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doi:10.2204/iodp.proc.314315316.217.2011

Introduction

Accretionary prism sediments are initially unlithified and have >60% porosity (e.g., Bray and Karig, 1985). Because of progressive sedimentation or underthrusting along the décollement, the unlithified sediments become lithified rocks at depth. The degree of lithification in a package of sediments can be examined quantitatively using physical properties. We focused on compressional and shear wave velocities to examine the state of lithification because these parameters are expressions of elastic moduli. The evolution of elastic moduli in sediments is strongly related to diagenetic and strengthening processes resulting from porosity decrease, fluid release, cementation, and strain history. Variability within the accretionary complex and along the décollement may have implications for wedge geometry, fluid migration, and seismogenesis (e.g., Bangs and Westbrook, 1991; von Huene and Scholl, 1991; Moore and Vrolijk, 1992; Erickson and Jarrard, 1998; Bilek and Lay, 1999; Moore and Saffer, 2001; Gettemy and Tobin, 2003; Saffer, 2007).

In this study, we describe the results of laboratory measurements of compressional and shear wave velocities and related physical properties such as shear and bulk moduli of sediments obtained from Integrated Ocean Drilling Program (IODP) Expedition 315 Site C0002 and IODP Expedition 316 Site C0004. The results can be compared with other velocity or physical property data of sediments from accretionary prisms (e.g., Gettemy and Tobin, 2003; Hashimoto et al., 2010; Raimbourg et al., 2011). All sites are located in the hanging wall and footwall of the megasplay fault in the Nankai accretionary prism (see the “Expedition 315 summary” chapter [Ashi et al., 2009] and the “Expedition 316 summary” chapter [Screaton et al., 2009]). Shear wave velocity for accretionary sediments has only rarely been reported (e.g., Gettemy and Tobin, 2003; Tsuji et al., 2006). Measuring the S-wave velocity of unlithified sediments at in situ conditions can be difficult, but S-wave velocity is needed to obtain elastic moduli. We conducted careful measurements and signal processing to obtain S-wave velocities.

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