On Relations Between Fault Zone Heterogeneity, Strength, and Fault Slip Style

Abstract:

Models for fault strength and slip behavior tend to simplify depth-dependent variation in composition, strain-rate, fluid pressure, and normal and differential stress. Geological descriptions of faults, on the other hand, highlight heterogeneities. Field observations document a spectrum of internal geometry from localised displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness. These observations indicate a large range of possible strain rates in natural faults, which may correspond to geophysically observed fault slip speeds that range from steady plate boundary creep through to earthquake slip.

We review geological observations and analyse numerical models of two-phase shear zones to discuss the effects of fault zone heterogeneity on bulk fault strength and active fault slip style. We assume there must be distinct conditions that produce earthquakes, creep, and slip at intermediate velocities. We also note that because intermediate slip styles occur over large ranges in depth and temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions to remain far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity.