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The physical properties of material in the accretionary prism and at tectonic subducting margin tectonic boundaries are fundamental to understanding the dynamic processes of plate subduction. In terms of large earthquakes in subduction zones, two possible major plate boundaries (approximately horizontal décollement extending to the trench and its branching megasplay fault, which are best exampled in the Nankai Trough) are the most important tectonic features. Since magnetic fabric analysis can provide reliable and important information about the formation and subsequent tectonic history of rock units, we aim to examine variations of sedimentary magnetic fabric characteristics across both the megasplay fault and the frontal thrust in the Nankai Trough, Japan, using samples from Integrated Ocean Drilling Program (IODP) Expedition 316.

Documenting the state of stress and strain in the accretionary prism is crucial for understanding the faulting process in a subduction zone. The triggers for catastrophic faulting at the plate boundary could be due to thermal condition, fluid pressure, lithification, and wedge shape. In order to constrain this problem, it is important to describe the observable stress and strain at the plate boundary. However, the large-scale structures within an accretionary prism and their links to the seismogenic process are still vague. For any field related to subduction zone process, information about the present status of strain and/or stress is fundamental. Difficulties often arise regarding the limited volume of material at any given depth when attempting to conduct structural and sedimentological analyses of marine sediments from drilled cores. The use of anisotropy of magnetic susceptibility (AMS) provides a quick and nondestructive analysis for small samples that enables us to grasp an overview of structure in drilled cores with systematic sampling. It has been demonstrated by previous ocean drilling expeditions that AMS analysis plays a key role in the investigation and understanding of deformation zones, for example, during Ocean Drilling Program (ODP) Legs 131, 156, 170, and 190 and the IODP NanTroSEIZE expeditions (Owens, 1993; Housen et al., 1996; Housen and Kanamatsu, 2003; Ujiie et al., 2003; Kitamura et al., 2010, 2014; Kanamatsu et al., 2012, 2014; Novak et al., 2014). In this report, we present the results of our AMS study of samples taken during IODP Expedition 316 at Sites C0004, C0006, C0007, and C0008 (Fig. F1). Intense sampling throughout the drilled cores provides an excellent overview of sediment deformation within and across an active subduction margin.