Multimechanism friction constitutive model for ultrafine quartz gouge at hypocentral conditions

Abstract

Occurrence of instability in crustal faults depends in part on the small‐magnitude dependence of frictional strength on slip rate and slip history. Rate dependence of friction reflects the operation of thermally activated mechanisms at points of contact along fault surfaces and is expected to change in space and time owing to variations in environmental conditions and slip rates during the seismic cycle. Several lines of evidence suggest solution‐precipitation processes in fault zones may be activated during interseismic periods when slip rates are small and may contribute to fault healing. We develop a constitutive model for faulting at hypocentral conditions that is capable of describing the variation in frictional properties as different slip mechanisms are activated in response to changes in temperature or slip rate. This model is based on the assumption that slip mechanisms are thermally activated and follow an Arrhenius relationship between temperature and slip rate, which allows the addition of temperature dependence to existing rate‐ and state‐dependent friction constitutive laws. Multiple slip mechanisms are treated as operating independently and concurrently, where each mechanism is described by the rate‐, state‐, and temperature‐dependent friction constitutive relation. The constitutive model is used to analyze triaxial friction experiments on ultrafine‐grained quartz gouge at temperatures to 600°C, effective confining pressure of 150 MPa, and water‐saturated or room‐dry conditions. These experiments investigated the stress relaxation response and slip history effects during slide‐hold‐slide tests with hold times up to 105s. The microstructure of the deformed quartz gouge and the transient friction behavior define at least two distinct frictional slip regimes: a low‐temperature regime characterized by cataclastic mechanisms with significant slip history effects, and a high‐temperature regime characterized by solution‐precipitation‐aided cataclastic flow with large‐magnitude rate dependence and insignificant slip history effects. In the model the parameters of the friction constitutive relation (e.g., a, b, and L) are treated as constants for each slip mechanism but are different for the different mechanisms. This model accurately describes the frictional behavior within each regime and across the transition between regimes. The analysis suggests that the greatest‐magnitude rate weakening behavior occurs at 100° to 300°C under wet conditions at laboratory slip rates. Significant solution‐precipitation is activated at temperatures above 300°C at laboratory slip rates or at lower slip‐rates and lower temperatures. The high‐temperature solution‐precipitation regime is described by a large‐magnitude rate strengthening (a − b = 0.03) and an apparent activation energy of approximately 44 kJ mol−1. The constitutive analysis suggests that the solution‐precipitation‐aided flow mechanism could be important during interseismic periods at hypocentral conditions and low shear stress but apparently is not characterized by significant slip history effects.