Fault Friction and the Upper Transition from Seismic to Aseismic Faulting

This paper discusses the processes and mechanical factors that define the seismogenic zone and the updip transition from seismic to aseismic faulting. We review friction laws for granular and clay-rich fault zones and discuss them in the context of the mechanics of stick-slip and the requirements for instability. Seismogenic faulting is driven by the unstable release of stored elastic energy, and thus explanations of the upper stability transition must be couched in terms of fault-zone rheology. We summarize several observations that define the seismogenic zone and its upper boundary, including microseismicity, earthquake afterslip, and the spatial distribution of seismic moment release during large earthquakes. We focus on two hypotheses for the updip limit of seismic faulting. The clay mineral hypothesis posits that a thermally driven transition from a smectite to an illite structure produces a transition from aseismic to seismic behavior. The fault gouge lithification hypothesis posits that the stability transition reflects a threshold consolidation, or lithification, state and a switch from pervasive to localized shear. Existing laboratory data, which cover a limited range of conditions, indicate that the smectite-illite transformation results in an increase in friction but not a transition from stable to unstable behavior. It appears that other depthand temperature-dependent processes may play an important role in altering the frictional properties of subduction faults and establishing the updip seismic limit.