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

[SCG46-P08] Microfabric variation in sediments associated with initial subduction processes across the Japan Trench (IODP Exp. 405)

*Takeru Yoshimoto¹, Taizo Uchida², Sara Satolli⁴, Toshiya Kanamatsu³, Yuzuru Yamamoto¹, Kohtaro Ujiie⁵, Marianne Conin⁶, Jamie Kirkpatrick⁸, Christine Regalla⁹, Patrick Fulton⁷, Shuichi Kodaira³, IODP Exp. 405 Scientists (1.Kobe University, 2.Kochi University, 3.JAMSTEC, 4.University of Chieti-Pescara, 5.University of Tsukuba, 6.University of Lorraine, 7.Cornell University, 8.University of Nevada, Reno, 9.Northern Arizona University)

Keywords:Anisotropy of Magnetic Susceptibility, Décollement, IODP, JTRACK

International Ocean Discovery Program (IODP) Expedition 405, Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK), successfully cored the plate boundary fault zone underneath the frontal prism (Site C0019) and the incoming plate sediments (Site C0026) in the Japan Trench subduction zone, NE Japan. To reveal microfabric variation in sediments, related to the initial subduction process, the anisotropy of magnetic susceptibility (AMS) was examined.
The magnetic foliation (F) in the frontal prism varies from 1.01 to 1.06, but the K_min inclination is constantly perpendicular to the bedding planes. The underthrust sediments beneath the décollement were characterized by vertical flattening (F = 1.06-1.09) compared to the same lithological unit in the incoming plate (F = 1.02-1.07). The porosity of the underthrust sediments was ~20% lower than that of the input sediments. These indicate that the frontal prism and underthrust sediments had been compacted during subduction due to the sediment overburden of the frontal prism.
In contrast, a consolidation fabric was apparently disturbed around the major fault zone; a decrease in the magnetic foliation from 1.04 to 1.01 without any change in magnetic lineation (L < 1.01) was observed in the 20 m-interval above the major fault zone at ~610 mbsf (meter below sea floor). Similarly, the magnetic foliation systematically decreases from 1.08 to 1.01, particularly in the 15 m interval above the plate boundary fault zone (826 mbsf in Hole C0019J and 846 mbsf in Hole C0019K).