Paper No. 4
Presentation Time: 8:45 AM

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


PLATT, John D., School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, BRANTUT, Nicolas, Department of Earth Sciences, UCL, London, RUDNICKI, John W., Departments of Civil/Environmental and Mechanical Engineering, Northwestern University and RICE, James R., School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, platt@fas.harvard.edu

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.