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doi:10.2204/iodp.proc.314315316.217.2011

Methods

The design of the experiment is similar to that employed in other studies of saturated marine sediments (e.g., Tobin et al., 1994; Tobin and Moore, 1997; Gettemy and Tobin, 2003; Hashimoto et al., 2010).

In the velocity measurements, two pumps (Teledyne ISCO 1000D syringe pump) were used to control pore fluid pressure and confine pressure. The pore pressure of 500 kPa was kept under drained conditions. Confining (effective) pressure was increased stepwise in the measurements. Velocity measurements were conducted under isotropic pressure conditions. Confining pressure was pressurized in 10 s and maintained for 24 h for the next step. The pressure intervals depended on the samples, ranging from 100 to 1000 kPa (Fig. F2). An in situ effective pressure was approximated for each sample from the sediment bulk density and hydrostatic pore fluid pressures at the depth of recovery (Table T1). The maximum effective pressure for each test was the in situ effective pressure. Lead zirconate titanate (PZT) shear wave transducers (1 MHz) were used in a source-receiver pair to measure wave speed. PZT in a shear orientation generates a weak compressional mode in addition to its primary shear mode. This allowed us to identify P- and S-wave arrivals in each test, although the S-wave arrival time was often difficult to locate precisely within the coda of the P-wave arrival. Axial displacements were measured during the experiments. Porosity changes were not obtained.

We found that the S-wave arrival was consistent across an array of waveforms at different effective pressures, although the error can be as large as ~2 µs (~5% error in shear wave velocity). Errors for P-wave velocity, bulk moduli, and shear moduli are ~2%, ~5%, and ~10%, respectively.

Three samples for each site were examined (Fig. F1C, F1D). At Site C0002, one sample is from forearc basin sediments (Core 315-C0002B-32R [Sample C2B-32R]) and two samples are from upper accreted sediments (Cores 315-C0002B-51R and 61R [Samples C2B-51R and C2B-61R]) (Fig. F1D). At Site C0004, one sample is from sedimentary breccia (Core 315-C0004C-12X [Sample C4C-12X]) and two samples are from underthrusted slope apron sediments (Cores 315-C0004C-41R and 49R [Samples C4C-41R and C4C-49R]), located below a fault (Fig. F1C).

Samples were formed into a cylindrical shape ~3.8 cm in diameter and ~5–6 cm in length.

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