桜島火山において発生する爆発地震の震源過程を波形インバージョン法により明らかにし, 爆発時に観測される空気振動, 地盤変動と比較検討することにより, 爆発的噴火の力学過程の考察を行った. 爆発地震の初動は押し波(P相)であり, 振幅の大きい引き波(D相)が続く. 初動から2～3秒後に最大振幅を持つ低周波振動(LP相)が見られる. 伝播速度, 振幅の距離減衰, 振動軌跡から, P, D相はP波, LP相はレイリー波と考えられる. P相は深さ2kmにおける等方膨張, D相は円筒収縮によって励起される. また, LP相は火口直下0.3kmにおける等方膨張と水平収縮によって励起される. 火口直下浅部における等方膨張は空気振動とほぼ同時に発生すること, また, そのモーメントは空気振動の振幅と相関が見られることから, 浅部の等方膨張が空気振動を発生させていることが考えられる. 浅部の等方膨張と水平収縮から得られた変位量は爆発時に観測される伸縮計, 傾斜計の step から推定された変位量とほぼ一致することから, 浅部におけるガス溜まりの破裂およびガスの放出によって大振幅のレイリー波が発生していると考えられる.

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Nekoma Volcano, situated between Bandai Volcano and the Aizu Basin in northeast Japan, is a composite volcano of andesite to dacite with a total eruption volume of 16 km³. A horseshoe-shaped caldera a few km in radius was formed at the top of the volcano, and the volcanic activity is divisible into the Old Nekoma Volcano established before the caldera forming event from ca. 1 Ma to 0.6 Ma and the New Nekoma Volcano established after the caldera forming event after ca. 0.5 Ma. Old Nekoma Volcano is subdivided into Oguninuma north lava, Hayama lavas, Hagidaira pyroclastic flow (block and ash flow) deposit, Main cone lavas, Oguniyama lavas and Ougigamine lavas, in ascending stratigraphic order. They formed a flat cone-shaped volcano. All but the Ougigamine lavas were produced by summit eruptions and the Ougigamine lavas formed monogenetic volcanoes from several vents on the western flank. New Nekoma Volcano, erupted after Oshizawa debris avalanche deposit, which related to the caldera forming event, is composed of Nekomagatake lavas and 1349 m lavas occurred at the horseshoe-shaped caldera margin.

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走時データのインヴァージョンによる速度構造解析では与えられた速度構造下における地震波の伝播経路と走時々問を求める必要がある. このため波線追跡法が用いられるが, 火山地帯などの高度に不均質な速度構造に対応できる波線追跡法として, 最短経路法 (Dijkstra, 1959) の結果をダウンヒルシンプレックス法により最適化するハイブリッド手法を開発し, 走時データトモグラフィー手法を構成した (Nishi, 2001). この論文ではこの手法を雲仙火山で1995年に実施された構造探査データに適用し, 浅部の3次元地震波速度構造を求めた. その結果, 従来多くの火山で指摘されているように火山の中央部は周辺部と比較して概して高速度領域であることがわかった. これはすでに2次元の屈折法によって得られている結果 (雲仙火山人工地震探査グループ・清水, 1997) とも調和的である. しかし, この高速度領域は水平方向にかなり不均質で雲仙火山の場合は地殻変動の圧力源に関連した低速度領域や高速度領域の中にもより高速度の領域が存在することが明らかになった.

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Usu volcano in SW Hokkaido began a lot of phreatic explosions, after 22 years of dormancy, on 31 March 2000. In the middle of April and May 2000, we observed an eruption using a piezo blast-sensor, visible and infrared image recorders for getting information on the volcanic blast, shape and scale of the ash cloud, distribution of ballistic fragments and the size of the volcanic crater. Based on a scaling-law established from the results of our field explosion experiments, the data were analyzed to clarify and understand the relations among explosion energy and depth, and the surface phenomena such as crater formation, propagation of volcanic blast and the ejection of ballistic fragment. A summary of the results is as follows. The scaling-law obtained from the field explosion experiment heldup well in the case of the Usu 2000 phreatic explosions. We could determine both the explosion energy and depth using observation data such as the maximum over-pressure of a volcanic blast and the duration time of the ash cloud ejection, and vice versa. The explosion energy of a firework type explosion ranged from 10¹⁰ J to 10¹² J and that of jet type 10⁸ J to 10¹²J, respectively. It was also determined that the firework type occurred just beneath the ground surface, while the jet type occurred at a few tens meters to 100 meters deep.

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An infrasonic microphone network detected clear infrasonic pulses associated with the 2000 eruption of Usu volcano, Japan. The pulses were excited at intervals of several seconds and were considered to be excited by phreatic explosions at active craters from the comparison to visual observation from helicopter. Using the records from the infrasonic network, source locations of the pulses could be precisely identified. The amplitude distribution shows strong azimuthal dependence, which is considered as the topographic effect. The frequency and amplitude of the infrasonic pulses reflected the activity of the phreatic eruption and they became lower and smaller with the decline of the phreatic explosions at the active craters.

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The phreatic eruptions continued at the Nishiyamanishi and Kompirayamanishi craters, northwest flank of Usu volcano, in the southwestern part of Hokkaido, Japan, followed the initial phreatomagmatic eruption on March 31, 2000. The phreatomagmatic- and phreatic ejecta is characterized by abundantly containing accidental and crystal fragrnents, and clay minerals originated in the Neogene and Plio-Pleistocene altered volcanic rocks. The fragments in the ash were correlated with subsurface geology; Miocene altered dacitic and andesitic rocks (Osarugawa and Sohshunaigawa Formations), Plio-Pleistocene argillic tuff (Yanagihara Formation), altered andesite (Lower Pleistocene Andesitic Rocks), and non-altered andesite and basaltic andesite (Usu Somma Lava). Alteration zones in this area were classified into seven zones (I-VII) by their mineral assemblages. They are Plio-Pleistocene alteration zones (e.g., I-IV) and Miocene alteration zones (e.g., V-VII), and the depth of the latter is greater than several hundreds meters. In zone IV, kaolinite is distinctively abundant, so that kaolinite/smectite ratio is an useful parameter to evaluate the degree of contribution of the zone as an origin of fragments in the ash. In the Nishiyamanishi craters area, there is another kaolinite zone (IV) in shallower level (＜ 200 m below sea level), which was resulted from diffusion of sulfuric acidic hydrothermal water related to Pleistocene FeS and S mineralization (Abuta mine).

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During the 2000 Usu eruption, a series of eruptions and/or explosions had intermittently continued since March 31. The initial eruption on March 31 had been the largest one and produced tephra fall with small pyroclastic flow. A scale of each eruption and explosion has decreased since early April. Since the middle of April, eruptions have spreaded trace amount of ash fall mainly around active craters. Considerable amount of juvenile materials have been recognized only in March 31 eruptives. Their volume percent in total eruptives had decreased abruptly from several tens % (March 31) to less than 1% (April 2). It had been hard to find juvenile materials in eruptive ash since then. Therefore, although the initial and following eruptions were phreatomagmatic, almost all the following eruptions since early April have been phreatic. Most characteristic feature in these phreatic eruptions was a discharge of mud bombs, because crater floors had been filled with mud. Juvenile materials of March 31 are dacitic pumice (SiO₂=69.0～69.9%) which are slightly less differentiated than those of 1977～78 eruptions. This is consistent with temporal variations in historic magma since AD 1663, suggesting that magma plumbing system has evolved since 1663 to 2000. However, compared with the former 1977～78 eruption, the 2000 juvenile materials show distinct, parallel chemical variation for example in SiO₂-TiO₂ diagram. This suggests that distinct and/or modified magma system has been active in the 2000 eruptions.

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Usu volcano, located in Hokkaido, Japan, erupted on 31 of March 2000. Phreato-magmatic eruptions occurred at two vent groupings-the Kompirayama and Nishiyama Craters-on the northwestern foot of the edifice. We observed the eruptive activity using infrared thermal camera imagery taken and broadcast by way of the Internet. Software was made to analyse temporal variations in the number of infrared image pixels showing thermal anomaly, these corresponding to the high temperature parts of the eruption plumes from each vent group. This measure acted as a proxy for the amount of eruptive material being ejected from the craters. Every few hours, the Nishiyama Craters were observed to greatly increase their activity levels for a few minutes from a low level, while the Kompirayama Craters generally stayed at a relatively high level. The two vent groups showed a negative relationship between their activity levels at a few tens of minutes time scale. Two theoretical models (a self stimulating compressive oscillation model and a self stimulating aquifer level oscillation model) were presented to explain the observed negative relationship between them.

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Extensive infrared (IR: 8.00〜11.00μm) and visible/near-infrared (VNIR: 0.51～2.35μm) images from airborne multispectral scanners (MSS: VAM-90A, AZM) were acquired over the Usu volcano in Hokkaido before and after the 2000 Usu eruption. IR images give the apparent ground-surface brightness temperatures of the geothermal area and its spatial distribution. VNIR images can be used to determine spectral signatures of the ground surface. The airborne MSS observations were carried out on 4 October 1999, 3 April 2000, 26 April 2000, 25 May 2000, 16 June 2000, 14 July 2000, I August 2000, 20 September 2000 and 24 October 2000. After the 2000 Usu eruption (31 March 2000), we could detect geothermal activities of the newly formed crater groups (Nishiyamanishi crater group and Konpirasan crater group). The ground-surface brightness temperatures around the newly formed crater groups were less than 60℃. The brightness-temperature in the craters were not able to be detected well because of the existence of a large amount of fumarolic gas. Only the observation on 24 October 2000 was able to measure the temperature of the N31 crater partially, and its temperature was 70 to 142℃. No observation revealed any significant temporal change of geothermal activities in the summit area of Mt. Usu. The ash fall area from the eruption can be estimated using VNIR images. The temporal changes in the estimated ash fall area indicate that the ash fell intensively in the first stage of the eruption from 3 April 2000 to 25 May 2000. We also discuss the applicability of airborne MSS techniques to identifying the influence of the terrestrial heat and the solar radiation on the ground-surface brightness temperatures.

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Usu volcano located in southwestern Hokkaido is one of the most active volcanoes in Japan. On March 31, 2000, it erupted at the western foot of Nishiyama with successive formation of new craters after 23 years of dormancy. On the following day, another eruption newly forming craters occurred at the western foot of Konpirayama located I km to the east of Nishiyama new craters. Monitoring of volcanic gases discharged from active craters will provide us with authentic information on the volcanic activity, however, due to intermittently violent explosions, vicinity of the new craters were not accessible. Analyses of water-soluble components adhering to volcanic ash were available for estimating composition of volcanic gases and monitoring eruption of Usu volcano. Volcanic ash samples were collected around the volcano for about eight months. All the ash was relatively depleted in SiO₂ and enriched in FeO in comparison with essential ejecta of the historic eruptions of Usu volcano. Smectite was a dominant clay mineral and kaolin was a minor constituent. All the ash samples were hydrated considerably. Temporal variation in the constituent minerals and chemical composition was not recognized. This result designated that variation of the water-soluble components adhering to the ash was not due to change of nature of volcanic ash but due to change of flux and chemical composition of volcanic gases.

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Small-scale pyroclastic surges were discharged at least three times during the 2000 eruption of Usu volcano. The first event was formation of dry surge derived from the collapse of low eruption column within initial 2 hours after the start of the eruption on March 31, 2000. This surge left moderately-sorted deposit with juvenile fragments. The second event occurred before the sunrise of April 7. The surge material was sticking on wall of buildings and tree trunks facing towards the source crater. An apartment house, which was located only within a few tens of meters from a new crater, was totally knocked down by the surge. Flow direction of the surge was controlled either by oblique discharge of the eruption column, wind direction or local topography. Maximum travel distance of this wet surge was about 600m from the source. Although exact date and source crater for the third event could not be specified, another wet surge was generated from a crater locating nearby residential area of Toyako hot-spring resort.

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Continuous soil CO₂ efflux measurement was carried out at the southern inner slope of Usu Caldera for about 5 months between May 2000 and October 2000. The result of continuous monitoring showed that temporal variations of atmospheric pressure and precipitation influence the CO₂ efflux at Usu volcano. Peaks of CO₂ efflux were usually observed after the precipitation of more than several mm/hr, which last several hours. We found two main trends of CO₂ flux during the observation period. In the period between June 20 and August 17, we observed decreasing trend of the CO₂ flux, that may be related to decreasing eruptive activity of the 2000 Usu eruption. In the period from September 19 to October 23, the CO₂ flux response was strongly negative-correlated with atmospheric pressure (time lag: 24 hours, γ=-0.74). Two trends of soil fluxes suggest that the degassing mechanism was different between the two periods.

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有珠山2000年噴火の18月前の1998年9月に, 山頂域, 麓域の150箇所で採取した土壌中のホウ素とアンモニアの定量を行った. その結果, 土壌中のホウ素含有量は1,300μg/kgに達し, アンモニア含有量は14mg/kgに達した. ホウ素含有量の空間分布は, 900μg/kg以上の極めて高い領域が高温噴気活動が見られる山頂カルデラ内のほかにも, 2000年の噴火地点に近い北西山麓に存在していた. また, アンモニア含有量の高い領域は, ホウ素含有量の高い領域に一致し, さらに昭和新山でも見られた. 土壌中のホウ素, アンモニアの高濃度異常域は, 二酸化炭素の土壌からの放出量が高い領域 (Hernandez et al.,2001) とよく合っており, これらの成分が共通の起源をもつことを示唆している. このことは,土壌中のホウ素やアンモニアが, 二酸化炭素と同様に,火山体における揮発性物質の挙動を知る上で有用な指標となることを示している.

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