地震波速度構造と実験室の速度のデータを比較することにより,日光火山群の地殻の温度分布を見積もった. 等温線とともに,地殻内S波反射面及び低周波微少地震の震源を3次元表示した. 900℃を超える高温域が火山フロントから背弧側に認められ,火山と高温域の間に,S波反射面と低周波微少地震の震源が認められた. 温度分布の結果は,日光火山群の地殻の顕著な低速度領域に,溶融域が存在することを示唆している. 延性的な下部地殻におけるマグマの移動が低周波微少地震を引き起こし,また,脆性的な上部地殻におけるマグマの貫入により地殻内反射面が形成されると考えられる.

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筆者らによる阿蘇カルデラのMT探査の結果,中岳火口下は低比抵抗であることが明らかになっている. 1989〜1990年の噴火に際して,火口周辺部の比抵抗構造,特に低比抵抗層の上面深度を明らかにすること,および火山活動に伴うこの低比抵抗層の変化の検出を目的として,阿蘇中岳火口を含む地域で,CSMT観測を1989年,1990年各1回実施した. これらのMT,CSMT観測の結果から次のことがあきらかになった. (1)中岳火口下の極めて低比抵抗な層は,火口周辺に限定されており,また,その低比抵抗層(層厚約200m)は,火口下の比較的浅部(約100m深度)から存在する. (2)また,この低比抵抗層は火山活動が活発であるときも,静穏時でも存在し,1989〜1990年の噴火に際しては,比抵抗の減少,その領域の拡大が見られた. この低比抵抗層は,火口下のイオンを多く含む帯水層であり,この帯水層の存在は,1989～1990年の噴火時に見られた地下水との相互作用で生じる小規模な水蒸気爆発や地磁気異常の変化と調和的である. このことは,この低比抵抗層のモニターが,阿蘇火山の噴火のプロセスの解明,および噴火予知にとって重要であること示す.

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Aokigahara Lava Flow erupted in A.D. 864 covers the northwestern foot of Fuji volcano. It consists of two flows: Ishizuka Lava erupted from a parasitic vent at the foot of Ohmuroyama, and Nagaoyama Lava effused from a cinder cone of Nagaoyama. Ishizuka Lava flowed down to Motosuko Lake, spreading laterally like an alluvial fan in front of the lake. Approximately 36 flow lobes that protruded from the aa front, pinch and swell, occasionally bifurcating into several branches from swollen portions of the lobes. Most lobes have a few longitudinal clefts with similar features to crease structures of rhyolitic and andesitic lava domes and to be banded on cleft walls on pahoehoe tumuli. The flow lobes are classified into three types on the basis of surface structures. Type-I lobes are characterized by well-developed clefts, which exhibit two distinct features: Stripe-type has alternating stripes of red and grey on the cleft wall parallel to the elcngation of the lobes. The stripes exist only for a depth a cm from the surface of the walls. The groundmass of the grey stripes contains abundant dendritic crystallites, while that of the red stripes consists of palagonitized glass; Blister-type develops blisters with pipe vesicles on the cleft wall. It does not have stripes, however, they may be present at different levels on the same cleft wall. Type-II lobes have clinkers with stripes similar to those of Stripe-type clefts of Type-Ilobes, however, they do not develop clefts. Type-III lobes are thinner than Type-I obes and are covered with red platy clinkers, giving the whole lobe a reddish appearance. Clefts are poorly developed. Continuous supply of lava into a flow lobe, which ceased advancement, causes tensile stress on solidifying crusts. We suggest that stepwise opening of cracks resulted in the difference in crystal density and habit of stripes on the clefts. Occasional opening of the crack and quenching hot semi-molten lava exposed at the crack tip under water resulted in the fomation of red stripes, followed by solidification of the new crust beneath the crack tip. Subsequent fracturing of newly formed brittle crust resulted in the formation of the grey stripes. The Blister-type clefts were formed when the supply of lava temporaily stopped and reslarted again. Solidification of the crust and the formation of blisters proceeded when lava within the lobe was static. We conclude that Type-I was produced by intermittent supply of lava into a static flow lobe. Type-II lobes were moving during inflation of the lobe. As a result, cleft walls with stripes were disrupted to produce clinkers. Type-III was produced by breaking of crust immediately after its fomationd uring rapid emplacement of the lobe. Therefore, the shear rates during the fomation of flow lobes increase from Type-I, II, to III lobes.

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