Fault rock structure-related stiffness contrast explains earthquakes in creeping faults

Abstract

Fault rocks exhibit structures resulting from different styles of shear deformation (either distributed or localized) during fault displacement. However, how the fault rock structures affect fault slip behavior remains poorly understood. We conducted shear experiments on thick and thin gouge layers of various mineral compositions, which simulate the predominant development of velocity-strengthening distributed deformation zones and velocity-weakening shear localized zones, respectively. Here, we show that deformation zones with contrasting structures and frictional properties can be developed together in a single fault. If the rheological stiffness of shear localization zones in the fault exceeds the elastic stiffness of neighboring wall rocks, stick-slip can intermittently occur along the localization zones while stable slip persists in the distributed deformation zones. These findings suggest that contrasting rheological stiffnesses resulting from fault rock structures may induce the simultaneous operation of multiple slip modes in a fault and explain the occurrence of earthquakes in creeping faults.