HR: 11:50h
AN: T42C-07    [Abstracts]
TI: Hydro-Mechanical Processes in Faulting: an Experimental Analysis of the Variability of Dilatancy in Simulated Fault Gouge
AU: * Samuelson, J
EM: jes447@psu.edu
AF: Penn State Department of Geosciences, Deike Building, University Park, PA 16802,
AU: Marone, C
EM: cjm38@psu.edu
AF: Penn State Department of Geosciences, Deike Building, University Park, PA 16802,
AU: Elsworth, D
EM: elsworth@psu.edu
AF: Penn State Department of Energy and Geo-Environmental Engineering, Hosler Building, University Park, PA 16802,
AB: Frictional hardening associated with the dilation of fault zones and subsequent depressurization of pore fluids levies an important control on the nucleation of earthquake slip in natural fault zones. The present work seeks to describe the dependency of dilatancy, described by Segall & Rice (1995), on key fault parameters. Velocity stepping tests were conducted in a double-direct shear configuration under true-triaxial loading conditions. Samples were jacketed and subjected to constant pore fluid pressure. Confining and pore pressures were maintained via high precision servo-controlled pressure intensifiers. Experiments described herein were run with constant pore pressure. Frictional contact area was 5 x 5 cm. Sliding velocity was systematically varied from a background loading rate of 1μm/s to speeds as high as 100 φm/s and concurrent dilation of the granular layer was measured to determine the dilatancy coefficient (ε = Δφ/Δln(v), where φ is porosity and v is shear velocity). The amount of dilation as a result of a change in shearing velocity was measured using both the physical dilation of the layer as measured by DCDT on the biaxial deformation apparatus, as well as by measuring the amount of fluid influx as delivered by the pore pressure intensifier. We report on the dependence of dilatancy on effective stress in the range 0.8 to 30 MPa, using pore pressures from 0.4 – 4 MPa. Each experiment, other than at 0.8 MPa, involved a high normal stress portion followed by a lower normal stress (e.g. 30 then 20 MPa, 15-10 MPa). Gouge layers were constructed using a precision leveling jig to an initial thickness of 4 mm prior to shear. Over this range of effective stresses ε increased from 5.8 x 10^-5 at 0.8 MPa effective stress to 5.0 x 10^-4 at 30 MPa. The effect of accumulated strain and gouge fabric development on ε was measured by repeated velocity steps between 1 μm/s and 10 μm/s over a displacement of ~18 mm at an effective stress of 15 MPa. Results indicate that the magnitude of dilation following a 10 fold increase in sliding velocity is insensitive to strain history. We investigated the hypothesis that ε is inversely proportional to gouge porosity (hence directly proportional to effective normal stress) by subjecting a sample to 40 MPa normal stress, so as to overcompact it, prior to shearing at 15 MPa normal stress. Results showed that the amount of dilation decreased as a function of increasing displacement after the drop in normal stress. After a slip of ~16 mm the measured dilation was indiscernible from the conventionally loaded sample. These results indicate that porosity of the gouge layer plays an important role in the magnitude of dilation and slip rate dependence of dilation. Our results show that the amount of dilation resulting from an upstep in shear velocity increases with increasing effective normal stress. Porosity of the gouge layer appears to be an important control on dilatancy as shown by increased dilation in samples that have been loaded in excess of the experimental effective stress. The insensitivity of dilation to increasing strain reveals that strain history has little influence on dilation. Further experiments will be conducted to examine the control that layer thickness imposes on dilation, the null hypothesis being that in cases of distributed shear, dilation scales with layer thickness, and in cases of localized shear the magnitude of dilation would be controlled by the shear band and so would not scale with layer thickness.
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting