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doi:10.2204/iodp.proc.343343T.203.2015

Results

Powdered samples

Values of a generally range between 0.003 to 0.009 for all samples except the fault zone sample from Section 343-C0019E-17R-1, the three pelagic clay samples from Section 20R-2, and the sample from Section 18R-1 from Unit 5 mudstones located ~2 m below the fault zone. These samples exhibited clearly lower values of a < 0.004 (Fig. F4; Table T2). Large scatter between –0.003 and 0.011 is observed for b₁ and b₂, with generally lower values occurring in the fault zone and pelagic clays. Instances of negative b are observed mostly in the white pelagic clay. For the critical slip distances, D_c1 is typically less than ~15 µm and generally smaller than D_c2, which mostly clusters between ~40 and 200 µm. Larger values of D_c1 ranging from ~20 µm up to several hundred micrometers are observed again for the fault zone, pelagic clay, and Section 18R-1 sample; however, in these cases the data were fit with one state variable, suggesting that the longer D_c is a typical value. Mostly velocity-weakening friction is observed throughout the borehole, with a-b approximately ranging between –0.01 and 0.005. Within the scatter, a-b appears to slightly increase from ~650 to 720 mbsf. The fault zone, pelagic clays, and 18R-1 sample tend to exhibit velocity-strengthening behavior (a-b up to ~0.004), but a few occurrences of velocity weakening are still observed in these samples. Initial friction values µ₀ are consistently between 0.43 and 0.58 and appear to slightly decrease from ~650 to 720 mbsf before becoming relatively constant downhole. Notably weak values of 0.18–0.29 are observed for the fault zone, Sample18R-1, and pelagic clay (Fig. F4, Table T3). The friction slip dependence parameter η shows a wide scatter between –0.025 and 0.01 mm^–1, but mostly negative values indicating slip-weakening friction. Again, exceptions include the fault zone, Sample 18R-1, and pelagic clay for which most of the slip-hardening is observed.

Intact samples

Due to the smaller number of samples, downhole trends are more difficult to identify for intact samples, but they exhibit many of the same characteristics as the powdered samples. No intact samples of the pelagic clay or Section 343-C0019E-18R-1 were tested, but one sample of the fault zone was tested in an intact state. For the intact wall rock mudstones (Units 3 and 5), a ranges from 0.003–0.007 but is <0.0024 in the fault zone (Fig. F5; Table T4). Both b₁ and b₂ range from 0.001 to 0.007; no negative values of b are observed. Values of b are clearly lower for the fault zone, ranging from 0.0004 to 0.0014. For mudstone samples, D_c1 is usually less than ~15 µm, and values <10 µm are common, whereas D_c2 values range from ~40 to 200 µm. Similar to observations from the powdered samples, the intact fault zone sample was well fit with one state variable; therefore, its D_c values of 35–90 µm match closely with the D_c2 values of the mudstone wall rock. Velocity weakening is common for intact samples, with a-b as low as –0.008. The fault zone sample is velocity neutral to velocity strengthening (a-b = 0–0.002); however, some instances of velocity strengthening are also observed in the wall rock. The range of initial friction values for the mudstones is ~0.40–0.55, slightly lower than the range for powdered samples (Fig. F5; Table T5). Initial friction values for the fault zone cluster tightly at µ₀ = 0.21. Finally, the friction slip dependence ranges from –0.02 to 0.02 mm^–1 and appears to increase with depth such that the fault zone and the mudstone below it (Section 343-C0019E-20R-1) exhibit only slip-hardening behavior, whereas the shallow samples exhibit slip weakening.

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