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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. 

International Ocean Discovery Program (IODP) Expedition 358 was carried out from October 2018 through March 2019 on the D/V Chikyu in 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, 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 the preliminary analysis. Coring from 400 to 571 mbsf yielded datable material and possible evidence for diapiric intrusion of sediments. 

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.