Strength Profile of the Inner Nankai Accretionary Prism at IODP Site C0002

Abstract

The strength of accretionary prisms is important for understanding their formation and the generation of earthquakes along the underlying plate boundary faults. We establish a continuous depth profile of rock strength and possible ranges of the minimum and maximum horizontal stresses (Shmin and SHmax) based on indentation tests on cuttings samples at the Integrated Ocean Drilling Program (IODP) Site C0002 in the Nankai accretionary prism. The rock strength (i.e., differential stress) increases with depth from 5.1 and 11.0 MPa at 975 meters below the seafloor (mbsf) to 22.8 and 59.6 MPa at 3005 mbsf in normal and reverse faulting regimes, respectively. The minimum value of Shmin and the maximum value of SHmax range from 33.2 and 49.4 MPa at 975 mbsf to 57.1 and 139.6 MPa at 3005 mbsf, respectively. Our estimated strength, in combination with Coulomb wedge model, may improve understanding of the evolution of accretionary prisms.

Mechanical Weakness of the Nankai Accretionary Prism: Insights From Vp Measurements of Drill Cuttings

“…When we combined the C 0 values determined in this study with C 0 depth profile data for Site C0002 reported by Kitamura and Hirose (2017) (Figure 5c), we found that, overall, the estimated C 0 values in Holes C0002N and C0002P tended to increase with depth, from ∼0.9–1.5 MPa at ∼1,000 mbsf to ∼11 MPa at ∼3,000 mbsf (see details in Kitamura et al., 2019). C 0 values in Holes C0002Q–S, estimated in this study, stayed in range between 3.1 and 18.2 MPa, at 2,822–3,262 mbsf (data with error bars in Figure 5c).…”

“…Although Kitamura et al. (2019) suggested that the strength of cuttings, as determined by indentation tests, showed little dependence on lithology above ∼3,000 mbsf in Holes C0002N and C0002P, we found that between 2,822 and 3,262 mbsf in Holes C0002Q–S, the C 0 values of sandy materials determined in this study tended to be higher than those of silty or clayey materials, except for two high C 0 values of siltstone from 2,822 and 2,862 mbsf in Hole C0002R.…”

“…We compared the V p ′‐ C 0 relationship for the cuttings at Site C0002 with V p ′‐ C 0 data obtained for shallow sediments from the Nankai accretionary prism and the Kumano forearc basin (Chang et al., 2010; Huffman et al., 2016; Kitamura et al., 2019), and other empirically obtained V p ′‐ C 0 relationships (e.g., Chang et al., 2006) (Figure 7). Overall, C 0 tended to increase with increasing V p ′.…”