Tephrochronological study of volcanic eruptions recorded in historical documents.

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In this study, we researched volcanic activities during thousands years BP by means of geomorphoiogy and tephrochronology. Confirmation of eruption, eruption point and scale and effect on environments were investigated by field survey and interpretation of historical documents. Our researching fields were izu volcanic islands in Shizuoka Prefecture and north Kanto districts of east Japan. There distributes many active volcanoes that erupted in historical age.In Mikura-jima island lie in Izu volcanic islands, the latest eruption of Oyama volcano occurred 5,200BP. Just above the deposits of last eruption, deposited peaty clay layer. Carbon 14age of the peaty clay is 3,640 yrs BP. At a depth 16-17cm from the surface, lies Iz-Kt tephra fall. This tephra fell when Tenjozan volcano erupted in 838AD at izu Kozushima-island. Pollen analysis of the peaty clay suggest that formation of Suzuhara moor in Mikurajima island might have been caused by the latest eruption of Oyama volcano.In northern Kanto district, Yoshinuma moor sited on Takahara volcano was surveyed. Hr-FP tephra fall from Haruna-Futatsu-Dake volcano, which erupted in middle 6th century deposited in the moor. From our investigation, Hr-FP tephra lie at a 1-3m depth from the surface of peat in Senjo-gahara moor in Nikko mountains, Karekinuma moor in Takahara volcano, Tashiroyama moor, Kinunuma moor. Tephra fall damages regional forests and varies the forest composition. The results of the pollen analysis by Yoshinuma indicate that the accumuiation rate differs in moor of Kanto district. The highest accumulation rate of peat seems to be 2 or 3 times larger among them.Hr-FP tephra eruption affects the environments, especially to forests. Effects of voicanic eruptions are different according to forest types whether the forest is composed of conifers or broadleaved, deciduous or evergreen.