Is complex fault zone behaviour a reflection of rheological heterogeneity?

Area of Science:

• Tectonophysics
• Seismology
• Geological Sciences

Background:

• Natural faults display a wide spectrum of slip speeds, ranging from steady plate boundary creep to rapid earthquake slip.
• Geological fault descriptions vary from localized displacement on discrete planes to distributed shearing within zones of finite thickness, implying diverse strain rates.

Purpose of the Study:

• To review geological observations and analyze numerical models of two-phase shear zones.
• To discuss the influence of fault zone heterogeneity on active fault slip style.
• To identify critical conditions governing earthquakes, creep, and intermediate slip velocities.

Main Methods:

• Review of geological observations of fault behavior.
• Analysis of numerical models simulating two-phase shear zones.
• Investigation of fault zone heterogeneity and its impact on slip style.

Main Results:

• Intermediate slip speeds occur across broad temperature ranges, suggesting control by fault properties and dynamic variables.
• The ratio of bulk driving stress to frictional yield strength and viscosity contrasts are identified as critical factors.
• Local frictional failure, though possible via stress amplification or fluid pressure, is spatially limited by shear zone heterogeneity.

Conclusions:

• Fault slip style is governed by a complex interplay of stress, frictional properties, and internal heterogeneity.
• Conditions for earthquake nucleation (frictional yield) differ significantly from those for steady viscous flow.
• Intermediate slip speeds likely result from localized frictional failures constrained by heterogeneous stress fields.