THE ROLE OF FAULT ZONE STRUCTURE AND MATURITY IN DETERMINING THE MODE OF FAILURE: PERSPECTIVES ON THE SPECTRUM OF FAULT SLIP BEHAVIORS (Invited Presentation)

Laboratory and field observations show that faults evolve with accumulated shear displacement. Geologically young, immature faults are structurally more complex and tend to fail in smaller earthquakes compared to mature fault zones with wide zones of wear material (fault gouge). Seismic and geologic data document these differences between mature and immature faults in various ways, including the depth frequency distribution of seismicity, fault roughness, and via the relation between fault zone structural complexity and earthquake frequency magnitude scaling. Laboratory friction data for fault gouge document related transitions as a function of shear displacement; albeit with some important differences. In lab experiments, gouge layers of finite width are commonly sheared between surfaces that do not evolve in roughness with displacement, and shear is initially stable. Unstable frictional shear begins when slip becomes localized on shear bands within the gouge. Experiments on initially bare rock surfaces, where gouge accumulates and roughness evolves slightly, corroborate the results from gouge experiments, often showing a frictional stability transition as a function of gouge accumulation. The lab data are broadly consistent with frictional stability theory, showing that the transition from stable to unstable frictional shear occurs as the rheologic stiffness, kc = (b-a)/Dc, drops below the loading stiffness k; where (b-a) is the friction slip rate parameter and Dc is the critical slip distance. Recent works show that complex frictional behavior occurs in the region around k/kc = 1.0. These experiments document in detail the transition from stable to unstable slip and in a few cases they include the full spectrum of fault slip behaviors ranging from aseismic slip to slow earthquakes and quasi-dynamic instability to dynamic stick-slip sliding. These data provide an important test of friction stability theory, and show that subtle variations in fault rheology can produce dramatic changes in the mode of faulting, such that a single fault zone could host multiple modes of frictional failure. I review results of these works with particular focus on how lab observations can help in learning to read the slip behavior of tectonic faults from geologic and geophysical data.