Quantitative measure of the variation in fault rheology due to fluid-rock interactions
We analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature ( T ) and pore water pressure ( P p ) on the sliding behavior of faults. Rate-stepping sliding tests were performed on laboratory faults in granite containing “gouge” (granite powder), both dry at 23° to 845°C [ Lockner et al. , 1986], and wet ( P p = 100 MPa) at 23° to 600°C [ Blanpied et al ., 1991, 1995]. Imposed slip velocities ( V ) ranged from 0.01 to 5.5 μm/s, and effective normal stresses were near 400 MPa. For dried granite at all temperatures, and wet granite below ∼300°C, the coefficient of friction (μ) shows low sensitivity to V , T , and P p . For wet granite above ∼350°, μ drops rapidly with increasing T and shows a strong, positive rate dependence and protracted strength transients following steps in V , presumably reflecting the activity of a water-aided deformation process. By inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate- and state-dependent constitutive law. One or two state variables were used to represent slip history effects. Each velocity step yielded an independent set of values for the nominal friction level, five constitutive parameters (transient parameters a , b 1 , and b 2 and characteristic displacements D c1 and D c2 ), and the velocity dependence of steady state friction ∂μ ss /∂ ln V = a - b 1 − b 2 . Below 250°, data from dry and most wet tests are adequately modeled by using the “slip law” [ Ruina , 1983] and one state variable ( a = 0.003 to 0.018, b = 0.001 to +0.018, D c ≈ 1 to 20 μm). Dried tests above 250° can also be fitted with one state variable. In contrast, wet tests above 350° require higher direct rate dependence ( a = 0.03 to 0.12), plus a second state variable with large, negative amplitude ( b 2 = −0.03 to −0.14) and large characteristic displacement ( D c2 = 300 to >4000 μm). Thus the parameters a , b 1 , and b 2 for wet granite show a pronounced change in their temperature dependence in the range 270° to 350°C, which may reflect a change in underlying deformation mechanism. We quantify the trends in parameter values from 25° to 600°C by piecewise linear regressions, which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip. The modeling results suggest that the succeptibility for unstable (stick-slip) sliding is maximized between 90° and 360°C, in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust.