3:45 PM - 4:00 PM
[SVC41-50] Tephrostratigraphy in central and northern Niijima Volcano, Izu Island: Necessity of reexamination of the eruption history for mitigation of volcanic hazard
Keywords:tephrostratigraphy, eruption history, volcanic hazard and mitigation, Niijima Volcano
1. Geomorphogical analysis using high resolution LiDAR DEM
We used the IN-YOU-ZU (Patent (Pat. No. 4379264) of Aero Asahi Corporation) generated from 1m resolution LiDAR DEM acquired by Geographical Survey Institute (GSI) for geomorphogical analysis. We recognized 6 tuff ring-maar craters around Wakago area in northern part of Niijima Island. In the central and northern part of Niijima Island, we estimated the sequential order of lava domes on the basis of degree of dissection. Mineji-yama is the oldest one, followed by Niijima-yama and Akasaki-mine. Miyatsuka-yama showing a flat and horizontal top was the latest one. Around the contact region of Miyatsuka-yama and Akasaki-mine, mounds with 20–30 m height were recognized. Lavas constituting those lava domes were covered with thick pyroclastic deposits, and the mounds on these edifices consist of pyroclastic deposit including volcanic blocks. Considering that, eruptions of each lava dome accompanied by pyroclastic products and had continued for a prolonged period (Kobayashi et al., 2018).
2-1 Miyatsuka-yama and Akasaki-mine area: At the Fujimi-touge where Yoshida (1992) described as the representative site of the tephro-stratigraphy, we can observe 9 tephra layers from Niijima and Kozushima Volcanoes (“Fujimi-touge tephra group”, F1 to F9 in descending order). F5 was defined as "Niijima-yama tephra", however this tephra is likely to be related to the eruption of the Miyatsuka-yama lava dome, as it is correlative with thick pyroclastic deposits directly covering the lavas that formed Miyatsuka-yama. F9 was defined as "Miyatsuka-yama tephra", however since it is correlated with thick pyroclastic deposits forming mounds on Akasaki-mine, it is most likely to relate to the eruption of the Akasaki-mine lava dome.
2-2 Wakago area: We recognized 6 tephra layers, corresponding to the ejecta of Kudamaki-Atchiyama eruption (AD856–857) (Tsukui et al., 2008), derived from the tuff ring-marr craters in Kudamaki and Awai-ura. These tephras are volcanic breccia deposits consisting of basaltic volcanic blocks, scoria, and pyroclastic surge deposits and so on, associated with the Atchi-yama rhyolitic lava dome. At the top of Miyatsuka-yama, we found ash fall deposits and volcanic blocks with a diameter of a few cm to 10 cm related to the Kudamaki-Atchiyama eruption.
2-3 Daisanyama site: At the Daisanyama site, we recognized at least 17 tephra layers, consisting of the tephras (including Fujimi-touge tephra group) from Niijima Volcano, the tephras from Kozushima Volcano (Nishizawa et al., 2018), and thin basaltic marker tephras. In addition, we detected volcanic glass shards from AT and K-Ah. At this site, pyroclastic deposits after ca. 30 ka are almost continuously exposed, it is possible to provide useful database for constructing the explosive volcanic eruption history in this area (Aoki et al., 2018).
3. Necessity of reexamination of the eruption history for volcanic hazard and mitigation
Results of our surveys, we concluded that reconstructing the tephrostratigraphy is necessary. It is most likely to judge that "Niijima-yama tephra" and "Miyatsuka-yama tephra" defined by Yoshida (1992) are the ejecta related to the eruptions of Miyatsuka-yama lava dome and Akasaki-mine lava dome, and also to judge that Niijima-yama formed at the same time as Akasaki-mine and Mineji-yama based on their degree of dissection.