Application of rate and state friction formalism and flash melting to thin permanent slip zones of major faults

Abstract

Strain within certain major exhumed fault zones concentrated within a ∼1 mm wide permanent slip zone. A surrounding 10 mm thick fault core is strongly damaged. Concentration of seismic strain rate within a very thin zone is expected both for rate and state friction and weakening of the slip zone by flash melting at real contacts. In both cases, the material at the rupture tip begins with a strength that is far above that for steady state sliding. The permanent slip zone is slightly weaker than its surroundings and begins to slip with a strain rate faster than the surrounding core. Damage from strain thus weakens the sliding zone more than the fault core further concentrating strain rate toward a minimum thickness that depends on the finite size of grains. Flash heating involves a weakening velocity that is the product of a material property, the weakening strain rate, and the slip zone thickness. There is a minimum strength associated with a sliding velocity ∼15 m s−1, where stresses at the crack tip just exceed the elastic limit. Higher sliding velocities imply large inelastic strains near the rupture tip that would add to macroscopic friction. Lower sliding velocities imply higher coefficients of friction, as flash weakening is velocity weakening. The width of the PSZ should organize to a finite value so its weakening velocity is that needed to give the sliding velocity for minimum strength.