Fault structure, frictional properties and mixed-mode fault slip behavior

Recent high-resolution GPS and seismological data reveal that tectonic faults exhibit complex, multi-modeslipbehavior including earthquakes, creep events, slow and silent earthquakes, low-frequency events and earthquake afterslip. The physical processes responsible for this range of behavior and the mechanisms that dictate faultslip rate or rupture propagation velocity are poorly understood. One avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the Earth's surface with laboratory experiments on the frictionalproperties of the fault rocks. Here, we show that fault zone structure has an important influence on mixed-modefaultslipbehavior. Our field studies depict a complex fault zone structure where foliated horizons surround meter- to decameter-sized lenses of competent material. The foliated rocks are composed of weak mineral phases, possess low frictional strength, and exhibit inherently stable, velocity-strengthening frictionalbehavior. In contrast, the competent lenses are made of strong minerals, possess high frictional strength, and exhibit potentially unstable, velocity-weakening frictionalbehavior. Tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons. With continued deformation, fault creep will concentrate stress within and around the strong and potentially unstable competent lenses, which may lead to earthquake nucleation. Our studies provide field and mechanical constraints for complex, mixed-modefaultslipbehavior ranging from repeating earthquakes to transient slip, episodic slow-slip and creep events