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The Tuaheni Landslide Complex is thought to have originated through multiple slip events offshore the Hawke’s Bay region, North Island (New Zealand). Cores were obtained from within and beneath the interpreted landslide complex to a maximum depth of ~188 meters below seafloor (mbsf) in Hole U1517C during International Ocean Discovery Program Expedition 372. This data report provides the results of 99 X-ray diffraction analyses of the clay-sized fraction (<2 µm spherical equivalent); sampling focused on the background lithology of hemipelagic mud. Normalized weight percent values for common clay-sized minerals (where smectite + illite + undifferentiated [chlorite + kaolinite] + quartz = 100%) do not vary markedly among the five lithostratigraphic units. Overall, the mean and standard deviation (σ) values are smectite = 47.1 wt% (σ = 5.2), illite = 34.0 wt% (σ = 3.4), chlorite + kaolinite = 8.8 wt% (σ = 1.9), and quartz = 10.2 wt% (σ = 4.0). Mineral proportions within the clay-sized fraction do change somewhat across the boundaries between units and near inferred slip surfaces (e.g., at ~31 mbsf), but those excursions are within the normal range of statistical scatter for the site. Likewise, indicators of clay diagenesis are relatively monotonous throughout the cored interval. The average value of illite crystallinity index is 0.53Δ°2θ (σ = 0.027). Smectite expandability averages 77.4% (σ = 4.9), and the average proportion of illite in illite/smectite mixed-layer clay is 8.7% (σ = 5.6). 

Submarine slope failures pose risks to coastlines because they can damage infrastructure and generate tsunamis. Passive margin slope failures represent the largest mass failures on Earth, yet we know little about their dynamics. While numerous studies characterize the lithology, structure, seismic attributes and geometry of failure deposits, we lack direct observations of failure evolution. Thus, we lack insight into the relationships between initial conditions, slope failure initiation and evolution, and final deposits. To investigate submarine slope failure dynamics in relation to initial conditions and to observe failure processes we performed physical experiments in a benchtop flume and produced numerical models. Submarine slope failures were induced under controlled pore pressure within sand–clay mixtures (0–5 wt% clay). Increased clay content corresponded to increased cohesion and pore pressure required for failure. Subsurface fractures and tensile cracks were only generated in experiments containing clay. Falling head tests showed a log-linear relation between hydraulic conductivity and clay content, which we used in our numerical models. Models of our experiments effectively simulate overpressure (pressure in excess of hydrostatic) and failure potential for (non)cohesive sediment mixtures. Overall our work shows the importance of clay in reducing permeability and increasing cohesion to create different failure modes due to overpressure. 

Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw ~9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4–5 km) succession of primarily Bengal-Nicobar Fan–related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate ~250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. The large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentary subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes at the Sunda subduction zone and globally at other subduction zones where submarine fan–influenced sections are being subducted. 

Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw ~9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4–5 km) succession of primarily Bengal-Nicobar Fan–related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate ~250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. Large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentary subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes, at the Sunda subduction zone and globally at other subduction zones where submarine fan–influenced sections are being subducted. 

Due to the availability of new site survey data and previous changes that defined proposed Sites SUMA-11C and SUMA-12A as the primary sites for Expedition 362, two new proposed alternate sites have been selected: SUMA-23A and SUMA-24A. This addendum provides the scientific objectives for proposed Sites SUMA-23A and SUMA-24A, regional and detailed maps, and seismic profiles for the two sites. The site priorities and drilling and coring strategy remain unchanged from the original Expedition 362 Scientific Prospectus. The operations time estimates for all alternate sites are presented. The new proposed alternate Sites SUMA-23A and SUMA-24A are located above Fracture Zone 7B, which is located south of the current primary and alternate sites. The sites are located close to the epicenter of one of the 2012 Mw >8 earthquakes. These sites are still part of the input section to the southern 2004 earthquake rupture region of the subduction zone. Proposed Site SUMA-23A provides a section of Unit 1 (thin trench wedge) and a significant part of Unit 2 (Bengal-Nicobar submarine fan deposits and interbedded hemipelagite) overlying Fracture Zone 7B and includes sampling of 10 m of basement atop the basement high. Proposed Site SUMA-24A provides a section of Unit 1 (thin trench wedge) and a thinner part of Unit 2 (Bengal-Nicobar submarine fan deposits and interbedded hemipelagite) than proposed Site SUMA-23A, which overlies Fracture Zone 7B, and includes sampling of 10 m of basement atop the basement high. The new site survey data were acquired on board the Schmidt Ocean Institute (CA, USA) research vessel (R/V) Falkor in 2015 during the MegaTera experiment, an international project between the Earth Observatory Singapore (EOS), the Indonesian Institute of Sciences, Schmidt Ocean Institute (SOI), and Institut de Physique du Globe de Paris (France). SOI provided the R/V Falkor for the experiment, and EOS funded the rental of the seismic equipment. 

The 2004 Mw 9.2 earthquake and tsunami that struck North Sumatra and the Andaman-Nicobar Islands devastated coastal communities around the Indian Ocean and was the first earthquake to be analyzed by modern techniques. This earthquake and the Tohoku-Oki Mw 9.0 earthquake and tsunami in 2011 showed unexpectedly shallow megathrust slip. In the case of North Sumatra, this shallow slip was focused beneath a distinctive plateau of the accretionary prism. This intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the North Sumatran subduction zone are a distinctive, thick (up to 4–5 km) sequence of primarily Bengal-Nicobar Fan–related sediments. This sequence shows strong evidence for induration and dewatering and has probably reached the temperatures required for sediment-strengthening diagenetic reactions prior to accretion. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggests the input materials are key to driving the distinctive slip behavior and long-term forearc structure. The aim of Expedition 362 is to begin to understand the nature of seismogenesis in North Sumatra through sampling these input materials and assessing their evolution, en route to understanding such processes on related convergent margins. Properties of the incoming section affect the strength of the wedge interior and base, likely promoting the observed plateau development. In turn, properties of deeper input sediment control décollement position and properties, and hence hold the key to shallow coseismic slip. During Expedition 362, two primary, riserless sites (proposed Sites SUMA-11C and SUMA-12A) will be drilled on the oceanic plate to analyze the properties of the input materials. Coring, downhole pressure and temperature measurements, and wireline logging at these sites will constrain sediment deposition rates, diagenesis, thermal and physical properties, and fluid composition. Postexpedition experimental analyses and numerical models will be employed to investigate the mechanical and frictional behavior of the input section sediments/sedimentary rocks as they thicken, accrete, and become involved in plate boundary slip system and prism development. These samples and downhole measurements will augment the internationally collected site survey bathymetric, seismic, and shallow core data that provide the regional geological framework of the margin.