Eruptions and earthquakes in and around Izu Arc area during the ninth century ; aiming the understandings of the tectonics

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Aiming to clarify the unique tectonics in and around to northern part of Izu Arc in the 9th century, we re-examined eruptions and seismicity occurred in the northern part of Izu Arc in the century.Based on geological, archaeological data and historical documents, we confirmed that eruptions and earthquakes were really vigorous in that period, as so far pointed out Big eruptions repeatedly took place in the 9th century, at Kozushima, Niijima, Miyakejima and Oshima volcanoes. Moreover, Fuji volcano was active and a couple of big earthquakes also occurred on Kanto inland. In addition, compiled results showed that seismic activity was also at a high level including surrounding area. Destructive earthquakes successively hit along the Itoigawa-Shizuoka active fault system, Akita Plains, Shonai Plains, Echigo Plains, Nagano fault system, and Nankai trough. Niigata-Yakeyama and Chokai volcanoes also erupted in that period. These activities in the middle of the 9th century were concentrated in Izu Islands and along boundary between the East and West Japan. These suggest a linkage of big eruptions and seismic activities over 800km long crossing Japan Arc. Geologically these events seem to express the East-West shortening along eastern margin of Amurian Plate over 800km, driven by the eastward motion of the plate.We point out the similarity of volcanic and seismic activities during the recent 50 years to those happened during the 9th century, in their source areas and manner. Some scientists that the central part of the Itoigawa-Shizuoka tectonic line active fault system and Fujigawa fault system are overdue to the average recurrence period. In this context we cannot discard the worst scenario that the collapse and eruption of Fuji volcano may follow the coming big earthquake along the eastern margin of the Amurian plate.