Shear‐induced dilatancy of fluid‐saturated faults: Experiment and theory

Abstract

[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 finegrained quartz fault gouge (d 50 = 127 mm). 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 (e = Df/Dln(v), where f is porosity and v is shear velocity) under a range of conditions: background shearing rate of 1 mm/s with steps to 3, 10, 30, and 100 mm/s at effective normal stresses from 0.8 to 20 MPa. We find that the dilatancy coefficient ranges from 4.7 Â 10 À5 to 3.0 Â 10 À4 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 tens of megapascals. 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, nonvolcanic tremors, and related phenomena.

The Mechanical Coupling of Fluid-Filled Granular Material Under Shear

“…Similar formulations also appear in Iverson [1993], Rudnicki and Chen [1988], Miller and Nur [2000], Snieder and van der Beukel [2004] and Samuelson et al [2009].…”

“…To study these differences we use again the value of the Deborah number. The definition of De in equation (32) may also be viewed as a velocity ratio between the velocity of deformation, u 0 , and the velocity of diffusion D/l [Samuelson et al, 2009]. When De ≪ 1, the deformation is slow enough to allow for a pore pressure front originating at any depth in the layer to reach the drained boundaries in the time scale of deformation.…”

“…The decreasing pore pressure causes the effective normal stress and shear resistance to rise. Such 'dilatancy hardening' may have important implications for nucleation of earthquakes along fault gouge [Scholz et al, 1973, Scholz, 1978, Lockner and Byerlee, 1994, Samuelson et al, 2009, Scholz, 2002, possibly retarding the onset of an earthquake instability. However, when the accumulating tectonic load will eventually reach the system shear resistivity, then the slip may potentially be more rapid.…”

“…When the fault is sealed, porosity increase (i.e. dilation) during shear is often suggested to prohibit unstable sliding via pore pressure reduction leading to strain hardening following equation (2) [Scholz et al, 1973, Scholz, 1978, Rudnicki and Chen, 1988, Segall and Rice, 1995, Moore and Iverson, 2002, Scholz, 2002, Samuelson et al, 2009, while compaction of under-compacted gouge was shown experimentally to lead to extreme weakening and unstable sliding [Blanpied et al, 1992].…”

Shear strength of sediments approaching subduction in the Nankai Trough, Japan as constraints on forearc mechanics

“…For these experiments, three samples were trimmed from intact whole round cores, the rest were remolded wet (cold pressed into the sample assembly) (Table S1). 3.5 wt% NaCl solution was also used as pore fluid for these experiments [see Ikari et al, 2009b;Samuelson et al, 2009 for further details].…”