Data report: frictional strength of mudstone samples from the Nankai Trough frontal thrust region, IODP Sites C0006 and C0007

We conducted shearing tests on samples from the frontal thrust region of the Nankai Trough offshore Kii Peninsula, Japan, recovered from Sites C0006 and C0007 during Integrated Ocean Drilling Program (IODP) Expedition 316. The samples are all silty clay recovered from 192–561 meters below seafloor and corresponding to Units II and III. Samples were sheared in a double-direct shear configuration in a true-triaxial pressure vessel that was confined and under fluid pressure control. We observed that residual shear strength values increase sublinearly over effective normal stresses from 5 to 35 MPa. Residual friction coefficients calculated from the measured shear strength and effective normal stress range from 0.22 to 0.56 and decrease as a function of increasing effective normal stress. For each sample, the data for coefficient of friction as a function of increasing effective normal stress are equally well fit by power law and logarithmic relations. Sample 316C0006F-19R-1 is noticeably weaker than the other samples, which can be attributed to its high clay mineral content of 67 wt%.