After 54 years’ dormancy since the last eruption in 1942, volcanic activity of Hokhaido-Komagatake volcano has obviously increased. Small phreatic eruptions sporadically occurred in 1996 and 1998, and several small phreatic eruptions intermittently continued from September to November in 2000. Eruptions on September 4 and October 28 were relatively large in the 2000 eruption, with a total eruptive mass of 10⁵ and 3×10⁴ tons, respectively. Volcanic ashes of the 1996, 1998, and 2000 eruptions contain fragments constituting the shallow levels of the volcanic edifice. No juvenile materials have been identified in the eruption products. However, ashes of the two major eruptions in 2000 contain fragments of hornblende and mafic andesite characteristic of the rocks seated at deeper levels of the edifice. In addition, as determined the water-soluble components adhered on the ashes, the Cl/S molar ratio increases up to 0.2 to 0.4 in 2000 eruptions while it remains＜0.1 in 1996 and 1998 eruptions. The depth of explosion has been getting deeper and the effect of magmatic gas has increased in 2000 eruptions than in 1996 and 1998 eruptions. We thus suggest that the vent system has been developed well toward a deeper magma storage system during the 2000 activity.

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Having investigated spatial variation of seismicity in the Pacific slab beneath northeastern Japan using JMA data since 1995, we found that number of earthquakes in the upper seismic plane decreases remarkably near the volcanic front and on its deeper side. The change in the seismicity is so clear that we can separate the active and inactive areas by a line. We show that the boundary line in the seismicity in the upper seismic plane delineates eastern boundary of the distribution of volcanoes and it runs almost parallel to the Volcanic Front. Further, a remarkable correspondence is observed between the boundary line and an active belt of shallow inland seismicity in northeastern Japan. In lwate Prefecture where the active seismic belt bends to the west the boundary line is curved to the west as well. We think the conspicuous decrease of the seismicity in the upper seismic plane beneath the volcanic front and on the deeper side is causally related to the formation of magma bodies along the subduction zone.

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The Chokai volcano is a giant andesitic volcano situated in the back arc side of northeast Japan arc. Newly obtained 19 K-Ar age data, with previous ones, revealed the development history of this volcano, whose activity was geologically divided into three stages; I, II, and III. Stage I: Around 0.6 Ma, volcanic activity began with eruptions of the Uguisugawa basalt and Tengumori volcaniclastic rock (earliest activity), those cover the basement rocks. During early and later activities, the Old Chokai volcanic edifice had been constructed until ca. 0.16 Ma without conspicuous dormancy (early to late activities), and attained its maximum size (2,000 m above sea level, with a volume of 47 km³) at ca. 0.4 Ma. Edifice collapses and renewal edifice constructions have taken place during ca. 0.4 to 0.16 Ma. Stage II: The eruptions started at ca. 0.16 Ma from many vents aligned in the WNW-ESE direction on western flank of the Old Chokai volcano, and the West Chokai volcanic edifice was formed (ca. 22 km³) in this stage. Activity of this stage began with the eruption of olivine-hypersthene-augite andesite and basalt (stage IIa: ca. 0.16-0.12 Ma) followed by the eruption of hornblende-bearing olivine-hypersthene-augite andesite lavas (stage IIb: ca. 0.10-0.09 Ma). Late in this stage, southwestern part of the West Chokai volcanic edifice collapsed resulting in the formation of the West Chokai horseshoe-shaped caldera, and andesitic lavas erupted within the caldera (stage IIc: ca. 0.09-0.02 Ma). Stage III: The East Chokai volcanic edifice was formed after ca. 0.02 Ma. This stage is subdivided to IIIa (3.5 km³) and IIIb (0.8 km³) stages, by construction of the East Chokai horseshoe-shaped caldera on the East Chokai volcanic edifice and the Kisakata debris avalanche deposit. Lava flows and lava domes were extruded within the caldera. The Shinzan lava dome (0.001 km³) was formed at 1801AD.

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青麻火山噴出物のフッ素含有量を測定した. 青麻火山の活動はカルデラの形成を境に, 前期と後期に分けられ, 後期噴出物は全て苦鉄質包有物を顕著に含む. 前期噴出物のフッ素含有量は105〜180ppm,後期噴出物は44〜298ppmである. 苦鉄質包有物のフッ素含有量は315～1564ppmで, 母岩と比べて高い値を示す. 玄武岩質安山岩および安山岩(苦鉄質包有物を除く)における青麻火山のフッ素含有量は, 青麻-恐火山列に属す他の火山の示す値とほぼ一致し, 那須火山帯(脊梁火山列)の示す値ともほぼ同じ値である.青麻一恐火山列の安山岩およびデイサイトのフッ素含有量はSiO₂量の増加に伴って増加せず,東北日本弧の他の火山列に比べ,比較的低い値を示す. 青麻火山のいくつかの苦鉄質包有物は,東北日本弧の他の火山と比べてはるかに高いフッ素含有量を示しており, このことは,これら苦鉄質包有物の成因を理解する上で重要な手掛かりを与えるものと考えられる.

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