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

Abstract

[1] 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, μ, for slow slip velocities (v = 0.26 mm/s) are relatively low (μ = 0.28–0.35) for velocity-strengthening samples compared with velocity-weakening samples (μ = 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.