著者

宮縁 育夫 増田 直朗 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.49, no.5, pp.267-282, 2004

参考文献数

31

被引用文献数

7

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The western part of post-caldera central cones of Aso Volcano, southwestern Japan, is characterized by wide chemical variation of lava flows (SiO₂=49-73 wt.%) and complicated topography. Geologic history of the area was reconstructed by stratigraphic relationships between lava flows and airfall tephra layers. Stratigraphy and chronology of lavas and tephra distributed on the western slope of the central cones are as follows: Ayugaerinotaki lava, Matsuhata pumice (MhP), Tochinoki lava (73±10 ka; K-Ar age), Aso central cone pumice 4 (ACP4), Tateno lava. Aso central cone pumice 3 (ACP3), Takanoobane lava (51±5ka), Eboshidake lava, Karisako lava, Akase lava (30 cal ka ; calibrated ¹⁴C age), Sawatsuno lava (27±6ka), Kusasenrigahama pumice (Kpfa; 31 cal ka), Otogase lava and Aso central cone pumice 1 (ACPI ; 4 cal ka), in ascending order. Between about 70,000 to 50,000 years ago Tochinoki lava, Tateno lava and Takanoobane lava accompanied precursor pumice-fall deposits. A catastrophic eruption occurred in and around the Kusasenrigahama crater at 31 cal ka. The eruption was initiated by the dacite Sawatsuno lava now and produced the largest plinian numice-fall deposit (Kpfa; 1.2 km³ in DRE volume). In the western part of post-caldera central cones of Aso Volcano, total discharge volumes of felsic and mafic magmas from about 70,000 to 30,000 years ago were estimated to be about 2.4 km³ and 0.47km³ respectively.

著者

宮縁 育夫

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-05-17

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2016年4月16日午前1時25分に発生したMj7.3の熊本地震(本震)によって熊本県から大分県を中心とする地域で甚大な災害が発生した.とくに熊本県阿蘇郡南阿蘇村では震度6強の揺れに襲われ,多数の建物倒壊とともに,100箇所以上の斜面崩壊が起こり,死者15名,行方不明者1名を出す大惨事となった.筆者はこの災害発生後から同村とその周辺域において現地調査を行い,2016年熊本地震によって発生した斜面崩壊の実態を明らかにしたので,その結果を報告する.4月16日未明の熊本地震による斜面崩壊の発生地域は,南阿蘇村を含む阿蘇カルデラ西部地域を中心としている.そうした斜面崩壊は阿蘇カルデラ壁斜面の崩壊と中央火口丘群斜面の崩壊に大きく区分することができる.前者については阿蘇カルデラの北西~西側壁の急斜面で大小さまざまな規模の崩壊が認められる.この地域の阿蘇カルデラ壁の標高差は300~450 m程度であり,大部分は先阿蘇火山岩類の安山岩からなる傾斜25度を越える斜面で崩壊が発生している.最大の崩壊は黒川に架かっていた阿蘇大橋の西側斜面で起こったもので,崩壊頂部の位置は標高710 m付近で,崩壊の高さは約300 m,幅130~200 mに達しており(いずれも土砂が堆積する部分を含む),国道57号線とJR豊肥本線を寸断した.遠望観察によると,明瞭なすべり面は認められず.崩壊面にはほぼ水平に堆積した先阿蘇火山岩類の溶岩や火砕岩などが確認できる.強い地震動によってカルデラ壁急斜面に存在した不安定な溶岩・火砕岩がクラックなどに沿って崩壊したのであろう.後者の中央火口丘群斜面の崩壊は今回の地震災害を特徴づける現象である.この崩壊は急斜面でも起こっているが,傾斜10度以下の緩斜面でも発生していることが特筆すべき点である.中央火口丘群西側斜面は,玄武岩から流紋岩に及ぶ広い組成の溶岩・火砕岩が分布しているが,そうした火山岩を厚さ数m~数10 mの未固結なテフラ層(おもにシルト質火山灰と土壌層)が覆っている.大部分の斜面崩壊は深さ4~8 m程度であり,溶岩を覆うテフラ層内で起こっていることが現地調査の結果,明らかとなった.また,崩壊した土砂は緩傾斜であるにもかかわらず,標高差の割に長距離(2016年熊本地震に伴って発生した斜面災害は,2012年7月などの豪雨による土砂災害とは異なった特徴を有している.強い地震動によっては,緩斜面であっても崩壊が発生して,その崩壊土砂が岩屑なだれ化して長距離運搬され,人命や建物に甚大な被害を及ぼすことが明らかとなった.

著者

木尾 竜也 成尾 英仁 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.67, no.3, pp.401-408, 2022-09-30 (Released:2022-10-27)

参考文献数

12

著者

小林 哲夫 奥野 充 長岡 信治 宮縁 育夫 井口 正人 味喜 大介

出版者

京都大学防災研究所

雑誌

京都大学防災研究所年報. B = Disaster Prevention Research Institute Annuals. B (ISSN:0386412X)

巻号頁・発行日

vol.53, no.B, pp.269-276, 2010-06-01

著者

星住 英夫 宮縁 育夫 宮城 磯治 下司 信夫 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.67, no.1, pp.91-112, 2022-03-31 (Released:2022-04-26)

参考文献数

71

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Aso volcano produced four huge ignimbrite-forming eruptions named Aso-1, 2, 3 and 4 in ascending order, among which Aso-4 is considered the largest eruption in Japan in the last 1 million years. This paper describes the tephra sequence between the Aso-4 and Aso-3 eruptions (Aso-4/3 tephra group). The reconstruction of the eruptive history for Aso-4/3 tephra group presented here provides a valuable contribution to the understanding of caldera volcanism by outlining the preparatory processes of a catastrophic ignimbrite eruption. The eruption sequence of the Aso-4/3 tephra group, which is composed of at least 37 units of pumice-fall, scoria-fall, and ash-fall deposits, is divided into five stages. Stage 1 is characterized by the eruption of mafic scoria (VEI 3-4) during 133-114.1 ka, after the eruption of Aso-3. Stage 2 is characterized by the frequent eruptions of mafic scoria and ash (VEI 3-4) during 114.1-108.4 ka. The magma composition became more felsic during explosive eruptions (VEI 3-4) from 108.4-104.7 ka (Stage 3). During the most active stage from 104.7-97.7 ka (Stage 4), voluminous felsic pumice-falls erupted (VEI 4-5). The ABCD tephra (97.7 ka) is the largest plinian pumice-fall deposit of Aso volcano. Stage 5 (97.7-88 ka) is a relatively dormant period, during which only a biotite dacite pumice-fall was deposited (VEI 4). The low number of eruptions during stage 5 suggests that the magma supply rate decreased during the 10 thousand years that preceded the Aso-4 ignimbrite eruption. The estimated total tephra volume for the Aso-4/3 tephra group is 23 km3, which corresponds to 10 km3 in dense rock equivalent (DRE). The estimated the long-range tephra discharge rate (0.23 km3 DRE/ky) is similar to that in the post-caldera stage of Aso-4 (0.2 km3 DRE/ky).

著者

西村 太志 石橋 秀巳 森 俊哉 地引 泰人 中道 治久 齋藤 武士 嶋野 岳人 吉村 俊平 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.66, no.3, pp.133-134, 2021-09-30 (Released:2021-10-29)

著者

宮縁 育夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.125, no.3, pp.421-429, 2016-06-25 (Released:2016-07-14)

参考文献数

21

被引用文献数

1 18

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The Kumamoto earthquake (Mj 7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types: landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25° at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300 m high, 130–200 m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (<600 m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important information for preventing or mitigating future landslide disasters in the Aso caldera region.

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

宮縁 育夫 飯塚 義之 遠入 楓大 大倉 敬宏

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.66, no.3, pp.157-169, 2021-09-30 (Released:2021-10-29)

参考文献数

19

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Prior to the onset of magmatic activity at the Nakadake first crater, Aso Volcano (SW Japan) from July 2019 to June 2020, multiple small eruptions occurred between April and May 2019. The May 3-5 eruption was one of the largest events during the pre-magmatic activity period. An ash-fall deposit from the early stage of that eruption (15 : 00-18 : 00 in JST on May 3) was distributed to the south of the source crater, whereas the ash erupted after 20 : 00 on May 3 was dispersed southwestwards. The May 3 15 : 00-18 : 00 ash was composed mainly of fine particles (<0.25 mm in diameter) and fell as accretionary lapilli (<0.8 mm). In contrast, ash after 20 : 00 on May 3 consisted mainly of 0.5 mm grains but lacked silt and clay content. Based on an isomass map, the total discharged mass of the May 3-5, 2019 eruption was about 700 tons. Although lithic (50 %) and altered glass (30-40 %) grains were dominant in both ash-fall deposits, they also included small amounts of black to pale-brown fresh glass shards (2-4 %) inferred to be juvenile material originating from newly ascending magma. After the May 3-5 event, small ash emissions occurred intermittently until July 2019. The proportions of fresh glass shards included in the May-July 2019 ash-fall deposits gradually increased; ash erupted in early July contained 7 % fresh glass grains. Small-scale magmatic activity began on July 26, 2019, and continued to mid-June 2020. The April to early July 2019 ash emissions at Nakadake first crater are inferred to be precursor phenomenon of the late July 2019 to mid-June 2020 magmatic eruptions. It is very important to clarify temporal variations in the mass and component characteristics of erupted materials for understanding the sequence of events and predicting future eruptive activity.

著者

穴井 千里 宮縁 育夫 宇津木 充 吉川 慎 望月 伸竜 渋谷 秀敏 大倉 敬宏

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.66, no.3, pp.171-186, 2021-09-30 (Released:2021-10-29)

参考文献数

29

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Nakadake volcano, the current active center of the Aso central cones (Kyushu), is one of the most active volcanoes in Japan. It has been active since ca. 22-21 cal ka, and has formed the old edifice (22-21 cal ka), the young edifice (around 5 cal ka) and the youngest pyroclastic cone (until present). The lava flows from the young edifice spread on the flank of the volcano several times around 5 cal ka. These lavas are supposed to give stratigraphic markers for constructing the eruptive history of Nakadake volcano, but the similarity in chemical composition and lithology hampers distinguishing and correlating them. We have conducted a paleomagnetic study to distinguish and correlate the lavas since the paleomagnetic secular variation (PSV) provides a high-resolution age information. If lava units have a temporal difference of more than 50 years, they could be distinguished by their paleomagnetic directions. The samples were collected from 9 lava flows and 8 agglutinate layers (welded scoria-fall deposits) and were subjected to the paleomagnetic and rock-magnetic measurements. These samples, from visual inspection, appear to be influenced by chemical alteration in the surface of the outcrop by sulfides of volcanic gases. To check a rock-magnetic effect of the chemical alteration of the lavas and agglutinates, thermomagnetic analyses were made on chip samples from the top (surface of rock) and bottom (inside of rock) of the collected paleomagnetic cores. The thermomagnetic analyses indicate that the core top and bottom samples show the same behaviors, in spite of the difference in color, and the carriers of magnetization of each core are titanium rich (titanium content, x, is about 0.6) and poor (x is about 0.1-0.2) titanomagnetites. The natural remanent magnetization of each sample shows a simple, single vector component in alternating field demagnetization experiments, which well defines the primary component. Site mean directions can be categorized into three different direction groups. These data suggest that the eruption producing lava flows and/or agglutinates occurred at three different ages. Furthermore, the paleomagnetic directions of one group is not consistent with the directions of the eruptive ages of Nakadake young edifice assigned from the previous stratigraphic studies. Comparing these directions with the paleomagnetic secular variation curve which has been drawn from basaltic volcanoes in the northwestern part of Aso central cones, the ages of the direction groups can be assigned to around 6.0-4.3 cal ka and 3.5 cal ka, respectively. This result demonstrates that paleomagnetic studies can greatly contribute for establishing the eruptive histories of volcanos.

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.62, no.1, pp.1-12, 2017-03-31 (Released:2017-03-28)

参考文献数

27

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Janoo Volcano (550-750 m in basal diameter;150 m in height) is located in the northwestern part of the post-caldera central cones of Aso Volcano, central Kyushu, southwestern Japan. The volcano had been thought to be a cinder cone composed entirely of scoria-fall deposit and older than 7.3 ka. Fieldwork in and around the volcano has re-examined the detailed tephra stratigraphy and eruption age of Janoo Volcano. A black humic paleosol divides an upper pumice-fall deposit from a lower scoria-fall deposit. The upper pumice-fall deposit shows only two pure pumice bed sections with pumice clasts scattered in a brown massive ash elsewhere in the deposit. The deposit is composed mainly of light gray well-vesiculated dacitic (SiO2=65.4-67.7 wt.%) pumiceous clasts containing biotite phenocrysts, and abundant banded pumices, suggesting a mixture of silicic and mafic magmas. Based on the phenocryst assemblage and age, the pumice-fall deposit is correlated to the Aso central cone pumice 1 (ACP1;4.1 ka), which is the only pumice-fall deposit erupted from Aso Volcano during Holocene time. The lower scoria-fall deposit is more than 30 m thick and constitutes most of the Janoo cinder cone. It includes brownish black to brown well-vesiculated basaltic andesite (SiO2=54.7-55.5 wt.%) scoriaceous clasts and cauliflower bombs with radially arranged cooling joints. The Akamizu lava (SiO2=57-59 wt.%) distributed west of the Janoo cinder cone, whose source was previously unknown, is attributed to Janoo Volcano based on the lava’s petrographic characteristics. A 14C age of 3830±30 years BP, which corresponds to 4.2-4.1 ka, was obtained from the humic paleosol interbedded between the ACP1 and Janoo scoria. The stratigraphy and characteristics of the tephra deposits suggest the following eruption sequence. The initial eruption at Janoo Volcano occurred at 4.9-4.3 ka and was strombolian in style forming the Janoo cinder cone. After lying in repose for a few hundred years, Janoo Volcano erupted again, and produced the ACP1 tephra containing abundant banded pumices and Akamizu lava at 4.1 ka. The southern half of the Janoo cinder cone was destroyed probably by the effusion of Akamizu lava. Volcanic activity forming Kishimadake, Ojodake, Komezuka and Kamikomezuka volcanoes in the northwestern part of post-caldera central cones at 4-3.3 ka was derived from basaltic to basaltic andesite magmas, whereas the eruption products of Janoo Volcano have a wide range in chemistry from basaltic andesite to dacite. Activity of Janoo Volcano is characterized by the presence of a dormant period (a few hundred years), allowing a paleosol to develop on the scoria-fall deposit, before ejection of both mafic and silicic magmas in the late eruption.

著者

宮縁 育夫 大丸 裕武 小松 陽一

出版者

日本地形学連合

雑誌

地形 (ISSN:03891755)

巻号頁・発行日

vol.25, no.1, pp.23-43, 2004-01-25

被引用文献数

3

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The 29 June 2001 rainstorm (total 238 mm; maximum hourly rainfall 98 mm) triggered more than 700 landslides and associated lahars at Aso Volcano, central Kyushu, SW Japan. Most of the landslides were concentrated in a 20-km^2 area that includes the northeastern slope of post-caldera central cones, the northern slope of Nekodake Volcano and the eastern caldera wall. We mapped landslides triggered by the storm using field observations and 1 : 20,000-scale aerial photography provided by the Kumamoto District Forest Office taken in December 2001. Most of the triggered landslides were shallow (about 1 m thick) soil slips in unconsolidated airfall tephra layers overlying lava flows and pyroclastic rocks. Average volume of the landslides was 550 m^3, but some had volumes exceeding 10,000 m^3. The rupture surfaces of most landslides were formed near the boundary between upper blackish and lower brownish tephra layers. These layers have differences in permeability, grain size and soil hardness. Most landslides mobilized completely into lahars, traveling a few kilometers along stream channels. Although some lahars eroded channel side slopes and transported boulders (2-3 m in diameter), most contained up to about 80 percent silt-to-clay-size particles, similar in composition to the original landslide debris. Sediment discharge volumes by the tephra-slip-induced lahars were estimated at an order of 10^3-10^4 m^3/km^2 using sediment volumes trapped by check dams. These volumes are similar or one-order of magnitude smaller than the volumes of the 1953 and 1990 landslide disasters in the same region. The characteristics of landslides and lahars and their estimated volumes provide important information about landslide and lahar hazards at Aso Volcano.

著者

宮縁 育夫 星住 英夫 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.49, no.2, pp.51-64, 2004-05-20 (Released:2017-03-20)

参考文献数

33

被引用文献数

5

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The stratigraphy and chronology of late-Pleistocene (after deposition of AT ash) tephra layers erupted from post-caldera central cones of Aso Volcano, SW Japan, are evaluated through study of the thick tephra sequence preserved mainly atop the pyroclastic-flow plateau east of the caldera. Twenty andesite to basaltic-andesite scoria-fall deposits (YmS20-YmS1 in ascending order) and two pumice-fall deposits (NbP2 and NbP1) were identified as maker beds for the interval ca. 29-13ka (calibrated 14C age). The tephra layers were produced mostly by scoria and ash eruptions of Nakadake, the only active post-caldera central cone, and their bulk volumes calculated from isopach maps range from 0.01 to 0.9km3. The radiocarbon ages of buried soils just below individual tephra layers reveal that YmS20 to YmS15 were erupted at ca. 22-21 ka and YmS20 to Ym-S1 at ca. 18-16ka. Total tephra volume during 29-13ka is estimated at about 1.3km3 (DRE). Production of tephra was mainly concentrated in two periods : 22-21 ka (0.8km3) and 18-16ka (0.5km3).

著者

宮縁 育夫 増田 直朗 渡辺 一徳

出版者

Japan Association for Quaternary Research

雑誌

第四紀研究 (ISSN:04182642)

巻号頁・発行日

vol.43, no.5, pp.353-358, 2004-10-01 (Released:2009-08-21)

参考文献数

23

被引用文献数

4 4

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阿蘇カルデラ西部に分布する立野溶岩の直下に,軽石層が存在することが認められ,その軽石層は阿蘇中央火口丘第4軽石(ACP4)に対比された.また,ACP4直下から未炭化の樹木根を採取し,加速器質量分析法による14C年代測定を行った.得られた年代は>43,200 14C yrs BP(>約46cal kaに相当)であり,立野溶岩の上位に存在する高野尾羽根溶岩のK-Ar年代(51±5ka)とも矛盾しない.阿蘇火山中央火口丘群の西部地域では,50ka前後に数100年程度の時間をおいて,立野溶岩と高野尾羽根溶岩というデイサイト~流紋岩質溶岩の流出が2回あり,それらの直前にはプリニー式噴火が発生し,多量の降下軽石が放出されていることが明らかになった.

著者

宮縁 育夫 星住 英夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.126, no.5, pp.581-593, 2017-10-25 (Released:2017-11-10)

参考文献数

34

被引用文献数

2 1

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Several volcaniclastic deposits discovered at the foot of the eastern wall of Aso caldera, central Kyushu, southwestern Japan, are divided into two types: lahar deposits (Lh1-Lh5 in descending order), which contain abundant subangular to subrounded lithic clasts ( 3.5 m in diameter) set in a sandy to silty matrix, and debris avalanche deposits (DA1 and DA2), which include numerous plastically deformed fragments of tephra (ash and scoria) and soil layers in a homogenous silty to clay matrix. DA2, which underlies a paleosol dated at 5.4 ka (calibrated 14C age), is the largest volcaniclastic deposit observed in the section (more than 2.5 m thick and about 70 m wide). Because the debris avalanche deposits display no evidence that they were transported by water, they are likely to have originated from landslides triggered by intense earthquakes. Tephra chronology and 14C-dating on paleosols along the succession suggest that lahars occurred once over 900 years (6.3-5.4 ka), three times over 1400 years (5.4-4 ka), once over 400 years (4-3.6 ka) and twice (including the 2012 lahar) in the last 3600 years. This evidence indicates that the lahars occurred at an interval of 400-1800 years. In contrast, two debris avalanche deposits exist in the same succession spanning the last 6300 years. Including landslides and associated debris avalanches triggered by the 2016 Kumamoto earthquake (Mj 7.3), landslides generating debris avalanches in the Aso caldera occurred at least three times in the last 6300 years. This may suggest the frequency of large earthquakes triggering debris avalanches in the central Kyushu region, which has many active faults.

著者

宮縁 育夫 飯塚 義之 大倉 敬宏

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.4, pp.441-452, 2022-12-31 (Released:2023-01-30)

参考文献数

12

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After the July 2019-June 2020 small-scale magmatic activity, surface unrest of the Nakadake first crater, which is located at the center of Aso caldera, SW Japan, had been mostly calm for fourteen months, and a lake had reformed inside the crater by late-August 2021. An eruption producing ballistic clasts and a tephra fall deposit occurred within the first crater of Nakadake at 04:44 on October 14, 2021. A large number of ballistic clasts were distributed from the west-northwestern rim of the Nakadake first crater to the southern rim of the second crater, with ballistics also reaching at least 450 m south of the center of the first crater. The largest clast (70×32×31 cm) was ejected a distance of 300 m SW of the center of the first crater. Several impact craters, which were<1 m in diameter, were observed in the surface ash layer at the crater rim. The ballistic clasts were dominated by basaltic-andesite lithic fragments of lavas and pyroclastic rocks, and were not thought to derive from a newly ascending magma. The tephra fall deposit was distributed to the southeast, extending approximately 30 km from the source crater. At the crater rim, the fall deposits were composed mainly of sand-size particles with small amounts of lapilli (<17 %), and were aggregated at sizes of a few mm to 1 cm. The aggregated muddy ash (a few millimeters in diameter) adhered to plant leaves and the surface of man-made constructions in the southeastern part of Aso caldera (4-10 km), indicating that the rising plume contained large amounts of condensed water vapor. Based on the isomass map, the total discharged mass of the October 14, 2021 eruption was calculated at approximately 2500 tons. Gray to white lithic grains (40-50 %) were dominant in the tephra deposits (0.125-0.25 mm fraction), while black to brown glass shards (8-16 %) were also observed. Although a very small proportion of glass particles appeared to be fresh, most of glass shards showed varying degrees of alteration based on microscope observation and electron micro-probe analysis. These combined lines of evidence suggest that the October 14, 2021 eruption of the first crater at Nakadake was probably a purely phreatic eruption.

著者

渡辺 一徳 本田 圭一 原 浩太郎 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.66, no.1, pp.21-34, 2021-03-31 (Released:2021-03-25)

参考文献数

36

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Aso Volcano in central Kyushu, SW Japan, is one of the largest caldera volcanoes in the world. Aso caldera, 25 km north-south and 18 km east-west, was formed by four gigantic ignimbrite eruptions: Aso-1 (270 ka), Aso-2 (140 ka), Aso-3 (120 ka) and Aso-4 (90 ka). The only outlet of the caldera, named as Tateno gorge, is located at the western edge of the caldera. Since the gorge’s formation history remains poorly studied, a detailed geologic study has been conducted to clarify the stratigraphic relationships between lavas and Aso ignimbrites, and evaluate the formation age of the gorge. The gorge walls are composed mainly of pre-Aso volcanic rocks (0.8-0.4 Ma), and densely welded Aso-2 pyroclastic flow deposits occur on both northern and southern walls of the gorge. Andesitic lava flows (Hokamaki lava) are found between the Aso-1 and Aso-2 ignimbrites in the gorge, and the lavas show potassium-rich characteristics that are indicative of the whole-rock chemistry of magmatic products from Aso Volcano. Additionally, welded Aso-1 blocks were observed in the gravel beds (lahar deposits) underlying the Hokamaki lava from borehole cores. The evidences suggest that the origin of Tateno gorge, which was draining a pre-existing caldera, formed after the Aso-1 ignimbrite eruption (270 ka), and thereafter Hokamaki lavas and the Aso-2 pyroclastic flows (140 ka) passed through the gorge.

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, 2010-10-31 (Released:2017-03-20)

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

宮縁 育夫

出版者

公益社団法人 東京地学協会

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.125, no.3, pp.421-429, 2016

被引用文献数

18

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The Kumamoto earthquake(Mj7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types:landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25°at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300m high, 130-200m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (<600m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important imformation for preventing or mitigating future landslide disasters in the Aso caldera region.

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.