Preliminary assessment of IODP Expedition 405 JTRACK in the Japan trench: investigating slip and tracking fault healing after a Mw9 earthquake

IODP Expedition 405 “Tracking Tsunamigenic Slip Across the Japan Trench” (JTRACK) was a challenging but successful 4-month expedition (September 6 to December 24, 2024 – 56 scientists) that revisited and drilled the large co-seismic slip region of the 2011 Mw 9.0 Tohoku-oki earthquake, 12 years after IODP Expedition 343 “JFAST” had done so. One of the expedition’s primary objectives is to evaluate temporal variations in stress state, fluid flow, and physical properties in the thirteen years since the Tohoku-oki earthquake. This will allow an assessment of how faults heal and reload after a major earthquake, and the role of fluids in such processes. The second objective is to investigate the compositional, structural, mechanical, hydrological, and frictional properties of the rocks in and around the shallow plate boundary, to assess the role of each of those components on the plate boundary location and slip behavior, and to understand the long-term evolution of this prism. Two sites were successfully drilled. At Site C0019, located ~8km landward of the trench axis, drilling intersected the frontal prism, décollement, and subducted plate at the JFAST location. Drilling at Site C0026 intersected the sediment sequence and underlying oceanic crust of the incoming Pacific Plate, thus serving as a reference site. Operations consisted of: 1) collecting logging while drilling (LWD) data at C0019 and C0026 from the seafloor to oceanic crust; 2) coring at C0019 through the entire frontal prism, décollement, and oceanic crust, and at C0026 through the incoming sediment sequence; 3) installing temperature sensors in two borehole observatories to characterize fault zone hydrogeology by re-instrumenting the existing observatory in borehole C0019D (JFAST observatory) and the developing and instrumenting a new observatory borehole C0019P (JTRACK observatory). Overall, Expedition 405 was a huge operational success. Under 7 km of water, it successfully recovered cores from multiple shallow hydraulic piston coring system (HPCS) holes at each site, as well as three deep small-diameter rotary core barrel (SD-RCB) holes at Site C0019 to ~950 mbsf and one SD-RBD hole at Site C0026 to ~300 mbsf. Together, the boreholes provide continuous records of the subsurface from the seafloor to the deep sedimentary rocks and mafic volcanic rocks of the oceanic crust, documenting the Pacific oceanic plate as never before. The plate boundary fault zone was drilled and sampled in multiple holes, providing a unique dataset from an active fault zone that constrains its lateral heterogeneity. Measurements and observations made provide key data to evaluate the controls on shallow tsunamigenic slip and the temporal variations in stress and physical properties and conditions that occur following a great subduction zone earthquake. Overall, the range of data gathered during the expedition is vast, encompassing sedimentary and volcanic processes, paleoseismology, paleoclimate and paleo-oceanography, earthquake mechanics, and tectonic processes at convergent margins.