9世紀にアムールプレート東縁に沿って起きた噴火・地震活動について

抄録

Based on geological and archaeological data as well as historic documents, we review eruptions and earthquakes which have occurred during the 9th century in central and east Japan. The results reconfirm vigorous activities on Izu Arc, Izu-Oshima(～838AD ＜ N₃, N₂, N₁ ＜ 886AD), Niijima(～857AD and 886AD), Kozushima(838AD), Miyakejima(832AD and 850AD), and at Fuji volcano(800AD, 838AD ＜ ＜ 864AD, 864AD) during the 9th century. Beside these events, a big eruption of Niigata Yakeyama volcano had likely occurred in 887AD. Chokai volcano also erupted in 871AD, and 810-823AD. Collapse of Yatsugatake volcano took place in 887, probably invoked by a strong shock in 887. In addition, earthquakes with a magnitude from 7 to 8 had taken place along the Itoigawa-Shizuoka active fault system (in 841 or 762AD), Nagano fault system(887AD), Echigo plains(863AD), Shonai plains(850AD), Akita plains(830AD) and Nankai trough(887AD). As a result, we can point out a linkage of big eruptions and seismic activities in the 9th century over 800km long crossing Japan Arc. Geologically this seems to be a surface expression of East-West compression along eastern margin of Amurian Plate over 800km which was driven by the eastward motion of the plate. It is noteworthy to mention that the very similar volcanic and seismic activities have been occurring in the last 50 years to those happened in the ninth century, in their source areas and manners.