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The change in physical properties from incoming sediment to accretionary prisms is a key to understanding mass balance of sediments, fluid flux, heat transportation, wedge architecture, and seismic behavior at a subduction interface (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). Although Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Experiment (NanTroSEIZE) Stage 1 was conducted targeting sediments in the wedge, slope basins, backarc basin, decollement, and shallow portion of the megasplay fault, incoming sediments at the reference sites located oceanward of the deformation front were drilled during IODP NanTroSEIZE Stage 2 Expeditions 322 and 333. Incoming sediments at the reference sites represent the state before subduction and in this respect are very important to understanding the change in physical properties after underthrusting. In addition, because the sediments at the reference sites are considered to be in a normal compaction state, the porosity-velocity relationship can be used to convert velocity into porosity and also into pore pressure (e.g., Saffer, 2007).

We measured compressional wave velocity at a variety of effective pressures to examine the relationship between porosity and velocity for sediments from Sites C0011 and C0012 from Expedition 333.