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1. Expedition 405 Preliminary Report Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK)

   Kirkpatrick, J; Regalla, C; Conin, M; Fulton, P; Ujiie, K; Kodaira, S; Okutsu, N; Maeda, L; Toczko, S; Eguchi, N (December 2025, International Ocean Discovery Program)

   The extremely large slip that occurred on the shallow portion of the Japan Trench subduction zone during the 2011 Mw 9.1 Tohoku-oki earthquake directly contributed to the devastating tsunami that inundated the Pacific coast of Japan. International Ocean Drilling Program (IODP) Expedition 405 aimed to investigate the conditions and processes that facilitated the extremely shallow slip on the subduction interface during the 2011 Tohoku-oki earthquake to improve understanding of the factors that slip to the trench on subduction zones. Expedition 405 implemented a combined logging, coring, and observatory operational plan at two sites: Site C0026 ~8 km seaward of the Japan Trench to characterize the input sediments to the subduction zone and Site C0019 ~6 km landward of the trench where the plate boundary fault zone is present at ~825 meters below seafloor (mbsf). At Site C0026, the input section was logged to ~430 mbsf with a logging-while-drilling (LWD) assembly that characterized the succession of sediments and rocks from the seafloor to the basaltic rocks of the oceanic crust. Cores recovered from four holes as deep as 290 mbsf contain a sequence of hemipelagic and pelagic sediments that will be input into the shallow subduction system and therefore control both the localization of the plate boundary fault zone and the slip behavior of the plate boundary. Site C0019 was previously drilled in 2012 during Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project [JFAST]), so revisiting this site allowed temporal variations in the frontal prism and plate boundary fault zone to be evaluated. The LWD data to ~960 mbsf characterized the frontal prism, plate boundary fault zone, and lower plate to the basaltic volcanic rocks. Cores were recovered from multiple holes that contain a variety of muds from the frontal prism and the plate boundary fault zone, as well as lower plate materials. Comparison with the sediments from Site C0026 provides a basis to interpret the tectonic and sedimentological processes operating in the dynamic environment of the frontal prism. Cores from the plate boundary fault zone provide a unique window into the structural complexity of an active plate boundary fault that is known to host large seismic slip. Two borehole observatories were installed at Site C0019 that contain temperature sensors deployed to measure temperature over a period of years and reveal the hydrogeologic structure of the shallow subduction system. These hugely successful drilling operations, combined with postexpedition work to measure the mechanical, frictional, paleomagnetic, and hydrogeologic properties of the core samples and to constrain the history of past seismic slip at Site C0019, provide an unprecedented opportunity to advance our understanding of shallow subduction systems. Outreach during the expedition leveraged and elevated the success of the operations by sharing the outcomes with diverse domestic and international audiences, including scientists, students, educators, stakeholders, and the general public. Thanks to the efforts of a large group of onboard outreach officers and their onshore support, activities included ship-to-shore broadcast events; interviews with science party members and crew; the publication of videos, blogs, magazine articles, and social media posts; and development of formalized classroom lesson plans and materials. 

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2. Expedition 405 Scientific Prospectus: Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK)

   https://doi.org/10.14379/iodp.sp.405.2023  

   Kodaira, S.; Conin, M.; Fulton, P.; Kirkpatrick, J.; Regalla, C.; Ujiie, K.; Okutsu, N.; Maeda, L.; Toczko, S.; Eguchi, N. (July 2023, Scientific prospectus)

   The 11 March 2011 M 9.0 Tohoku-oki earthquake was one of the largest earthquakes ever recorded and was accompanied by a devastating tsunami. Slip during the earthquake was exceptionally large at shallow depth on the plate boundary fault, which was one of the primary factors that contributed to the extreme tsunami amplitudes that inundated the coast of Japan. International Ocean Discovery Program Expedition 405 aims to investigate the conditions and processes that facilitated the extremely shallow slip on the subduction interface in the 2011 Tohoku-oki earthquake. Proposed work includes coring and logging operations at two sites in a transect across the trench. The first site, located within the overriding plate, will access the fault zone in the region of large shallow slip, targeting the plate boundary décollement, overlying frontal prism, and subducted units cut by the décollement. The second site, located on the Pacific plate, will access the undisturbed sedimentary and volcanic inputs to the subduction zone. A borehole observatory will be installed into the décollement and surrounding rocks to provide measurements of the temperature in and around the fault over the following several years. Sampling, geophysical logs, and the observatory temperature time series will document the compositional, structural, mechanical, and frictional properties of the rocks in the décollement and adjacent country rock, as well as the hydrogeologic structure and pore fluid pressure of the fault zone and frontal prism—key properties that influence the effective stress to facilitate earthquake slip and potential for large slip. Results from Expedition 405 will address fundamental questions about earthquake slip on subduction zones that may directly inform earthquake and tsunami hazard assessments around the world. 

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3. Expedition 358 Scientific Prospectus: NanTroSEIZE Plate Boundary Deep Riser 4

   https://doi.org/10.14379/iodp.sp.358.2018  

   Tobin, H.; Kimura, G.; Kinoshita, M.; Toczko, S.; Maeda, L. (July 2018, Scientific prospectus)

   null (Ed.)

   The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) program is a coordinated, multiexpedition drilling project designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies. The fundamental scientific objectives of the NanTroSEIZE project include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system. Site C0002 is in the Kumano forearc basin above the seismogenic, and presumably locked, portion of the plate boundary thrust system. The Kumano Basin sedimentary sequence and uppermost part of the accretionary prism were drilled, logged, and sampled during Integrated Ocean Drilling Program Expeditions 314 (logging while drilling [LWD] to 1401.5 mbsf), 315 (coring to 1057 mbsf), 338 (LWD to 2005 mbsf and coring to 1120 mbsf), and 348 (LWD to 3058.5 mbsf, with limited coring from 2163-2218.5 mbsf). International Ocean Discovery Program (IODP) Expedition 358 aims to reach and sample the megasplay fault/plate boundary fault at Site C0002 by extending the riser borehole (Hole C0002P) established during previous Integrated Ocean Drilling Program NanTroSEIZE expeditions. Evidence suggests that the drilling target at the megasplay will reach a region where megathrust seismogenic processes are active. A new borehole will be “kicked off” from Hole C0002P and will be extended to ~5000 meters below seafloor (mbsf) and then across the high-amplitude seismic reflector identified as the main plate boundary fault. Continuous LWD with drilling mud gas analysis and limited coring at the anticipated plate boundary fault depth, will leave the cased borehole completed at ~5200 mbsf. A suite of analyses on cuttings, mud gases, and limited cores will address the four primary scientific objectives: (1) determine the composition, stratigraphy, and deformational history of the Miocene accretionary prism; (2) reconstruct its thermal, diagenetic, and metamorphic history; (3) determine horizontal stress orientations and magnitudes; and (4) investigate the mechanical and hydrological properties of the upper plate of the seismogenic plate boundary. The main scientific objective is to log and sample the hanging wall, the fault zone, and into the footwall. The main contingency plan is to leave a cased borehole in good condition for future installation of a long-term borehole monitoring system (LTBMS). These operations will extend drilling conducted during Integrated Ocean Drilling Program Expeditions 326, 338, and 348. The entire expedition will cover a period of 164 days, beginning on 7 October 2018 and ending on 21 March 2019. 

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4. Expedition 358 Preliminary Report: NanTroSEIZE Plate Boundary Deep Riser 4: Nankai Seismogenic/Slow Slip Megathrust

   https://doi.org/10.14379/iodp.pr.358.2019  

   Tobin, H.; Hirose, T.; Ikari, M.; Kanagawa, K.; Kimura, G.; Kinoshita, M.; Kitajima, H.; Saffer, D.; Yamaguchi, A.; Eguchi, N.; et al (October 2019, Preliminary report)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 358 was carried out from October 2018 through March 2019 on the D/V Chikyuin an attempt to reach a plate boundary fault zone at seismogenic depths for the first time in scientific ocean drilling. The goal was to extend Hole C0002P from ~2900 to ~5200 meters below seafloor (mbsf) and cross the seismically interpreted main décollement fault zone with logging while drilling (LWD), downhole stress measurements, cuttings sampling, mud gas sampling, and partial coring by drilling a sidetrack to create a new hole (C0002Q). Although drilling reached 3262.5 mbsf, the deepest to date in all of scientific ocean drilling, the effort to drill to and sample the target—the megathrust fault zone—was not successful. Operational challenges in establishing sidetrack holes and advancing them at reasonable rates of penetration limited the new cased hole interval to less than 60 m total at a depth shallower than the previously established casing depth of 2922 mbsf. Combined, the cuttings, logs, and ~60 cm of recovered core from sidetrack Holes C0002Q–C0002T revealed hemipelagic sediments and fine silty turbidites consistent in lithology and physical properties with those recovered in the same depth interval at the same site during Integrated Ocean Drilling Program Expedition 348. Cuttings revealed evidence of only weakly deformed rock, with relatively common calcite veins but few other structural indicators. Because no downhole leak-off tests were made and very little borehole imaging was performed, no further insight into the tectonic context was acquired. After riser drilling at Site C0002 was terminated, drilling at alternate contingency Sites C0024 and C0025 was carried out. Site C0024 targeted the frontal thrust region to sample and log hanging wall rocks and the shallow portion of the décollement zone, and Site C0025 accessed sediments in the Kumano fore-arc basin. At Site C0024, a dedicated logging hole was drilled and a very complete suite of logs were acquired from 0 to 869 mbsf. Preliminary interpretation of log response and images suggests the frontal thrust zone was encountered from about 813 mbsf to the base of the hole, with a zone of notably low resistivity and steep bedding from 850 mbsf to the bottom of the hole. Core samples revealed lithologic units interpreted to be hemipelagic and turbiditic basin fill, trench fill, and Shikoku Basin sediments and encountered deformation potentially associated with a back thrust imaged in seismic reflection data. However, coring had to be terminated at about 620 mbsf, well short of the frontal thrust zone. Site C0025 recovered fore-arc basin sediments underlain by those interpreted to have been deposited in a trench-slope basin setting; no clear transition into older, inner accretionary wedge material was identified during preliminary analysis. Coring from 400 to 571 mbsf yielded datable material and possible evidence for diapiric intrusion of sediments. 

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5. Expedition 375 Scientific Prospectus: Hikurangi Subduction Margin Coring and Observatories

   https://doi.org/10.14379/iodp.sp.375.2017  

   Saffer, D.M.; Wallace, L.M.; Petronotis, K. (January 2017, Scientific prospectus)

   null (Ed.)

   Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expedition 375 aims to investigate the processes and in situ conditions that underlie subduction zone SSEs at northern Hikurangi through coring of the frontal thrust, upper plate, and incoming sedimentary succession and through installation of borehole observatories in the frontal thrust and upper plate above the slow slip source area. Logging-while-drilling (LWD) data for this project will be acquired as part of Expedition 372 (beginning in November 2017; see the Expedition 372 Scientific Prospectus for further details on the LWD acquisition program). Northern Hikurangi subduction margin SSEs recur every 2 years and thus provide an excellent setting to monitor deformation and associated chemical and physical properties surrounding the SSE source area throughout the slow slip cycle. Sampling material from the sedimentary section and oceanic basement of the subducting plate and from the primary active thrust in the outer wedge near the trench will reveal the rock properties, composition, and lithologic and structural character of the material transported downdip to the known SSE source region. A recent seafloor geodetic experiment shows the possibility that SSEs at northern Hikurangi may propagate all the way to the trench, indicating that the shallow fault zone target for Expedition 375 may lie within the SSE rupture area. Four primary sites are planned for coring, and observatories will be installed at two of these sites. Expedition 375 (together with the Hikurangi subduction component of Expedition 372) is designed to address three fundamental scientific objectives: (1) characterize the state and composition of the incoming plate and shallow plate boundary fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth; (2) characterize material properties, thermal regime, and stress conditions in the upper plate above the SSE source region; and (3) install observatories at the frontal thrust and in the upper plate above the SSE source to measure temporal variations in deformation, fluid flow, and seismicity. The observatories will monitor deformation and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of slow slip events and their relationship to great earthquakes along the subduction interface. 

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