Laboratory measurement of shear induced fault zone dilatancy, and numerical estimation of its influence on friction constitutive parameters in quasi-undrained scenarios

Chapter 1. Pore fluid pressure plays an important role in the frictional strength and stability of tectonic faults. We report on laboratory measurements of porosity changes associated with transient increases in shear velocity during frictional sliding within simulated fine-grained quartz fault gouge (d50=127 microm). We use our measurements to model transient pore fluid depressurization in response to dilation resulting from step changes in shearing velocity. Dilatant hardening requires undrained response with the transition from drained to undrained loading indexed by the ratio of the rate of porosity change to the rate of drained fluid loss. Undrained loading is favored for high slip rates on low-permeability thick faults with low critical slip distances. Although experimental conditions indicate negligible depressurization due to relatively high system permeability, model results indicate that under feasible, but end-member conditions, shear-induced dilation of fault zones could reduce pore pressures, or correspondingly increase effective normal stresses, by several 10's of MPa. Our results show that transient increases in shearing rate cause fault zone dilation. Such dilation would tend to arrest nucleation of unstable slip. Pore fluid depressurization would exacerbate this effect and could be a significant factor in generation of slow earthquakes, non-volcanic tremors, and related phenomena.;Chapter 2. We use numerical simulations to investigate how fault zone dilatancy and pore fluid decompression influence shear strength behavior in the context of rate and state friction constitutive laws. Dilatant hardening can change the friction response and the effective critical stiffness, Kcr, which delineates the transition from stable to unstable sliding in an ultimately velocity weakening fault zone. We study the frictional shear strength response to velocity step tests and show that in cases where the duration of pore fluid decompression is long compared to the time necessary for friction to evolve (as dictated by the rate-and-state-dependant friction law) both the critical slip distance (DC) and the direct effect (A) are effectively increased. We vary the permeability of the fault zone (k), the dilatancy coefficient (epsilon), and the magnitude of the shearing velocity of the fault zone ( vlp), and also compare results using both the Aging and Slip laws for the evolution of the state variable. We show that over the range from k=10-14 m2 to 10-21 m2 DC is effectively increased from 25 microm to ∼1 cm, and A is increased from 0.15 MPa to over 4 MPa. We also vary epsilon from 10-5--10 -3, and the size of the velocity step from 3 to 1000x and find large increases in the effective values of DC and A, which may lead to inhibition of unstable, stick-slip sliding.;Chapter 3. We describe laboratory experiments on dilatancy and friction constitutive properties of granular fault zones. We focus in particular on the dilatancy coefficient epsilon defined in the context of rate/state friction theory by the change in porosity Delta&phis; resulting from a perturbation in shearing velocity Deltav: epsilon=Delta&phis;/Deltaln( v). We find a strong positive correlation between initial grain-size and epsilon at sigma = 5 MPa but the correlation disappears at higher normal stress, consistent with grain comminution. For Westerly Granite epsilon varies from 2.0x10-4 at 5 MPa to 1.2x10-4 at 30 MPa, while for illite shale and the ODP sample, epsilon is ∼1.4x10-4 under the same conditions. The SAFOD shale showed the highest value for epsilon at all normal stresses, varying from 5.5x10 -4 at 5 MPa to 2.9x10-4 at 30 MPa. Our experiments show that dilation correlates strongly with mean grain-size when sigma is 10 MPa or less, suggesting that immature fault gouge may exhibit greater dilation than mature fault gouge that has undergone significant comminution. Our experiments also show that mineral composition of the gouge may play an important role in fault zone dilatancy, in particular the SAFOD shales exhibit strong dilatancy which may be a factor in explaining the stable sliding typical of the creeping section of the San Andreas Fault. (Abstract shortened by UMI.)...