Book ChapterExpedition 405 Scientific Prospectus Addendum: Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK)Kodaira, S; Conin, M; Fulton, P; Kirkpatrick, J; Regalla, C; Ujiie, K; Okutsu, N; Maeda, L; Toczko, S; Eguchi, NA new component to the drilling operations has been added to the International Ocean Discovery Program (IODP) Expedition 405 schedule following approval of an Ancillary Project Letter (1013-APL). The new operations involve deploying a borehole observatory into Hole C0019D with a temperature sensor string. This hole and observatory infrastructure (i.e., casing) was previously drilled as part of Integrated Ocean Drilling Program Expedition 343/343T in 2012 (Expedition 343/343T Scientists, 2013). Site C0019 is the same site as Site JTCT-01A, described as part of Expedition 405 in Kodaira et al. (2023). Installation of a new instrument string in Hole C0019D at the beginning of Expedition 405 operations will allow the passive observation of anticipated subsurface hydrologic effects caused by nearby drilling (e.g., Kinoshita and Saffer, 2018). Together, the new drilling around Site C0019/JTCT-01A and the resulting observatory temperature observations in Hole C0019D will constitute a series of cross-borehole experiments that enable the determination of large-scale hydrogeologic properties around the plate boundary fault and overlying damage zone. The site priorities and drilling and coring strategy at the primary sites for Expedition 405 (Sites JTCT-01A and JTCT-02A) remain unchanged from the original Expedition 405 Scientific Prospectus (Kodaira et al., 2023).International Ocean Discovery Program2024-07-1510523420Scientific prospectus4052332-1385https://doi.org/10.14379/iodp.sp.405add.20241326927International Ocean Discovery ProgramIODPChikyuExpedition 405Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK)Earth in MotionSite JTCT-02ASite JTCT-01ANational Science Foundation