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

[SCG46-04] Lithological overview of IODP Expedition 405 (JTRACK) Site C0019 and C0026

*山口 飛鳥¹、Bellanova Piero²、Brunet Morgane³、Castillo Marissa⁴、Chang Yu-Chun⁵、Everard Jenna⁶、福地 里菜⁷、Gough Amy⁸、Nicholson Uisdean⁸、Pizer Charlotte⁹、Turel Cédric¹⁰、Conin Marianne¹¹、Kirkpatrick Jamie¹²、小平 秀一⁵、Fulton Patrick¹³、Regalla Christine¹⁴、氏家 恒太郎¹⁵、 IODP Expedition 405 Scientists、前田 玲奈⁵、奥津 なつみ⁵ (1.東京大学大気海洋研究所、2.RWTH Aachen University、3.IFREMER、4.The Ohio State University、5.JAMSTEC、6.Lamont-Doherty Earth Observatory, Columbia University、7.鳴門教育大学、8.Heriot-Watt University、9.University of Innsbruck、10.UMR Géoazur、11.University of Lorraine、12.University of Nevada, Reno、13.Cornell University USA、14.Northern Arizona University、15.筑波大学)

キーワード：日本海溝、JTRACK、ちきゅう

International Ocean Discovery Program (IODP) Expedition 405 (JTRACK) sailed from September to December, 2024, to investigate the properties, processes, and conditions within subduction zones that promote large slip to the trench. One of the objectives of this expedition is to characterize the geologic composition and structure of the frontal prism, décollement, and subducted oceanic crust. Lithological observations contribute to understanding how these materials control localization of strain over geologic timescales, fault and earthquake mechanics, and provide evidence of past earthquakes and deformation.
In this presentation, we report key lithological findings from JTRACK, summarized as follows:
1. The complete lithology of both the input site (Site C0026) and the prism site (Site C0019) is described.
2. The largest population of sediments is classified as siliceous vitric mud in a modified ternary plot of sediment grain size and composition.
3. The frontal prism is primarily composed of chaotic and inclined beds.
4. Soft-sediment deformation structures were commonly observed, especially in ashy intervals, possibly formed by ancient earthquakes.
5. Continuous surface sediment samples for paleoseismological investigations were obtained.
6. Lithological variations around the plate boundary fault zones were identified.
7. Basalt, chert, and limestone from the Pacific plate, which are potential fluid sources to the subduction zone system, were sampled beneath the prism.
8. Although the mass flux of each component is still unclear, the frontal prism appears to be formed through a mixture of material from landward slope failures, possibly including reworked prism material, trench-fill sediments, and sediments from the incoming plate.
Postcruise research will enable further understanding and quantification of these processes.