Data report: frictional healing and compressibility of sheared sediment from fault zones, Sites C0004 and C0007

Abstract

We conducted shearing tests on samples spanning key fault zones recovered from Integrated Ocean Drilling Program (IODP) Sites C0004 and C0007 from Expedition 316 to the Nankai Trough offshore Japan. Results of slide-hold-slide and normal stress stepping procedures allow quantification of frictional healing, time-dependent compaction, and compressibility of sheared gouge layers. Samples were sheared as thin layers in the double-direct shear configuration at 25°C in a true-triaxial pressure vessel at constant effective normal stress of 25 MPa and pore pressure of 5 MPa using a water-based brine saturated with 3.5 wt% NaCl. We monitored the poromechanical drainage state during shear by holding the pore pressure at the upstream end of the sample at 5 MPa while monitoring the downstream pore pressure. Friction constitutive behavior was determined using slide-hold-slide tests in which hold times ranged from 1 to 1000 s. Frictional healing rates range from ~0.003 to 0.008 per decade, with slightly higher rates measured for samples from Site C0004 than from Site C0007. Both sites exhibit similar compaction rates of ~0.002–0.007 per decade during holds. Both frictional healing and compaction correlate very well with the logarithm of hold time, with R 2 > 0.95. For Site C0004 samples, compressibility was found to be ~1–4 GPa –1 . At Site C0004, compaction rates and compressibility are elevated in the vicinity of the fault zone. At Site C0007, healing rates decrease, and compaction rates increase, with increasing depth.