Depth dependence of the frictional behavior of montmorillonite fault gouge: Implications for seismicity along a décollement zone

Abstract

To understand the seismogenic potential of shallow plate-boundary thrust faults (décollements) in relatively warm subduction zones, water-saturated Na-montmorillonite gouges were sheared at a pore fluid pressure of 10 MPa, effective normal stresses (σneff) of 10–70 MPa, temperatures (T) of 25–150°C, and axial displacement rates of 0.03–3 μm/s. The Na-montmorillonite gouges were frictionally very weak at all conditions tested (steady state friction coefficient μss = 0.05–0.09). At T ≤ 60°C, Na-montmorillonite showed a transition from velocity-strengthening to velocity-weakening behavior with increasing σneff, whereas at T ≥ 90°C it was largely velocity neutral or velocity strengthening, irrespective of σneff. The rates of frictional healing (β) showed extremely low values (mostly <0.001) at all temperatures. Our results suggest that the existence of Na-montmorillonite in the décollement zone at Costa Rica and Nankai promotes aseismic slip, particularly at shallow depths, forming weakly coupled regions.