著者

山科 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.58, no.1, pp.103-114, 2013-03-29

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桜島火山の噴火活動について見通しを得るため,北部鹿児島湾とほぼ位置が重なる姶良カルデラの周りの地殻変動について,特に水準点2474番と2480番の相対的な高さの変化に注目して検討を加えた.この区間では,これまで注目されることがほとんどなかったが,1914年の桜島火山の大きな噴火に先立って明瞭な膨張が生じていたであろうことが確かめられた.これにより,大きな噴火に先立って姶良カルデラ地下に蓄えられたマグマの1914年時点の限界量が推測される.1914年の噴火によって姶良カルデラでは顕著な収縮が生じたが,その後膨張が進んで1900年(大噴火まで14年)の水準を超え,1970年代初め頃には1914年の限界値にかなり近づいたと考えられる.桜島では,1970年代から80年代にかけて爆発的活動が活発に続いた.この期間,姶良カルデラの膨張は停滞およびやや後退していたが,桜島の爆発的な活動が低下すると再び変動が進み始めた.近年,ここで推測された1914年の限界値にさらに近づきつつあり,大きな噴火の可能性があり得ることを示している.一方,姶良カルデラ地下のマグマ量の増減と桜島からの噴出物量の収支については課題があり,今後の検討に委ねられる.

著者

町田 洋

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.4, 1991-12-25

著者

鎌田 浩毅 宇都 浩三 内海 茂

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.33, no.4, pp.305-320, 1988-12-28

被引用文献数

4

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中部九州,豊肥火山地域中央部の涌蓋(わいた)山地域には,輝石安山岩質の溶岩流の上に角閃石安山岩質の溶岩流と溶岩円頂丘から成る火山群が形成されている.涌蓋山地域は,耶馬渓(やばけい)火砕流の噴出に伴って形成された猪牟田(ししむた)カルデラの南西縁に位置する.涌蓋山地域に分布する火山岩の地質層序をたて,火山岩のK-Ar年代測定を行なった.涌蓋山地域の火山岩はそれぞれ噴出中心の異なる単成火山群の特徴をもち,猪牟田カルデラの後カルデラ火山活動の産物と考えられる.これらの火山活動は,耶馬渓火砕流噴出直後の1Maから0.3Maまで継続したことが判明した.また火山岩の噴出量と噴出岩石の組成変化を検討した結果,火山活動は0.7Ma付近を境とし,1回の噴出量の多い輝石安山岩質の活動から,噴出量の少ない角閃石安山岩質の活動へと変化したことが判明した.このことは猪牟田カルデラ直下のマグマ溜りの温度が時間とともに低下したことを示唆する.涌蓋山地域の火山活動は0.7 Ma 付近を境として総噴出量が減少し,火山体の形態も溶岩台地卓越型から少数の成層火山体を伴った溶岩円頂丘卓越型へと変化した.涌蓋山地域の火山岩の変化は豊肥火山地域全体のテクトニクスの変化を反映すると考えられる.

著者

宝田 晋治

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.36, no.1, pp.11-23, 1991

被引用文献数

3 3

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Iwasegawa debris avalanche deposit is distributed along river channels in the southeastern foot of Tashirodake Volcano, northern Japan. The volume of the deposit is estimated to be 0.1 km³. Debris avalanche block (DB) in Iwasegawa debris avalanche deposit is composed of materials, e. g. altered tuff breccia, lava with jigsaw cracks, derived from the source area ; and materials captured during flowage from the basement formations and river deposits. DB and debris avalanche matrix (DM) show lateral variation, where the maximum size of DB decreases, and sorting of DM becomes better from the source area towards the distal end. Volcanic clasts within DM show normal grading. However, wood fragments within DM show reverse grading. The result of preferred orientation measurements on 273 pieces of wood fragments coincide with the local flow direction. Plug flow model is used to explain the flow mechanism for the Iwasegawa debris avalanche, wherein a rigid plug and a laminar boundary layer exist as distinct parts of the flow. As the Iwasegawa debris avalanche flowed down the steep slope at high speed, clasts of basement formations were eroded and incorporated into the laminar boundary layer under a strong shear stress field. DM was produced by shear stress either in between two DB or DB and basement formations. Fragile DB were carried within the rigid plug without any major ruptures. The size of DB decreases down stream due to the progressive collision of each DB. Collision between DB and basement formations permitted additional fracturing during flowage. Wood fragments included within the DM in the laminar boundary layer were rotated and aligned themselves parallel to the flow direction. Lithic clasts having higher densities than those around DM tend to settle toward the bottom, whereas wood fragments of lower density tend to float toward the top of the flow. When the shear stress in the laminar boundary layer became smaller than the yield strength of the flow due to deceleration, Iwasegawa debris avalanche was freezed and stopped thixotropically.

著者

荒牧 重雄

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.32, no.1, pp.57-72, 1987-04-30

被引用文献数

3

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Lake Nyos is a maar-type volcano formed by phreatomagmatic eruption along the fissure. The rock is weakly alkaline olivine basalt very rich in crustal xenoliths and mantle-derived spinel-lehrzolite nodules. Stratiied surge deposits made of scoria and ash drape the pre-Cambrian(?) granitic basement along the lake shore. Lake Nyos is a part of Cameroon Volcanic Line which contains many fields of monogenetic volcanoes. Among them is another maar-type crater lake Monoun, which erupted similar lethal gas(CO_2) in 1984 and killed 37 people. The rocks of these young monogenetic volcanoes range from weakly to strongly alkaline basalts and basanitoids. The older(Tertiary?), plateau-forming basalts which constitute immediate basement are also alkaline basalts and basanitoids.

著者

吉池 雄藏

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, 2005-12-30

著者

八木 健三

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.16, no.1, pp.28-35, 1971

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Calderas are classified into salic and mafic types according to their association with either salic pyroclastic flows or mafic lava flows. The salic type corresponds to the low gravity anomaly type, and the mafic type to the high gravity anomaly type of YOKOYAMA. When the longer diameters of the two types of calderas in Japan and the world are plotted against the SiO₂ contents of the pyroclastic flows, or lava flows, they are distributed in diverging U-shaped areas, i.e., the size of salic type calderas becomes larger with increasing SiO₂ content, and that of mafic type calderas increases with decreasing SiO₂ content. The colossal amounts of salic magmas and their high explosivity explain the larger size of the more salic type calderas, while the lower viscosity of the more mafic magmas explains the larger size of the more mafic type calderas. The relation between the diameters and the depths of the salic type calderas of Japan is examined. For smaller calderas, less than 7 km in diameter, there is some linear relation between the two parameters, but for the larger ones the depths are nearly constant at 500~700 m, irrespective of their diameters. Comparison is made with artificial explosion and lunar craters, and their genesis is discussed.

著者

村山 広道 鎌田 政明

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.21, no.3, 1976-12-15

著者

伴 雅雄

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.36, no.2, pp.255-267, 1991-07-15

被引用文献数

4

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Nasu volcanoes, the southern part of the volcanic front of Northeast Japan arc, comprises 10 small volcanic centers. Minamigassan is one of these centers and is located at the southern end of this volcanoes. A petrological model of magmatic proccess of Minamigassan volcano is presented based on the mineralogy and whole-rock chemistry. The eruption products of Minamigassan volcano can be divided into five units ; E-1, E-2, L-1, L-2, and L-3 units from lower to upper. The caldera collapse occured at the beginning of L-1 unit. The E-1 unit belongs to medium-K tholeiite, E-2 to low-K tholeiite, L-1 to medium-K calc-alkaline, L-2 to medium-K tholeiite, and L-3 to medium-K calc-alkaline categories, respectively. L-1 unit is petrologically similar to E-1 unit. Least squares subtraction calculation for major elements and Rayliegh fractionation model calculation for trace elements indicate that the chemical variation within E-1 and E-2 unit can be interpreted by fractional crystallization of phenocrystic minerals. On the other hand calc-alkaline suites, L-1 and L-3 units suffered magma mixing. These units formed by mixing between basic magma which resembles E-1 unit magma and felsic magma which is derived from early stage magma through crustal contamination or crustal melt. Distinct differences in LIL/HFS ratio among E-1 and E-2 units cannot be explained either by the difference in degrees of partial melting of a common source or by any fractional crystallization process. The difference between E-1 and E-2 units have been originated in chemical difference of source material.

著者

大野 希一 山川 修治 大石 雅之 高橋 康 上野 龍之 井田 貴史

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.535-554, 2005-12-30

被引用文献数

1

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A cloud height generated by a volcanic eruption reflects the immensity and/or magnitude of the eruption; thus a measuring of the height's temporal variation during the event is very significant in judging whether the activity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we must take information about the cloud's height by means of the pyroclastic deposits. In general, the total time taken for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T_1); the time for the pyroclastic materials to be transported laterally by the eruption cloud (T_2); and the time for pyroclastic materials to fall and be deposited on the ground (T_3). Since T_3 can be calculated from the settling velocity of pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given value for T_1 is assumed, the most suitable wind velocity to explain T_2 can be determined. Thus the height at which pyroclastic materials separate from the eruption cloud can be determined by using the vertical profile of wind velocity around the volcano. These ideas were applied to the eruption occurred at 19:44 (JST) on September 23, 2004, at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 7.2×10^6kg. Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mud raindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall was estimated to be 2.2-3.1mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at 20:03 in the Kitakaruizawa area (about 9km north-northeast from the source). Assuming 2-5 minutes for T_1 and 11.5-12.0m/s of average lateral wind velocity, the height at which the mud raindrops separated from the eruption cloud can be estimated at 3,430-3,860m (3,610m on average). From this conclusion, the transportation and depositional process of the pyroclastic materials generated on September 23, 2004, at the Asama volcano can summarized as follows : the explosion occurred at 19:44 and the eruption cloud rose to 3,610m while blowing 2.49km downwind from the source. The cloud moved laterally for 4.51km with generating raindrops. At 19:54, mud raindrops separated from the cloud 7.0km north-northeast from the source, then fell to the ground at 20:03 after being blown 2.0km downwind by a lateral wind.

著者

日下 部実

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.32, no.1, 1987-04-30

著者

山元 孝広

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.51, no.4, pp.257-271, 2006-08-31 (Released:2017-03-20)

参考文献数

43

被引用文献数

4

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The Sashikiji 2 (S2) Member in the products of Izu-Oshima volcano was formed by an explosive eruption accompanied with caldera depression. This member is characterized by breccia called as a “low-temperature pyroclastic flow deposit”. In this paper, the S2 breccia is re-examined based on stratigraphy, grain fabrics, grain-size distributions and modal compositions. The S2 Member is divided into six units from S2-a to S2-f in ascending order. The S2-a unit consists of scoria, bomb and aa lava flows from flank fissures. The S2-b unit is made up of well-bedded ash and fine-lapilli from the summit. The S2-c unit is composed of matrix-supported breccia, locally filling valley bottoms and containing abundant deformed soil fragments and woods. The S2-d unit consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in the whole island. The S2-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. The S2-f unit is clast-supported breccia with and without ash matrix. New 14C ages of wood fragments in the S2 Member have been determined as about Cal AD 340. Although the S2-c and -d units are previously interpreted to the low-temperature pyroclastic flow deposit, these units are quite different in sedimentological features as follows. The grain fabric measurements have revealed that the S2-d unit has a-type imbrication showing the longest axis of grains parallel to the flow direction. On the other hand, the S2-c has random fabric of grains. The grain size distribution of the S2-d unit shows a bimodal nature having subpopulations at phi -1.0 to 1.0 and coarser than phi -2.5. The bimodal nature and a-type imbrication suggest that the two transport processes overlap; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in a cast-dispersion mass flow. The S2-c unit shows a polymodal grain size distribution with multi subpopulations from coarse to fine. The poor sorting, massive appearance, valley-confined distribution, and random grain fabric of the S2-c unit are characteristic of deposition from a cohesive flow without formation of traction-related bedforms or sorting of different grain sizes by turbulence. The modal composition measurements have indicated that the S2-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units are derived from steam explosions due to outburst of highly-pressurized geothermal fluid within the edifice. The S2-c unit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection of condensed water from a plume or heavy rainfall. The S2-d unit was a pyroclastic density current deposit resulted from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are 150 to 30m/s based on suspended grain sizes.

著者

須藤 茂 斎藤 英二 渡辺 和明 安田 聡

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.3, pp.283-292, 2003-07-10 (Released:2017-03-20)

参考文献数

44

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Ground deformation around Iwate and Mitsuishi-yama volcanoes was monitored by Electro-optical distance measurement (EDM), global positioning system (GPS), and leveling from 1998 to 2002. Repeated GPS observation data, which showed a four to six centimeter eastward movement of the summit of Iwate volcano in 1998 and 1999, are concordant with the interferometry SAR analysis by the NASDA, which showed the expansion around the Mitsuishi-yama in 1998. The leveling on the north side of Iwate and Mitsuishi-yama volcanoes also showed the upheaval around the Mitsuishi-yama between 1998 and 1999, and depression between 1999 and 2000. The automatic continuous GPS observation and data transfer system was developed in this study, and the data showed the depression of the Mitsuishi-yama area from 2000 to 2002. The direction of the depression was not straight downward, but the south or south-south-east at the rate of one to three centimeters a year. According to the extent of the deformation area, their source is thought to be at a depth around eight kilometers. A higher temperature granitic body of five hundred degrees centigrade or more has been already detected directly by drilling in the Kakkonda geothermal field, just south of Mitsuishi-yama. The addition of new magma to the granitic body or separation of gas from the magma possibly caused the expansion in this area in 1998 and 1999, but it may be difficult to explain the depression of this area from 2000 to 2002.

著者

鹿野 和彦 大口 健志 林 信太郎 宇都 浩三 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.47, no.5, pp.373-396, 2002-11-29 (Released:2017-03-20)

参考文献数

82

被引用文献数

5

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An alkali-rhyolite tuff-ring is newly identified in the western end of the Oga Peninsula and named as Toga volcano in this paper. The existence of this maar-type volcano at the Toga Bay has been suspected for a long time because of the elliptical embayment reminiscent of a maar and the distribution of the Toga Pumice localized along the bay coast. The Toga Pumice is cornposed mainly of pumice and non- to poorly-vesicular glass shards, but many pumices of lapilli size are rounded and fines are poor giving a sandy epiclastic appearance to the deposit. In our latest survey along the bay coast, the Toga Pumice is found to be in direct contact with the basement rocks. The contact steeply inclines at 40-50° and envelopes an elliptical area 2.0 km×2.4 km covering the bay and bay coast to form a funnel-shape structure. The basement rocks at the contact are brecciated to a depth of several tens of centimeters, or collapsed into fragments to be contained in the Toga Pumice. The beds inside the inferred crater incline toward the center of the crater at 10-30° or much smaller angles, presumably reflecting a shallow concave structure infilling the more steeply sided crater. The deposit is thinly to thickly bedded to be parallel- to wavy- or cross-stratified, inversely to normally graded with many furrows, rip-up clasts and load casts, and is sorted as well as fines-depleted pyroclastic flow deposits and/or pyroclastic surge deposits. These features are characterisitic to turbidites and indicate the place of emplacement was filled with water. Constituent glass shards are, however, commonly platy or blocky and likely to be phreatomagmatic in origin, and pumice lapilli are interpreted to have been originally angular but rounded by repeated entrainment and abrasion in multiple phreatomagmatic eruptions and succeeding emplacement in the crater lake. A pyroclastic surge deposit (Oga Pumice Tuff) correlative in composition and age to the Toga Pumice occurs at Anden and Wakimoto, 11 km and 15 km east of Toga, respectively. The juvenile pumice lapilli are angular to subrounded, in contrast with the pumice lapilli of the Toga Pumice.

著者

藤原 智 三木原 香乃 市村 美沙 石本 正芳 小林 知勝

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.161-169, 2023-09-30 (Released:2023-11-02)

参考文献数

11

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In the Atosanupuri volcanic complex in the Kussharo caldera, eastern Hokkaido, Japan, short-term uplift followed by subsidence around 1994 was detected using interferometric SAR (InSAR) analysis. In this study, an InSAR time series analysis from 2014 to 2022 using the ALOS-2 satellite revealed continued long-term subsidence of the entire Atosanupuri volcanic complex. The subsidence followed an exponential trend, with a relaxation time constant of several decades. However, long-term data are required to determine future displacement convergence due to frequent temporary and unsteady stagnations and uplifts. In contrast, the northwestern part of the Rishiri lava dome showed a constant subsidence rate without fluctuations. The results of the InSAR time-series analysis from 2016 to 2020 demonstrated that a horizontal sheet-like crust (sill) located 5.3 km below the surface of the Atosanupuri volcanic complex is shrinking at a rate of −1.44 million m3/year, whereas another sill at a depth of 700 m below the surface of the northwestern part of the Rishiri lava dome is shrinking at a rate of −21,000 m3/year. Although the residuals after subtracting these pressure source models indicate displacements of a few millimeters per year, these are most likely systematic errors inherent in InSAR. The InSAR time series analysis proved to be highly accurate in capturing temporal changes and spatial distribution, even when the displacement is less than 1 cm per year, and the results were not easily confounded by various errors. Therefore, data accumulation is crucial for InSAR time-series analysis.

著者

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.44, no.1, pp.23-40, 1999-03-05 (Released:2017-03-20)

参考文献数

47

被引用文献数

8

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Rishiri volcano is situated off the northern coast of Hokkaido, Japan. Based on mode of eruption, migration of main vents, petrological features of the eruptive products and new K-Ar ages, the activity of the volcano can be divided into three main stages: Early, Middle, and Late. The Early and Late stages can each be further subdivided into two stages. The Early stage (<0.2-0.04 Ma) was characterized by the formation of a stratovolcano and flank lava domes which were aligned along a 17km-long NW-SE trend. The stratovolcano consisted of thin lava flows and pyroclastics, mainly of andesite with a small amount of basalt, whereas lava domes were dacite. During the Middle stage (ca. 0.04 Ma), thick lava flows and pyroclastics erupted from central vents and formed a main stratovolcano. The products were mainly andesite, with lesser dacite. After formation of the stratovolcano (<0.04 Ma), many flank vents (15 km-long NW-SE trend) were active in different eruption types, such as plinian, strombolian, and phreatomagmatic. The rocks were mainly basalt, with subordinate andesite and rhyolite. The most recent eruptions occurred several thousands years ago, but no volcanic activity including fumaroles can be observed at present. Eruption rate increased from >0.1 to >0.4 km3 (DRE)/ky during the Early stage, and also >0.4 km3 (DRE)/ky in the Middle stage. The rate then decreased from >0.35 to >0.09 km3 (DRE)/ky in the Late stage. Estimation of magmatic temperature was made from the presence or absence of hornblende in dacites. This suggests that magmatic temperature increased from the Early to Middle stages, and then decreased in the Late stage. Temporal variations in both eruption rate and magmatic temperature suggest that the volcanic activity of Rishiri volcano can be explained by ascent and cooling of a single heat source (diapir). Based on the diapir model and dormancy of activity during recent several thousands years, it seems that the life of Rishiri volcano as an active volcano is virtually over.

著者

野津 憲治

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.253-258, 2016-03-31 (Released:2017-03-20)

著者

坂口 弘訓 須藤 靖明 沢田 順弘 吉川 慎

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.5, pp.143-149, 2008-10-31 (Released:2017-03-20)

参考文献数

14

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Aso Volcano is one of the active volcanoes in Japan. Seismic wave associated with volcanic activity had been recorded by Wiechert seismograph at Aso Volcanological Laboratory, Kyoto University from 1928 to 2000. Some records of volcanic tremors related to large-scale explosive events had been already analyzed, however, many events were not yet examined. In this study, the previous volcanic activities with explosion are re-examined and classified into the following four types based on the seismographic record: (A) The amplitude of tremor was small prior to an explosion. After the explosion, the amplitude of tremor increased; (B) A phreatic explosion suddenly took place without any precursory signal. The tremor amplitude was less than 3μm before the phreatic explosion, and then decreased to be less than 0.5μm after the explosion; (C) An explosion occurred after decreasing in amplitude of volcanic tremor. After explosion, volcanic activity had been increasing; (D) The volcanic tremor was increasing and changed into continuous tremors. An explosion occurs among continuous tremor. In Nakadake crater, types C and D are major types of volcanic activity after 1963. The above classification could be an important criteria for the prediction of eruption at Aso Volcano.

著者

Sri HIDAYATI 石原 和弘 井口 正人

出版者

The Volcanological Society of Japan

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.52, no.6, pp.289-309, 2007-12-28 (Released:2017-03-20)

参考文献数

41

被引用文献数

11

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山頂噴火活動が低下し,姶良カルデラの地盤が膨張に転じた1993年以降,桜島とその周辺では火山構造性地震の発生頻度が漸次高まった.2003年11月からは桜島南西沖の6〜9kmの深さで地震が多発し,従来ほとんど発生が認められなかった姶良カルデラ北東部でも地震が発生した.翌年末にはGPSによりカルデラの地盤の膨張が観測されたが桜島の噴火活動に顕著な変化はこれまでのところ認められていない.1998~2005年に発生した火山構造性地震の震源と発震機構を求め,火山活動およびマグマ供給系との関係を検討した.(1)桜島およびその周辺の火山構造性地震の震源は姶良カルデラから桜島を通ってその南西側にかけて分布し,これらは,桜島南岳直下の深さ0〜4km,南西沖深さ6〜9kmおよび姶良カルデラ内深さ4〜14kmの3つの領域に分けられる.南岳下の深さ2kmまでの発震機構は逆断層型が卓越するが,2kmより深い部分では横ずれ型が卓越する.(2)桜島南西沖の火山構造性地震は張力軸が西北西-東南東方向の正断層型であり,(3)姶良カルデラ内の火山構造性地震の節面の方向は構造線の方向に〜致しており,いずれもこの地域のテクトニクス場と調和的である.(4)桜島南西沖の地震活動が姶良カルデラから桜島を横切るマグマの貫入イベントに関連しているのではないかという仮説にたって,地殻変動データを吟味してその可能性を検討するとともに,新たなマグマ供給系モデルを提示した.

著者

立山 耕平 成毛 志乃 佐々木 寿 福井 拓哉 山田 浩之

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.65, no.2, pp.41-51, 2020-06-30 (Released:2020-07-06)

参考文献数

26

被引用文献数

1

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In recent years, casualties caused by the impact of ballistic ejecta from sudden phreatic eruptions have drawn much attention, as observed with Mt. Ontake in September 2014 and Mt. Moto-shirane in January 2018. Hence, improvement of evacuation facilities (shelters) that protects against ballistic ejecta is expected as a forthcoming volcanic disaster prevention initiative. In many cases, the utilized evacuation facilities are outfitted by strengthening existing facilities such as mountain huts. Therefore, it is necessary to understand the baseline impact resistance against ballistic ejecta of the existing mountain huts. In the case of Japanese wooden buildings, Japanese-style rooms with tatami (Japanese-style thick straw mats) are often used. In this study, we focused on the impact resistance of tatami used for flooring. We conducted tests which simulated the impact of ballistic ejecta on various types of tatami, in order to assess the penetration limit of tatami. Three types of bodies of tatami (tatamidoko) were prepared: straw tatamidoko, non-straw tatamidoko type III, and straw sandwich tatamidoko. The projectile was simulated ballistic ejecta with a diameter of 128mm and a mass of 2.66kg, made using a vitrified grinding wheel. This object was launched at a speed of 22 to 69m/s using a pneumatic impact test apparatus. From the impact test, non-straw tatamidoko type III did not prevent penetration, even at an impact energy of 0.63kJ. Therefore, if non-straw tatamidoko type III was to be used in a mountain hut, it cannot be expected to protect against ballistic ejecta. On the other hand, the minimum energy of penetration of straw tatamidoko and straw sandwich tatamidoko were 4.9 and 4.1kJ, respectively, and they had sufficient impact-resistance against ballistic ejecta compared to the mountain hut roof. Thus, it was shown that the downstairs of straw tatamidoko and straw sandwich tatamidoko can be designated as “a safer place in the building”.