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

[SCG46-01] From JFAST to JTRACK: Investigating the Spatiotemporal Evolution of the Japan Trench Tsunamigenic Fault

*Shuichi Kodaira¹, Kohtaro Ujiie², Marianne Conin³, Christine Regalla⁴, James Kirkpatrick⁵, Patrick Fulton⁶, Lena Maeda¹, Natsumi Okutsu¹, Nobuhisa Eguchi¹, IODP Exp.405 Science Party (1.Japan Agency for Marine-Earth Science and Technology, 2.University of Tsukuba, 3.University of Lorraine, 4.Northern Arizona University, 5.University of Nevada, 6.Cornell University)

Keywords:Tohoku-oki earthquake, Japan Trench, seismogenic fault, IODP

The 2011 Mw9.1 Tohoku-oki earthquake was one of the largest recorded earthquakes, generating a devastating tsunami due to exceptionally large slip at the shallow portion of the Japan Trench subduction zone. To investigate the conditions and processes that facilitated this extreme slip and to clarify the spatial and temporal variations of the structural, material, and stress state of the plate boundary fault, International Ocean Discovery Program (IODP) Expedition 405 (JTRACK) conducted a multidisciplinary drilling program including coring, logging, and observatory installation at two sites across the trench. Site C0019, located ~6 km landward of the Japan Trench, was previously drilled during IODP Expedition 343 (JFAST) in 2012. JFAST provided important insights into the frictional properties of the fault that enabled large coseismic slip, but only a limited number of samples were recovered from the fault vicinity due to time constraints. On the other hand, JTRACK successfully obtained core samples from the frontal prism down to the oceanic igneous crust through the plate boundary fault, capturing the complexity of the accretionary prism, fault, and lower plate materials. This allowed for a more detailed characterization of the structural and lithological properties of the fault zone and the frontal prism. JTRACK also extended logging while drilling (LWD) operations to ~960 mbsf, providing key data to assess the present stress state around the fault. Additionally, two borehole observatories were installed to monitor long-term temperature variations, offering critical data on the hydrogeologic structure and fluid pressure conditions within the fault system. Site C0026, located ~8 km seaward of the Japan Trench, was drilled to investigate input sediments to the subduction system. LWD data extended to ~430 mbsf, and coring recovered sediments from depths up to 290 mbsf. Input sediments, composed of hemipelagic and pelagic deposits, will ultimately be incorporated into the subduction zone, influencing the mechanical and frictional properties of the plate boundary fault. The successful execution of Expedition 405 provided valuable data and geological samples to clarify the spatial and temporal variations of the structural, material, and stress state of the plate boundary fault. Combined with post-cruise analyses of mechanical, frictional, paleomagnetic, and hydrogeologic properties, these findings will provide unprecedented insights into the mechanisms and processes of large shallow coseismic slip.