Abstract:We analyzed refractive index, microstructure, vesicularity of glass from an alien tephra in the upper weathered volcanic ash soil horizon of Fuji Volcano, Izu peninsula, Miyakejima, and Niijima Volcanoes. The glass size increases toward Izu Islands. Based on the vesiculality of glass, the Kozushima-Tenjosan tephra (AD 838) can be distinguished from the Niijima-Mukaiyama tephra (AD 886). Thus, we determined from where the tephra of low refractive index on the flank of Fuji Volcano comes from. Next, using the stratigraphic correlation of the tephra, the detailed stratigraphy of Fuji Volcano during the period of AD 700-1,000 was clarified. It was difficult to determine exact age of each eruption by 14 C dating with 2 sigma error bar (around ±40 years). However, the KozushimaTenjosan tephra horizon can classify Fuji Volcano products those below the tephra and those above the tephra. The Obuchi scoria (OBC), the Kansuyama scoria (KNS), the Nishi-Futatsuzuka scoria on S and SE flanks, the Takamarubi lava flow (Tam) and the Hinokimarubi 2 lava flow (Hnm 2), the scoria associated with the Oniwa-Okuniwa 2 lava flow (Onw 2) on NE and N flanks are below the Kozushima-Tenjosan tephra. The Fudosawa lava flow (Fud), the Obuchimarubi lava flow (Obu), the Higashi-Usuzuka south lava flow (Hum), the Mizugatsuka-hinokimarubi lava flow (Him) on S and SE flanks, the Kenmarubi 2 lava flow (Ken 2), the Tenjin-Igatono scoria (TNG), the Aokigahara lava flow and the Nagaoyama scoria on the NE and N flanks are above the Kozushima-Tenjosan tephra. The Kozushima-Tenjosan tephra was discovered just below the S-24-6 scoria on E flank. The stratigraphy of E flank is rearranged as follows in ascending order: the S-24-5 scoria, the S-24-5-1 scoria (newly defined), the S-24-5-2 scoria (newly defined), the S-24-5-3 scoria (newly defined), the Kozushima-Tenjosan tephra, the S-24-6 scoria, the S-24-7 scoria, the S-24-9 scoria, the S-24-10 scoria.

The Sengen-yama hills, located at Yamakita town, Kanagawa Prefecture, Japan, represent the upthrown block along the E-W trending Hinata fault. The roughly -km long Hinata thrust fault is a segment of a long bell-shaped splay of faults the Tanna, Hirayama, Hinata and Kozu-Matsuda faults developed at the collision zone of Honshu island and Izu-Mariana arc. We found old fluvial gravel deposits and an overlying pumice fall deposit near the top of the hills. The pumice fall deposit is identified to be the Fuji-Yoshioka tephra F-YP; -ka on the basis of phenocryst assemblages, refractive index of the heavy minerals and facies association. Based on geology and tephra correlation, the onset of emplacement of the Sengen-yama hills is determined to be -ka. According to previous studies, the initiation of the faults occurred about ka. Close spatial correlation and close onset ages of these faults suggest that they were formed and developed in the same tectonic regime.CThe Geological Society of Japan 2005 111