[S-SS11] Active faults and paleoseismology

[SSS11-P06] Holocene crustal movements recorded in fossil calcareous assemblages around the Jogasaki Coast, the eastern part of Izu Peninsula, Japan

*Yuzuko Tsukahara¹, Toshihito Saito², Toshimichi Nakanishi³, Nariaki Nishiyama¹, Natsuko Fujita¹, Makoto Kawamura¹, Koji Umeda⁴ (1.Tono Geoscience Center, JAEA, 2.Izu Jogasaki Nature School, 3.Museum of Natural and Environmental History, Shizuoka, 4.Hirosaki University)

Keywords:crustal movement, fossil calcareous assemblage, Holocene, Izu Peninsula, 14C, uplift

Paleo-sea levels estimated from the elevation and age of emerged shoreline features and fossil calcareous assemblages are one of the useful tools for reconstructing the tectonic features of the Holocene. In this study, we reconstruct the paleo-sea levels around the eastern part of Izu Peninsula using fossil calcareous assemblages to verify case studies in areas with active volcanism and seismic activity. In addition, we will infer the characteristics of crustal deformation through comparison with previous studies. Shishikura et al. (2023) surveyed emerged shoreline features and fossil calcareous assemblages around the coast in the vicinity of Ito, the northeastern part of the Izu Peninsula, to clarify the emerged process. They concluded that the cause of the emergence is associated with a volcanotectonic uplift due to subsurface magma movement. In order to verify the representativeness of this crustal feature, we conducted a field survey of the Jogasaki Coast, 8-9 km south of the area studied by Shishikura et al. (2023).
We found four fossil calcareous assemblages consisting of Serpulidae, Barnacle and shells at three sites on the Jogasaki Coast. We collected 21 samples from fossil calcareous assemblages and dated them to ¹⁴C using JAEA-AMS-TONO-5MV at the Tono Geoscience Center, and calibrated them to calendar years, to estimate the emergence age. The assemblages were attached at altitudes of approximately 2.5-3.6 m, 3.3-3.5 m, 2.5-2.8 m, and 3.5-3.8 m. The emergence ages were estimated to be 1222-1561 CE, 1244-1596 CE, 1130-1490 CE, and 1225-1577 CE. These ages are generally consistent with those of Shishikura et al. (2023) for the upper section of Zone 2. On the other hand, comparing the height distribution, the upper heights are up to 1.35 m higher than those of the upper section of Zone 2. There is a possibility that the difference of heights indicates a regional change in tectonic activity. Thus, in an area where fossil calcareous assemblages are preserved at multiple sites, it is possible to compare and interpret them. It is expected that this research will be an effective tool for more precisely understanding the characteristics of crustal movements specific to the region and will contribute to clarify the mechanisms.
This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2022)”.

Reference Shishikura et al.(2023) Tectonophysics, 864.