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Many of the casualties in the phreatic eruption of Mt. Ontake, which is located in central Japan, on September 27, 2014, were climbers near the summit. The main cause of death was injury by lapilli and block ejecta (ballistic ejecta). The absence of shelters at the summit and along the climbing trails may be a major factor in the large number of casualties attributable to ballistic ejecta, and establishing and improving shelters on active volcanoes is an urgent need. The climbers who evacuated to wooden mountain huts survived the 2014 Mt. Ontake eruption, this research focuses on the impact characteristics of wooden materials. We therefore performed an experimental investigation into the safety of wooden materials under impact by simulated ballistic ejecta with a mass of 2.66 kg (equivalent to a diameter of 128 mm) as by assembling cylindrical abrasive travelling at up to 300 km/h (approximately 83 m/s). With abrasives acting as ballistic ejecta, the simulated shelter facility components serving as targets were wooden structures composed of Japanese cedar, which found on volcanoes at high elevations in Japan. The components of the fabricated roof were Japanese cedar with a thickness of 15 mm as roofboard installed on the surface waterproof sheet and 0.4 mm thick Galvalume® steel sheet with an additional rafter installed. The impact absorption energy of the fabricated roof increased with simulated ballistic ejecta impact velocity because the localization of bending deformation progressed at the impact part and the frequency of splitting failure increased with increasing impact velocity. This is attributable to variation in the mode of splitting failure of cedar boards and the dependence of the material strength on projectile velocity. We also experimentally investigated simple reinforcement methods in order to obtain basic data on conversion of mountain huts to shelters. The reinforced wooden structure could withstand simulated ballistic ejecta impacts of up to approximately 13 kJ by incorporating two layers of aramid fabric between a waterproof sheet and a cedar board. Review of the roof structures of existing wooden buildings based on these results promises further advances in safe impact-resistant design.

Mount Ontake erupted at 11:52 am on September 27, 2014, which generated pyroclastic density currents, ballistic projectiles, ash falls, and a small-scale lahar that spouted directly from craters formed by the eruption. Because this lahar may have been generated by water released from within these craters, we refer to this lahar as a “syneruptive-spouted type lahar” in this study. The lahar of the 2014 eruption was small relative to the other syneruptive type lahars reported in the past that were snowmelt type or crater lake breakout type lahars. Nevertheless, in the 2014 event, the syneruptive-spouted type lahar extended approximately 5 km downstream from the Jigokudani crater via the Akagawa River, with an estimated total volume of ~1.2 × 105 m3. We have reviewed other representative syneruptive-spouted type lahars that have been reported in Japan. The syneruptive-spouted type lahar attributed to the September 2014 eruption had the longest runout distance and largest volume of all cases studied. The mineral assemblage identified from samples of the lahar deposits is similar to that of ash-fall deposits from the same eruption. Previous workers deduced that the ash was derived mainly from shallow depths (within 2 km of the surface). The syneruptive-spouted type lahar deposits are therefore also considered to have originated from shallow depths. A syneruptive-spouted type lahar is a small-scale phenomenon that causes little direct damage to infrastructure, but has long-term influence on water quality. Increases in turbidity and decreases in pH are expected to occur in the Mount Ontake area downstream of Nigorisawa after heavy rainfall events in the future. Therefore, the potential indirect (but long term) damage of syneruptive-spouted type lahars should be considered for hazard mapping and planning volcanic disaster prevention measures.

浅間火山2004年9月1日噴火（以下，9/1噴火と略す）について未解明であった火口西側地域を中心に現地踏査を行った．さらに，高分解能衛星画像の判読結果や室内分析および火山監視画像の解析から9/1噴火による本質物（パン皮状火山弾）を特定し，それらの分布を明らかにした．地層を形成しにくいブルカノ式噴火噴出物の分布を現地で把握するために，高分解能衛星画像が有効であった．現地踏査単独ではブルカノ式噴火噴出物の把握は難しいため，平常時より高分解能衛星画像や空中写真の整備など，噴火前後の微地形変化を比較できる準備が必要である．特定した本質物（パン皮状火山弾）を用いて，9/1噴火をもたらしたマグマの実態を推定した．含水量測定の結果より算出されたマグマ深度の差から，マグマ柱は鉛直方向に少なくとも180 m程度の高さであったと考えられる．

The Hakkoda-Towada caldera cluster (HTCC) is a typical Late Cenozoic caldera cluster located in the northern part of the Northeast Japan Arc. The HTCC consists of five caldera volcanoes, active between 3.5 Ma and present time. The felsic magmas can be classified into high-K (HK-) type and medium- to low-K (MLK-) type based on their whole-rock chemistry. The HK-type magmas are characterized by higher K2O and Rb contents and higher 87Sr/86Sr ratios than MLK-type magmas. Both magmas cannot be derived from fractional crystallization of any basaltic magma in the HTCC. Assimilation-fractional crystallization model calculations show that crustal assimilation is necessary for producing the felsic magmas, and HK-type magmas are produced by higher degree of crustal assimilation with fractional crystallization than MLK-type magmas. Although MLK-type magmas were erupted throughout HTCC activity, HK-type magmas were erupted only during the initial stage. The temporal variations of magma types suggest the large contribution of crustal components in the initial stage. A major volcanic hiatus of 3 my before the HTCC activity suggests a relatively cold crust in the initial stage. The cold crust probably promoted crustal assimilation and fractional crystallization, and caused the initial generation of HK-type magmas. Subsequently, the repeated supply of mantle-derived magmas raised temperature in the crust and formed relatively stable magma pathways. Such a later system produced MLK-type magmas with lesser crustal components. The MLK-type magmas are common and HK-type magmas are exceptional during the Pliocene–Quaternary volcanism in the Northeast Japan Arc. This fact suggests that exceptional conditions are necessary for the production of HK-type magmas. A relatively cold crust caused by a long volcanic hiatus (several million years) is considered as one of the probable conditions. Intensive crustal assimilation and fractional crystallization promoted by the cold crust may be necessary for the generation of highly evolved HK-type felsic magmas.

Towada volcano is an active caldera volcano located at the northern part of the Northeast Japan arc. We re-examined the tephra stratigraphy of the post-caldera stage (15 ka to present) of Towada volcano, and carried out AMS radiocarbon dating of six soil samples taken from directly below the tephra layers. The most suitable ages of eruptive episodes A, B, C, D, D', E, F, G and Ninokura Scoria are estimated to be 1.1, 2.8, 6.2, 7.6, 8.3, 9.4, 10.1, 10.6, and 11-15 ka, respectively, on the basis of the thickness of soil layers between tephras and the radiocarbon ages determined by present and previous studies. This result shows that the eruption intervals have become longer with time, and the frequency of eruptions has decreased with time. Although the cause of this change is not known as yet, these are important data for predicting long-term eruptive activity of Towada volcano.

This study shows the stratigraphy, petrography, whole-rock chemistry and paleomagnetic polarity of pyroclastic flow deposits during the latest Pliocene to Middle Pleistocene in the southeastern foot area of the Hakkoda Caldera, Northeast Japan. Six pyroclastic flow deposits are identified in this area : Kumanosawa Pyroclastic Flow Deposits (Ks), Takatoge Pyroclastic Flow Deposits (Tk), Osegawa Pyroclastic Flow Deposits (Os), Hakkoda-Ose Pyroclastic Flow Deposits (Hto), Hakkoda 1st-stage Pyroclastic Flow Deposits (Ht1) and Hakkoda 2nd-stage Pyroclastic Flow Deposits (Ht2), in order of decreasing age. These pyroclastic flow deposits have dacitic to rhyolitic compositions, and show distinct modal compositions and whole-rock major element chemistry in each. Based on stratigraphy, topography and paleomagnetic polarity, eruptive ages of the pyroclastic flow deposits are estimated to be as follows : Ks, 1.95-1.77 Ma; Tk, 1.77-1.07 Ma ; Os and Hto, 0.99-0.78 Ma ; Ht1, 0.76 Ma ; Ht2, 0.40 Ma. The source calderas of the Ks, Tk, Os, and Hto can be estimated from petrological features. The source of the Ks is neither the Hakkoda Caldera nor Okiura Caldera, and is probably an unidentified caldera. The source of the Tk may be a low gravity anomaly area “Nenokuchi Caldera” located northeast of Towada Caldera. The source of the Os is not the Okiura Caldera. The source of the Hto is the Hakkoda Caldera. This study suggests that the two large-scale pyroclastic flow deposits during 1-2 Ma erupted from previously unidentified calderas (one is possibly Nenokuchi Caldera). This proposes a revised volcanic history in which the activities of caldera volcanoes overlap each other in the Hakkoda-Towada Volcanic Region.

平成15年十勝沖地震 (2003年9月26日発生; Mj8.0) により, 震央から約230km離れた北海道網走支庁常呂郡端野町で, シラスの液状化に起因する土砂流動災害が発生した. 変状は, 火砕流台地上において谷埋め盛土により造成された農地で発生したもので, 地表部に陥没と亀裂を伴う数千m3以上の土砂流動現象が生じ, 近傍の農地と小河川を埋積した. この現象は, 陥没域の低所側末端部に生じた数箇所の噴砂孔から, 多量の火山灰が絞り出されるように流出する間, 陥没域表層部の非液状化層が顕著に側方へ流動した痕跡がなく, 陥没に伴う地表亀裂からは噴砂がほとんど見られないことが特徴である. これは既往の地震時の液状化に起因する斜面変動と異なる形態的特徴と考えられ, 本報告では「絞り出し流動現象」と呼ぶ.今回変状が発生した谷埋め造成地は, 起伏量が小さく傾斜のゆるい丘陵地内にあり, 空中写真判読や地形図読図の手法で人工改変地形の範囲を特定することは簡単ではない. 本調査で実施した, 航空機レーザ地形計測による細密DEMに基づく地形量の視覚表現は, 微地形の再現性がよく, 大縮尺地形図のない丘陵地などの緩傾斜地において造成農地の形状を把握する手段として期待できる.

We applied C-band SAR data acquired by RADARSAT-2 to examine ground surface changes in the active volcanic field. In the active volcano area, the observation of secondary disaster with rainfall is also important as well as direct influence by the volcanic eruption. By continuous monitoring, we generated a change detection image from the Spotlight mode data (1m resolution). By using the change detection images and also by field survey, sand movements with debris flow and ground surface change by eruption were extracted. We concluded that continuous monitoring using the SAR is effective for preventing volcanic disaster.