LABORATORY MEASUREMENT OF SHEAR INDUCED FAULT ZONE DILATANCY, AND NUMERICAL ESTIMATION OF ITS INFLUENCE ON FRICTION CONSTITUTIVE PARAMETERS IN QUASI-UNDRAINED SCENARIOS

Open Access
Author:
Samuelson, Jon Erik
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 29, 2009
Committee Members:
  • Chris Marone, Dissertation Advisor
  • Chris Marone, Committee Chair
  • Derek Elsworth, Committee Member
  • Demian Saffer, Committee Member
  • Kamini Singha, Committee Member
Keywords:
  • friction
  • earthquake
  • dilatancy
  • fault zone
  • pore fluid pressure
Abstract:
<b>Chapter 1:</b> 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 (d<sub>50</sub>=127 um). Experiments were conducted in a novel true-triaxial pressure vessel using the double-direct shear geometry. Shearing velocity step tests were used to measure a dilatancy coefficient (<i>&#949; = &#916;&#966;/&#916;</i>ln(<i>v</i>) , where &#966; is porosity and <i>v</i> is shear velocity) under a range of conditions: background shearing rate of 1 µm/s with steps to 3, 10, 30, and 100 µm/s at effective normal stresses from 0.8 to 20 MPa. We find that the dilatancy coefficient ranges from 4.7x10<sup>-5</sup> to 3.0x10<sup>-4</sup> and that it does not vary with effective normal stress. 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.<p> <b>Chapter 2:</b> 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, <i>K<sub>cr</sub></i>, 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 (<i>D<sub>C</sub></i> ) and the direct effect (<i>A</i>) are effectively increased. We vary the permeability of the fault zone (<i>k</i>), the dilatancy coefficient (<i>&#949;</i>), and the magnitude of the shearing velocity of the fault zone (<i>v<sub>lp</sub></i> ), 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 <i>k</i>=10<sup>-14</sup> m<sup>2</sup> to 10<sup>-21</sup> m<sup>2</sup> <i>D<sub>C</sub></i> is effectively increased from 25 um to ~ 1 cm , and <i>A</i> is increased from 0.15 MPa to over 4 MPa. We also vary <i>&#966;</i> from 10<sup>-5</sup> – 10<sup>-3</sup>, and the size of the velocity step from 3 to 1000x and find large increases in the effective values of <i>D<sub>C</sub></i> and <i>A</i>, which may lead to inhibition of unstable, stick-slip sliding.<p> <b>Chapter 3:</b> We describe laboratory experiments on dilatancy and friction constitutive properties of granular fault zones. We focus in particular on the dilatancy coefficient <i>&#949;</i> defined in the context of rate/state friction theory by the change in porosity <i>&#916;&#966;</i> resulting from a perturbation in shearing velocity <i>&#916;</i><i>v</i> : <i>&#949;</i>=<i>&#916;&#966;/&#916;</i>ln(<i>v</i>) . Velocity stepping experiments were conducted in the double-direct shear geometry on a range of materials including quartz sand, illite shale, Westerly Granite, ocean drilling core (ODP 1173-71x), and samples from the San Andreas Fault (SAFOD Phase III). Tests were conducted dry, at room temperature and humidity, at normal stresses <i>&#963;</i>,&#61472;from 5 to 30 MPa and slip velocities of 3 to 300 µm/s. Experiments on quartz sand explored the influence of particle size on <i>&#949;</i>. We find a strong positive correlation between initial grain-size and <i>&#949;</i> at <i>&#963;</i>&#61472;= 5 MPa but the correlation disappears at higher normal stress, consistent with grain comminution. For Westerly Granite <i>&#949;</i> varies from 2.0x10<sup>-4</sup> at 5 MPa to 1.2x10<sup>-4</sup> at 30 MPa, while for illite shale and the ODP sample, <i>&#949;</i> is ~ 1.4x10<i>-4</i> under the same conditions. The SAFOD shale showed the highest value for <i>&#949;</i> at all normal stresses, varying from 5.5x10<sup>-4</sup> at 5 MPa to 2.9x10<sup>-4</sup> at 30 MPa. Our experiments show that dilation correlates strongly with mean grain-size when <i>&#963;</i> 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.