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

a r t i c l e i n f o Recent high-resolution GPS and seismological data reveal that tectonic faults exhibit complex, multi-mode slip behavior 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 fault slip 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 frictional properties of the fault rocks. Here, we show that fault zone structure has an important influence on mixed-mode fault slip behavior. 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 frictional behavior. In contrast, the competent lenses are made of strong minerals, possess high frictional strength, and exhibit potentially unstable, velocity-weakening frictional behavior. 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-mode fault slip behavior ranging from repeating earthquakes to transient slip, episodic slow-slip and creep events. A traditional interpretation of tectonic faults is that stress is relieved either as earthquakes resulting from sudden frictional insta-bilities or by continuous aseismic frictional sliding and fault creep (e.g., Scholz, 1998). In this view, crustal faults have a stable region near the surface (0–3 km), owing to the presence of loosely consolidated material (Marone and Scholz, 1988), and at depth (15–20 km), owing to the onset of viscous deformation of fault rocks with increasing temperature (Brace and Kohlstedt, 1980). Between these depths, within the seismogenic zone, fault slip is envisaged to occur primarily by earthquakes. Frictional processes and the parameters that dictate the stability of frictional sliding are therefore critical for the physics of earthquakes. Several lines of evidence suggest that most seismically active faults are statically strong structures with friction, μ, in the range 0.6–0.85 (e.g. Byerlee, 1978; Scholz, 2000). Geological evidence along seismic faults suggests: 1) that earthquake ruptures are localized within principal slip zones less than …