北八甲田火山群における最近5000年間の噴火史

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  • Eruptive history of Kita-Hakkoda volcano group in Japan during the past 5000 years

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The eruptive history of Kita-Hakkoda volcano group in the northeast Japan arc during the past 5000 years has been determined by tephra-stratigraphy and AMS ^<14>C dating. Kita-Hakkoda volcano group comprises eleven small-scale stratovolcanoes. We identified seven tephra layers (Hk-J1,Hk-J2,Hk-J3,Hk-1,Hk-2,Hk-3 and Hk-4 in descending order) erupted from this volcano group. The Hk-J1,Hk-J2,and Hk-J3 tephras were erupted from Jigokunuma Pond (100m in diameter) situated on the southwesterm foot of Odake volcano. The distributions of these tephras are limited the area within 300 m from the vent. These tephras were derived from phreatic eruptions. The magnitude of each eruption is less than 10^4 m^3 in volume. Judging from stratigraphic relation with the Towada-a tephra (AD 915), the eruption age of Hk-J1,Hk-J2 and Hk-J3 is estimated to be 500-400 years ago. The Hk-1,Hk-3 and Hk-4 tephras were erupted from the summit crater of Odake volcano. The source vent of the Hk-2 tephra may be Odake or ldodake volcano. The distribution of each tephra is restricted in 4-7 km from the vents. These tephras were derived from phreatic or vulcanian eruptions. The volume of these tephras ranges from 10^6 to 10^5 m^3. On the basis of radiocarbon dates, their ages are estimated to be 2.1 kyr BP for Hk-1,3.0 kyr BP for Hk-2,3.8 kyr BP for Hk-3 and 4.2 kyr BP for Hk-4.

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