Friction velocity dependence of clay-rich fault material along a megasplay fault in the Nankai subduction zone at intermediate to high velocities

The frictional properties of clay-rich fault material collected from a megasplay fault within the Nankai accretionary complex were examined in frictional experiments performed at a normal stress of 5 MPa under water-saturated conditions for slip velocities from 0.0026 to 260 mm/s with > 250 mm of displacement. Our results reveal that the fault material can show two stages of velocity weakening behavior: weakening at slow velocities (v < 2.6 mm/s), characterized by a small degree of friction velocity dependence (the absolute value of (a - b) is typically < 0.005), and a dramatic weakening at high velocities (v > 26 mm/s). Such a process of fault weakening may provide important constraints on models of faulting along a megasplay fault. At slip velocities from 0.026 to 2.6 mm/s, there exist both velocity-weakening and velocity-strengthening fault materials. The frictional coefficient values, mu, for slow slip velocities (v = 0.26 mm/s) are relatively low (mu = 0.28-0.35) for velocity-strengthening samples compared with velocity-weakening samples (mu = 0.38-0.49). Microstructural analyses reveal that velocity-strengthening samples show homogeneous deformation textures in which the entire gouge layer is deformed, whereas velocity-weakening materials show evidence of shear localization in which deformation is concentrated along narrow subsidiary shears. Citation: Tsutsumi, A., O. Fabbri, A. M. Karpoff, K. Ujiie, and A. Tsujimoto (2011), Friction velocity dependence of clay-rich fault material along a megasplay fault in the Nankai subduction zone at intermediate to high velocities, Geophys. Res. Lett., 38, L19301, doi: 10.1029/2011GL049314.