The role of viscous deformation mechanisms in controlling fault weakening during the propagation of earthquakes.

Abstract

Faults become transiently weak during the propagation of earthquakes. Fast, efficient lubrication of the sliding portion of seismic faults promotes slip acceleration, rupture propagation and radiation of potentially hazardous waves. Indeed, laboratory experiments show that fault materials dramatically weaken when sheared at seismic velocities (> 0.1 m s-1). Several thermally-activated mechanisms, triggered by shear heating, have been proposed to explain the coseismic weakening of faults. However, a unifying law describing the thermal weakening observed for different rock types is still lacking.
We explored this problematic by performing friction experiments in a rotary shear apparatus on powders of a range of rock-forming minerals (calcite, dolomite, anhydrite, sodium chloride and olivine), at seismic velocities up to v = 1.4 m s–1 and normal stresses uptoσn =25MPa.
When the experimental fault is weak, sheared gouges consistently develop a well- defined, porosity-free principal slip zone (PSZ) with finite thickness (a few tens of micrometres). PSZ is composed of fine-grained (< 5 μm) crystalline aggregates, which display textures typical of sub-solidus viscous flow. These are similar to those found in natural ultramylonites of ductile, aseismic shear zones active in the lower crust.
Calcite experiments were explored in-detail through the monitoring of acoustic emissions and with further microstructural analysis, which integrated SEM EBSD technique and TEM imaging. We show that, at dynamic conditions, thermally-activated viscous mechanisms (e.g. grainsize sensitive and insensitive creep) compete with brittle processes in controlling fault strength.
We also show that local temperatures attained during seismic slip control the strength of simulated faults according to an Arrhenius-type law. Our results demonstrate that coseismic lubrication of faults is operated by viscous processes, which can be described by are general, material-dependent Arrhenius-type constitutive equation. Therefore, our essay offers a unifying approach to the quantification of the dynamic strength of faults and to numerical modelling of earthquakes.