STRAIN LOCALIZATION IN FLUID-SATURATED GOUGE DURING SEISMIC SHEAR AND IMPLICATIONS FOR FIELD OBSERVATIONS, DYNAMIC WEAKENING AND THE ONSET OF MELTING

We have studied how micron-scale strain localization during seismic shear can be explained using a model for a gouge layer undergoing thermal pressurization and thermal decomposition sheared between two undeforming half-spaces. A linear stability analysis combined with numerical simulations leads to a formula for the localized zone thickness as a function of the gouge properties. Using parameters modeling a typical centroidal depth for a crustal seismogenic zone we predict thicknesses comparable to laboratory (Brantut et al., 2008; Kitajima et al., 2010) and field (Chester and Chester, 1998; De Paola et al., 2008) observations. As straining localizes the frictional heating is focused into a narrower zone, leading to a larger temperature rise than expected for a uniformly sheared gouge layer and a rapid acceleration in dynamic weakening.

Laboratory observations show a thickening of the highly localized material with slip and a distinct banded structure within the highly localized material (T. Mitchell, priv. comm.; Kitajima et al., 2010), suggesting that the deforming zone migrates during shear. We show how thermal pressurization and thermal decomposition lead to this migration for a gouge layer with uniform properties. In addition we show that straining migrates towards pre-existing regions within the gouge layer that generate or trap pore pressures more efficiently. Thus, the distribution of shear strain throughout the gouge layer may be largely controlled by pre-existing structures within the gouge layer.

The migration outlined above has three important consequences: (1) Migration must be taken into account when inferring the width of the deforming zone from field observations. Even when the zone of localized straining is only a few tens of microns wide, migration can lead to a final strain profile with a zone of roughly uniform strain on the order of a millimeter wide. (2) The properties of pre-existing structures that are most susceptible to localization control the initial dynamic weakening. (3) Migration of the localized zone distributes heating over a broader region, leading to a much lower temperature rise when compared with a stationary shear zone. Our results rarely show temperatures above the melting temperature, providing a plausible explanation for the scarcity of melt observed on mature faults.