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

摘要

To understand the seismogenic potential of shallow plate-boundary thrust faults (decollements) in relatively warm subduction zones, water-saturated Na-montmorillonite gouges were sheared at a pore fluid pressure of 10MPa, effective normal stresses (sigma(eff)(n)) of 10-70MPa, temperatures (T) of 25-150 degrees C, and axial displacement rates of 0.03-3m/s. The Na-montmorillonite gouges were frictionally very weak at all conditions tested (steady state friction coefficient (ss)=0.05-0.09). At T60 degrees C, Na-montmorillonite showed a transition from velocity-strengthening to velocity-weakening behavior with increasing sigma(eff)(n), whereas at T90 degrees C it was largely velocity neutral or velocity strengthening, irrespective of sigma(eff)(n). The rates of frictional healing () showed extremely low values (mostly <0.001) at all temperatures. Our results suggest that the existence of Na-montmorillonite in the decollement zone at Costa Rica and Nankai promotes aseismic slip, particularly at shallow depths, forming weakly coupled regions.