Construction of late Quaternary eruption history in and around the Izu Volcanic Islands, off Tokyo

Kaori Aoki1, Makoto Kobayashi2, Masanori Murata3, Fumikatsu Nishizawa4, Takayuki Takahashi3 and Takehiko Suzuki3, (1)Tokyo Metropolitan University, Research Center for Volcanic Hazards and Their Mitigation, Tokyo, Japan, (2)Mt.Fuji World Heritage Center, Shizuoka, Japan, (3)Tokyo Metropolitan University, Research Center for Volcanic Hazards and Their Mitigation, Japan, (4)Kanagawa Prefectural Museum of Natural History, Japan
Izu-Bonin Islands area off Tokyo has 21 active volcanoes (defined by JMA) including 7 submarine volcanoes. In northern islands area, there are Niijima Volcano, Kozushima Volcano, and Omurodashi submarine caldera volcano, which lively hydrothermal vent is discovered, originated from felsic magma, and Izu-Oshima Volcano and Toshima Volcano originated from basaltic magma on the Zenisu Ridge. For over 25,000 residents in this area, volcanic disasters are always the threat. We have worked on clarifying the eruption history and the distribution of eruptive products in this area for the volcanic hazard mitigation and the anti-disaster measures. Here, we report the latest scientific results.

We have recognized 21 eruptive events at least on Niijima Volcano, and some of them are derived from Kozushima and other distal volcanoes during late Quaternary. Kozushima Volcano, based on previous studies, would have erupted extensively basement lavas and overlying pyroclastic deposits at 70-64 ka, 52-35 ka, 32-23 ka. After that, we recognized 8 eruptive events at least on the Kozushima Volcano, and some of them are correlated to tephras from Niijima and other distal volcanoes.

The felsic maker tephras provided from Niijima and Kozushima volcanoes are also found at Izu-Oshima and Toshima volcanoes. The Izu-Oshima Volcano has a record of 12 major eruptions occurred after 1.8 ka, which were the caldera-forming activity. We re-examined the chronological framework based on 14C dating of eruption history of the Younger Edifice of pre-caldera before 1.8 ka. As a result, eruptive frequency of Izu-Oshima from 17 ka to 1.8 ka is shorter (67–75%) than previous estimation, suggesting higher frequent occurrence of the eruptions in this period.

Also, we investigate tephrostratigraphy in the marine core which length is 190.38 m off the Boso Peninsula. The upper 30 m of this core includes felsic tephras provided from Niijima, Kozushima, and Omurodashi volcanoes since 20 ka.