Insight into Submarine Landslide Flow Behavior from Particle Size Analysis and 3D Seismic, Site C0021, IODP Expedition 338

I propose to conduct a downhole particle size analysis for Site C0021. Grain size distribution is a fundamental rock property that I will use to (1) document vertical variation in grain size distribution of the entire cored interval including the large landslide deposit (Figure 1) and (2) gain insight into the release mechanism of the landslide deposit (fast vs slow). Furthermore, grain size is a basic rock property dataset that will be available to other investigators for petrophysical/geotechnical/sedimentological studies and aid in correlation control across other NanTroSEIZE sites (e.g., C0018). The proposed specific tasks will illuminate magnitude and occurrence of submarine landslides along active subduction zone margins and their role in triggering catastrophic tsunami hazards and in sediment mass-transfer and margin evolution. The proposed work will result in two publications: 1) the particle size analysis will be published as a data report to be published in the expedition proceedings, and 2) a follow-up manuscript that builds on the particle size analysis will be submitted to a peer-reviewed journal. Grain size distribution within the landslide unit will provide insight into the release mechanism (fast vs slow), which is a critical factor in determining the magnitude of slide-generated tsunami (Harbitz, 1992). Site C0021 is ideally located near the up-dip source area (Figure 1). Particle size distribution at this site will therefore represent near pre-failure composition, as opposed to the downdip site (C0018), which could have experienced significant sorting during the landslide process. I will combine the grain size analyses into a detailed core-log-seismic integration using the physical properties data, LWD data, and 3D seismic data to shed light on flow behavior of landslides at Nankai. I will do this by calculating a Flow Factor for the landslide penetrated at C0021 (and C0018) after the methodology described in Sawyer et al, 2012. Sawyer et al (2012) showed how composition (grain size and water content) and shear stress relate to the acceleration of the released slide in laboratory experiments. The ratio between the shear stress that drives flow ( D ƒä ) and the shear strength that resists flow ( R ƒä ) is defined as the Flow Factor (Ff) and yields insight into the flow behavior of the landslides (Sawyer et al., 2012). RELATION TO EXPEDITION OBJECTIVES The proposed work will shed light on the sedimentary evolution of the slope basin seaward of the megasplay fault, which is characterized by stacked deposits of submarine landslides. We aim to make fundamental scientific contributions in linking the tectonic activity of the splay fault with the resulting sedimentary deformation processes, which fits within NanTroSEIZE objectives. Furthermore, the proposed work directly addresses key questions defined in the NanTroSLIDE project proposal: ¡§What are the dynamics of large submarine landslides and can we infer their tsunamigenic potential?¡¨ We will address this with the proposed work by linking the rock properties with slope geometry to constrain sliding dynamics and tsunamigenic potential. METHODOLOGY We will first measure grain size via hydrometer (ASTM, 2007). We then conduct a mapping exercise of the landslide unit and regional structure in the 3D seismic data. We will then conduct an infinite slope calculation of the driving shear stress, D ƒä , which depends on slide thickness, bed slope, and bulk density. We will determine bed slope and slide thickness from mapping in 3D seismic. We will constrain prefailure bulk density from moisture and density data of the tophole sediments. The landslide unit is buried and its porosity is not representative of pre-failure conditions when it was at the surface. Therefore, tophole sediment will be more representative of actual pre-failure properties of landslide, and appear to be very similar lithology (Figure 2) to those of the landslide unit. We will use vane shear strength (both peak and residual) to constrain the resisting shear strength, R ƒä . RELATION TO OTHER INVESTIGATORS To my knowledge, no other scientists are conducting particle size analyses at Site C0021. The particle size analysis will complement the work being proposed by Strasser (1. Sand Provenance; 2. Kinematics of slides) and Tudge (Correlation from C0018 to C0021 and MTD characterization) and perhaps projects by others. The proposed Flow Factor analysis will also complement studies of slide behavior but will not be repeat works of others. The Flow Factor methodology is a unique approach. FACILITIES, EXPERTISE, AND PERSONNEL My research lab at the University of Kentucky is equipped with seismic workstations and laboratory facilities for sedimentological analyses, where the work will be conducted. I have conducted a similar particle size study in support of IODP Expedition 308, Gulf of Mexico (Sawyer et al., 2008). I have advanced experience in interpretation of 3-dimensional seismic data of underwater landslides. The grain size analyses and seismic mapping will be performed directly by my graduate student in support of their graduate degree.