part of the northern Japan Alps, central Japan. Yakedake volcanic hazard map was published in March 2002, and in June 2002, it was distributed to the inhabitants of Kamitakara village, Gifu prefecture, where is located 4-20km west from the volcano. In January 2003, the questionnaire survey was carried out on the inhabitants in order to know their attitudes to the volcanic hazard map and the level of their understanding of the contents of the hazard map. The Kamitakara village office distributed the questionnaires to 1,102 families through the headman of each ward, the headman collected 802 answers. The results of analysis were as follows. 89% of the respondents knew the existence of the hazard map and 35% read it well, but about 11% have not read the map at all. The elders have a tendency to have deeper understanding of the hazard map than younger ones, especially in elders who have experiences to meet some kinds of natural hazards. And the people who once attended the explanatory meeting of the hazard map, which was held for the residents living inside the disaster-prone area four times after the publication of the hazard map, also tend to have more proper understandings. The people who are engaged to the tourism give more attention to the volcanic hazard than others. The respondents have strong tendency to require more knowledge about the volcanic activities and hazards. We can say that the further activities by scientists, engineers and administrative officers are expected in order to establish an informed consent, that is, there should be a decision-making by inhabitants themselves and support by officers in charge with detailed explanations.

著者

西村祐一 宮地直道

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.43, no.4, pp.239-242, 1998-08-31

被引用文献数

1

著者

辻 智大 岸本 博志 藤田 浩司 中村 千怜 長田 朋大 木村 一成 古澤 明 大西 耕造 西坂 直樹 池田 倫治 太田 岳洋 福岡 仁至

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.68, no.3, pp.129-160, 2023-09-30 (Released:2023-11-02)

参考文献数

49

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Kuju volcano, located within Beppu-Shimabara graben central Kyushu, Southwest Japan, has been active in the recent 200,000 years. The 54 ka Handa eruption, as large as VEI 5 or 6 and the largest one of the volcano, released large-scale pyroclastic flow deposits (Handa pfd; Kj-Hd) and a wide-spread tephra (Kj-D ash and Kj-P1 pumice fall deposits) that has been reported at more than 500 km from the source. The stratigraphic relationships among the deposits from the Handa eruption are important for volcanology and disaster prevention, and have been studied in various studies, but there is no consensus on the stratigraphy. In this study, we examined the stratigraphic relationships and the eruption history based on the stratigraphic and petrographic studies around Kuju volcano, as well as on Shikoku and Honshu Islands. As the results, the stratigraphic relationships were revealed as follows. 1) The pumice fall deposit, that has been named Kj-Yu, was previously included in Kj-D ash layers, but is revealed to be a much older ejecta than Kj-D ash, along with the tephras newly named Kj-Tb1 and 2. 2) The clay-rich layer just below Kj-D was previously considered to be soil, but it contains a large number of volcanic ash particles so that it is defined as Kj-Y ash layer. 3) Three light brown fine ash layers, newly named Kj-D-U2, 4 and 6, sandwich between the blue grey sandy ash layers i.e. Kj-D-U1, 3, 5 and 7, are revealed to be the co-ignimbrite ash derived from Kj-Hd 1, 2 and 3 pfd, respectively. It suggests that the Kj-Hd1, 2 and 3 pfd are interbedded with Kj-D-U ash layers. 4) Kj-P1 overlies on Kj-D-U7 ash layer that mantled the reworked deposit of Kj-Hd3. 5) Kj-P1 is divided into lower and upper units based on the grain-size analysis, petrography, the chemical composition of glass shards and the isopach maps. Kj-S pfd was formed in the same time as the upper unit. Based on the results, the eruption history is assumed as follows. Pre-Handa eruption: the activity was low and the small-scale explosive eruptions that had released the pumice and volcanic fragments in loam (Kj-Y), followed by a relatively large explosive eruption that had formed Kj-AL. Early phase: the eruption started with phreatic eruption, sub-plinian eruption that deposited the lower unit of Kj-D ash. Subsequently, the eruption changed to vulcanian eruptions that ejected Kj-D-U. This eruption continued for a long period time. During the time, three large-scale pyroclastic flow eruptions happened and has formed Kj-Hd1, 2 and 3. Their co-ignimbrite ashes generated from the Kj-Hd pfds were deposited as Kj-D-U2, 4 and 6. Lahar were generated after Kj-Hd2 and 3 deposition. This phase was terminated by the deposition of Kj-D-U7 ash. Late phase: the plinian plumes occurred twice and deposited lower and upper lalyers of Kj-P1. The second one is the largest plinian eruption in the whole volcano history, with a large umbrella plume producing a wide-spread tephra at more than 500 km from the source and an intraplinian pyroclastic flow (Kj-S).

著者

及川 輝樹 谷 健一郎

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.65, no.3, pp.83-87, 2020-09-30 (Released:2020-10-13)

参考文献数

18

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The activity of Toshima Volcano, Izu Islands in Japan, is divided into two stages: younger (the lava flows of parasitic craters) and older (the main stratovolcano) stages. The 14C age of “Kajiana” crater lava, the earliest lava of the younger stage, was approximately 11 ka (cal BP). Based on the results of radiocarbon dating and topographic analysis, we conclude that all three magmatic eruptions of the younger stage of Toshima Volcano occurred during the Holocene.

著者

宮地 直道 富樫 茂子 千葉 達朗

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.49, no.5, pp.237-248, 2004-10-29 (Released:2017-03-20)

参考文献数

34

被引用文献数

5

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A large-scale collapse occurred at the eastern slope of Fuji volcano about 2900 years ago, based on calibrated 14C age of a wood sample collected in the resulting debris avalanche deposit. The collapsed slide deposit, called “Gotemba debris avalanche deposit” (Goda), is distributed on the eastern foot of the volcano covering an area of more than 53 km2 The source amphitheater is not preserved because it became covered by younger tephra erupted from the summit crater. This avalanche deposit is overlain by the “Gotemba, mudflow deposits” (Gomf) emplaced repeatedly after the avalanche. Some now units of the Goda and Gomf entered pre-existing rivers and were finally emplaced as fluvial deposits. The Goda is composed of debris-avalanche blocks, showing jigsaw cracks, along with smaller blocks ranging from several tens of centimeters up to l m in diameter. The debris-avalanche matrix is a mixture of smaller nieces of blocks and ash-sized materials due to mainly shearing and fragmentation of large blocks. Igneous rocks include fresh and altered gray basaltic lava, weathered tephra including red scoria and white clay. Petrographical and geochemical data indicate that most blocks were derived from the Older Fuji volcano. The volumes of the Goda and Gomf are about l.05km^3 and 0.71km^3 respectively, based on presently available geological and borehole data. Since the blocks of Goda are composed mostly of the products of the Older Fuji volcano and the older stage lavas of Younger Fuji volcano do not extend to the eastern foot of Fuji volcano, a bulge of Older Fuji volcano must have existed in the eastern flank of Fuji volcano preventing the older stage lavas to now to the east. This bulge collapsed in the form of three blocks from the foot of the mountain. The abundance of hydrothermally altered deposits in the Goda and the absence of fresh volcanic products within the Goda suggest its origin as a rupture inside the altered deposits possibly triggered by a large earthquake or phreatic eruption.

著者

林 信太郎

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.43, no.4, pp.207-212, 1998-08-31 (Released:2017-03-20)

参考文献数

19

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Kampu volcano is a small stratovolcano situated at the central part of Oga Peninsula, Akita Prefecture. In 1810, an earthquake as large as M 6.5 occurred near this volcano. Yoshimasa Satake, the lord of the Akita clan, wrote two official reports to the Tokugawa shogunate. They included eruption records of Kampu volcano: “Yamayake” and “Yamayakekuzure”. These words were usually used for the eruption during Edo period and mean that the mountain was firing. Several reliable documents, which was written at Oga Peninsula included no eruption record. In addition, there is no eruption record in the note of Yoshimasa Satake, which is thought to have used for making the two official reports. It is concluded that the eruption descriptions of 1810 Kampu is false and created by Yoshimasa Satake at Akita clan office at Edo (Tokyo). The false eruption might have been created to make the exaggerated damage report of earthquake.

著者

中道 治久

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.207-220, 2022-06-30 (Released:2022-07-28)

参考文献数

57

著者

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.43, no.5, pp.349-371, 1998

参考文献数

53

被引用文献数

3

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Reevaluation of places, type, magnitude, and influences of the 800-802 A.D. eruption (Enryaku eruption) of Fuji Volcano, Japan, was made through tephrochronology and analyses of historical records. The Nishi Kofuji fissure on the northeastern slope is newly recognized as a crater of the 802 A.D. flank eruption. The Nishi Kofuji fissure ejected fallout scoria toward ENE and lava flows, which can be correlated with Takamarubi and Hinokimarubi 11 Lavas on the northeastern foot. The Tenjinyama-lgatonoyama fissure on the northwestern slope probably erupted during the Enryaku eruption and ejected fallout scoria and lava fiows. A series of historical documents and paintings (Miyashita documents), which are unauthorized, personal records and are regarded to be unreliable by many historians, includes many detailed descriptions of paleogeogra-phy around Fuji Volcano and of the Enryaku eruption. Although some of the descriptions were exaggerated and conflict with geological observations, some of them are concordant with geologic data. The Enryaku eruption probably gave serious damages to ancient traffic routes particularly on the northwestern-northeastern foot of Fuji Volcano. The Gotenba area, which is located on the eastern foot, was also damaged by thin ash-fall and probably by lahars. This caused a temporal, southward relocation of the offical trafiic route, which had passed through the Gotenba area.

著者

安井 真也 高橋 正樹 石原 和弘 味喜 大介

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.52, no.3, pp.161-186, 2007-06-29

被引用文献数

3

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The 1914-1915 Sakurajima eruption was the largest eruption in Japan in the 20th century and erupted andesitic magma was about 1.5km^3 DRE (Dense Rock Equivalent) in volume. Pumice fall and lava flows were generated from the fissure vents on the western and the eastern flanks of the volcano and pyroclastic cones were formed around the vents. Eruptive style changed with time. It is divided into three stages. After the initial, vigorous, Plinian eruption of about 36 hours (Stage 1), extrusion of lava associated with intermittent ash-emitting eruptions with or without detonations lasted for about 20 days on both sides (Stage 2), followed by an outflow of lava for more than 1.5 years on the eastern side (Stage 3). Consequently, the vast lava fields, which consist of a number of flow units formed on both sides of the volcano. Some units of lava show evidence of welded pyroclastic origin, suggesting clastogenic lava. In the western lava field, surface blocks characteristically consist of pyroclastic materials which show variable degrees of welding even within a single block. Typical eutaxitic textures and abundant broken crystals are also recognized under the microscope. Some flow units can be traced upstream to a pyroclastic cone. These features indicate that many flow units of lava on the western flank are clastogenic, which were generated by the initial, Plinian eruption of Stage 1. In the eastern lava field, evidence of pyroclastic origin is rarely discernable. However, the content of broken crystals varies widely from 20% to 80% in volume. Most lava flows, which were erupted in Stage 2 associated with frequent ash-emitting eruptions, contain broken crystals more or less than 50%. This fact indicates that magma in the conduit experienced repetitive fragmentation and coalescence due to intermittent explosions prior to outflow. Lava flows of Stage 3 contain much smaller amounts of broken crystals indicating gentle outflow of coherent lava. Relatively large-scale lava deltas developed toward the sea in the eastern lava field. Eyewitness account at that time reports that ocean entry of lava from several points started several months after the beginning of Stage 3. Although small-scale breakouts formed at the flow fronts of some lava on both sides, a large volume of the deltas can not be accounted for by secondary breakouts of ponded lava within the precedent flow lobes. It is considered that lava tube system fed lava to form the lava deltas.

著者

杉下 七海 常松 佳恵 伴 雅雄 佐々木 寿

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.77-89, 2022-03-31 (Released:2022-04-26)

参考文献数

27

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Ballistic projectiles are large pyroclasts (>0.1 m in diameter) traveling through the air without being affected by the flow of gas. This phenomenon is harmful (and potentially fatal) when a volcanic eruption suddenly occurs as the ballistic velocity is quite high, sometimes reaching several hundred meters per second. Therefore, it is important to simulate the trajectory of ballistic projectiles in an affected region. We have estimated the ejection conditions of the 1895 Zao eruption by visually comparing simulated results using a numerical model called “Ballista” to actual block distributions obtained from field observations and aerial photographs. Interestingly, around Goshikidake (northeast of the Okama crater) the farther blocks were from the crater, the larger the block size was. The ejection direction was estimated to be 120° from the north (southeast direction), because the deposit blocks are spatially dense in this direction. The ejection angle was estimated to be 10°, and the ejection velocity was estimated to be 110-120 m/s. The estimated eruption velocity of the 1895 Zao eruption was similar to that of the 2014 Ontake eruption and within the range of small vulcanian eruptions. Although we often worry that a magmatic eruption will occur after a phreatic eruption, it is also possible that a vigorous block emission will occur with a considerably high ejection velocity during a phreatic eruption.