Quantitative characterization of permeability reduction associated with compactive cataclastic flow

Abstract

Pore pressure excess has been frequently invoked in understanding seismogenic processes. Many working hypotheses for generating high pore pressure are proposed. However, one important ingredient still missing in these models is quantitative knowledge of the rate of change of permeability and porosity of rocks under various conditions of stress, temperature and pore fluid. We formulated a cumulative damage model to characterize the effect of stress on permeability of granular materials undergone compactive cataclastic flow. Laboratory measurements demonstrate that in the cataclastic flow regime, application of nonhydrostatic stress induces significant porosity reduction if the applied differential stress exceeds a threshold. This critical stress state is referred to as the onset of shear-enhanced compaction C*. Before C*, permeability and porosity reduction are controlled mainly by the effective mean stress. At C*, the deviatoric stress exerts primary control over permeability and porosity evolution. The increase in deviatoric stress results in drastic permeability and porosity reduction and considerable permeability anisotropy. Beyond C*, permeability and porosity reduction becomes gradual again. Microstructural observations reveal that stress-induced microcracking and pore collapse are the primary forms of damage during compactive cataclastic flow. A cumulative damage model was proposed based on these observations. By introducing stress sensitivity coefficients, this model provides a quantitative measure of the stress-induced permeability evolution. The first-order relation between permeability and stress formulated here can be easily incorporated in earthquake models. Coupled with recent advances in laboratory characterization of transport properties within fault zones, our model should provide necessary constraints to earthquake slip weakening mechanisms.