[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-P01] Paleoseismological record around the JTRACK site, Japan Trench

*金松 敏也¹、池原 研²、熊 衎昕¹、Chang Yu-Chun¹、新井 和乃² (1.海洋研究開発機構、2.産業技術総合研究所)

キーワード：古地震学、JTRACK、タービダイト、日本海溝、巨大地震

During the 2011 Tohoku-oki earthquake, a large slip of frontal wedge occurred around 38°N off Tohoku in northern Japan. This slip is thought to have occurred on the horst-graben system of the subducting plate and was accompanied by extensive mass transport. Indeed, previous studies had reported that mass transport deposits is the dominant lithology on the surface of the landward slope of the frontal wedge. On the other hand, it was found that gravity flow deposits were formed in the basin of the Japan Trench floor in relation to the 2011 Tohoku-oki earthquake, and moreover thick event layers corresponding to past huge earthquakes of similar magnitude to the 2011 Tohoku earthquake were also documented. Event layers corresponding to the 2011 Tohoku earthquake, the 1454 CE Kyotoku earthquake and the 869 CE Jogan earthquake were found to be well preserved and widely distributed in the trench floor basin. A comprehensive study of the stratigraphy of the Japan Trench in the vicinity of JTRACK site was conducted, employing sediment samples obtained using a conventional piston corer. In addition to thick event layers, it was found that several layers, with smaller thickness but distinct upward fine-grained deposits, are intercalated between event layers of the 2011 Tohoku-oki earthquake and the 1454 CE Kyotoku earthquake. They were confirmed at least five sites in the Japan Trench basins around 38°N. Their distribution extends about 200 km north-south off Miyagi. Their spatial distribution appears to partially overlap with that of the 2011 Tohoku-oki earthquake event layers. Their averaged recurrence interval is estimated to be about 100 years, although this estimation involves a large uncertainty due to the lack of precise ages in these interval. If these event layers were induced by earthquakes, what earthquakes correspond to the event layers? The historical record suggests that large interplate earthquakes have occurred in the vicinity of the trench (e.g. M7.9 in 1793 AD, M7.7 in 1897 AD), based on the distribution of tsunamis. These earthquakes probably correspond to the event beds identified in the piston core samples. These geological data are likely to be important in estimating long-term stress release and accumulation in the Miyagi-oki, Japan trench subduction with huge earthquake records.