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