Numazawa volcano is a small caldera volcano in NE Japan, and comprises dacitic lava domes, the Mizunuma Pyroclastic Deposit, and the Numazawako Pyroclastic Deposit, in ascending order. The Mizunuma eruption involving 1-2 km³ dacitic magma occurred at about 51 ka. This eruption started with phreatic explosions (unit I), followed by a Plinian eruption of inversely-graded pumice fallout (unit II), and terminated with generation of Pyroclastic flows (unit III). The Mizunuma Unit III pyroclastic flow deposit was made up of densely-welded, thick, pumiceous tuff breccia confined to the valley. The recent Numazawako eruption at 5.0 ka, involving 2.5 km³ dacitic and andesitic magmas, started with major pyroclastic-flow generation (unit I), and followed by a minor Plinian phase producing normally-graded pumice fall deposits (unit II). The Numazawako Unit I pyroclastic flow deposit consists of the massive pumice flow facies filling topographic lows and the stratified surge facies covering topographic highs : this flow devastated an area of ＞450 km² around the volcano to the maximum distance of ＞20 km from the vent. The basal part of the Unit I deposit is the coarsest and contains abundant hydrothermally-alterated lithic fragments. As a result of this eruption, 2-km-wide Numazawako caldera was formed. The Units II and III of the Mizunuma Pyroclastic Deposit represent the normal sequence of pyroclastic-flow eruption : the transition from fall to flow activity was resulted by the extrusion of magma with progressively lower gas content as the eruption proceeded. In this case, effective emission of heated groundwater around the conduit before the magmatic eruption, produced the Unit I phreatic deposit and inhibited violent interactions between external water and ascending magma. On the other hand, the Numazawako eruption is unusual, as it started with the generation of destructive pyroclastic flow. Its mass-eruption rate is sufficient for rapid column collapse, in order to produce a high-speed flow at the beginning of the eruption. This eruption was probably initiated by phreatic explosions which disrupted large parts of the conduit sealing compressed steam and sudden decompression of magma which generated a highly-discharged column.

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雲仙岳の火山活動に伴って発生した地震のS波コーダ部分の振幅減衰から,雲仙岳周辺のQ構造を推定した.コーダQ^-1(Qc^-1)は等方1次散乱近似の仮定で推定された.雲仙岳西方の地震を雲仙岳西山腹の観測点で観測した場合と,山頂直下の地震を近傍の観測点で観測した場合に,高Qc^-1値が得られた.低Qc^-1;値は島原半島東部の地震で得られた.また,雲仙岳東部で観測された地震も,低Qc^1-値であった.不均質性と非弾性の異なる性質の2種類の媒質を仮定して,多重散乱と内部減衰による地震波の振幅減衰を,モンテカルロ法でシミュレーションした結果と,観測から推定されたQ構造との比較を行った.シミュレーションでは,地震波の振幅減衰は,強い不均質性と非弾性の領域の内部か,または,近傍で顕著になることがわかった.このことから,雲仙岳の西側及び山頂付近が,東方部分よりもより強い不均質性と非弾性を示すといえる.

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