Previous   |    Next

doi:10.2204/iodp.proc.314315316.217.2011

Results

Change in compressional wave velocity with effective pressure is shown in Figure F2. Velocities from Sites C0002 and C0004 ranged from ~1430 to ~1910 m/s and ~1700 to ~2010 m/s, respectively, at low effective pressures as high as 500 kPa (Fig. F2). Velocity increases with effective pressure. Although Sample C2B-32R shows higher velocity than Sample C2B-51R at low effective pressure, the slope in wave speed versus effective pressure is relatively flatter. The slopes for Samples C2B-51R and C2B-61R are almost parallel. For Site C0004, the relationships between wave speed and effective pressure for forearc basin sediments and underthrust sediments have similar slope angles (Fig. F2).

Porosity was corrected to remove smectite effects from onboard porosity measurements using Brown and Ransom (1996)’s method; smectite content was estimated by X-ray diffraction analysis (Guo et al., 2009). Representative velocities were taken under hydrostatic conditions assuming the in situ setting is consistent with hydrostatic pore pressure. Porosity, compressional and shear wave velocities, and related properties are shown in Table T1.

Corrected porosity is ~37.4%–27.0% for Site C0002 and ~46.8%–38.9% for Site C0004. Compressional wave velocity is ~2010–2370 m/s for Site C0002 and ~1790–2200 m/s for Site C0004 under hydrostatic pore pressure conditions. The relationship between porosity and compressional wave velocity is shown in Figure F3A. Porosity and compressional wave velocity for Site C0001 (Hashimoto et al., 2010) are also included in Figure F3A for comparison. Open symbols represent cover sediments (slope-apron sediment or forearc basin sediments) for all sites; solid symbols represent sediments below the cover sediments, such as accreted sediments for Sites C0001 and C0002 and underthrust sediments for Site C0004. At Site C0001, the relationship between porosity and compressional wave velocity for slope-apron sediments has a lower slope angle than that for other sediments, although the data for Sites C0002 and C0004 are more limited.

Shear wave velocity is ~720–950 m/s for Site C0002 and ~650–940 m/s for Site C0004 under hydrostatic pore pressure conditions. The relationship between porosity and shear wave velocity is presented in Figure F3B. Because the number of samples for shear wave velocity measurements is very limited, it is difficult to compare velocity-porosity relationships between forearc basin sediments and accreted sediments at Site C0002 or between slope-apron sediments and underthrust sediments at Site C0004. Porosity at Site C0004 is higher than that at Site C0002 in samples with almost the same shear wave velocities.

V_P/V_S is ~2.5–2.8 for Site C0002 and ~2.3–2.7 for Site C0004, with a mean of ~2.6 (Fig. F3C; Table T1). V_P/V_S for forearc basin sediments (Site C0002) and slope-apron sediments (Site C0004) are slightly higher than those for other sediments.

From compressional and shear wave velocities, shear modulus (µ) and bulk modulus (κ) at specific effective pressures and porosities can be obtained, assuming that samples are isotropic and linearly elastic bodies, using the following formulas:

where ρ_bw is wet bulk density from moisture and density data.

The computed shear modulus is ~1.0–1.88 GPa for Site C0002 and ~0.79–1.76 GPa for Site C0004. The bulk modulus is ~6.60–9.24 GPa for Site C0002 and ~4.22–7.17 GPa for Site C0004 (Table T1). Both are under hydrostatic fluid pressure conditions.

Top of page   |    Previous   |    Next