Frictional and hydrologic properties of a major splay fault system, Nankai subduction zone

We report on laboratory experiments examining the frictional and hydrologic properties of fault gouge and wall rock along a borehole transect that crosses a major out‐of‐sequence thrust splay fault within the Nankai accretionary complex. At 25 MPa effective normal stress, the fault zone material is frictionally weak ( μ ≤ 0.44) and exhibits low permeability after shearing ( k < 5.5 × 10 −20 m 2 ). Fault zone samples are weaker and less permeable than the surrounding wall rocks, consistent with their slightly higher clay abundance. All samples exhibit velocity‐strengthening frictional behavior over most of the experimental conditions we explored, consistent with aseismic slip at shallow depths. The fault and wall rock both exhibit prominent minima in the friction rate parameter ( a − b ) for sliding velocities of 1–10 μ m/s (∼0.1–1.0 m/day), comparable to rates for slow slip events. This suggests that the frictional properties of fault zone material play a central role in governing the mode of slip on subduction megathrusts.