EFFECTS OF SLIP, SLIP RATE, AND SHEAR HEATING ON THE FRICTION OF GRANITE

The stability of fault slip is sensitive to the way in which frictiona l strength responds to changes in slip rate and in particular to the e ffective velocity dependence of steady state friction Delta mu(ss)/Del ta ln V. This quantity can vary substantially with displacement, tempe rature and slip rate. To investigate the physical basis for this behav ior and the possible influence of shear heating, we slid initially bar e granite surfaces in unconfined rotary shear to displacements of hund reds of millimeters at normal stresses, sigma(n), of 10 and 25 MPa and at room temperature. We imposed step changes in slip rate within the range 10(-)2 to 10(3.5) mu m/s and also monitored frictional heating w ith thermistors embedded in the granite. The transient response of mu to slip rate steps was fit to a rate- and state-dependent friction law using two state variables to estimate the values of several parameter s in the constitutive law. The first 20 mm of slip shows rising fricti on and falling Delta mu(ss)/Delta ln V; further slip shows roughly con stant friction, Delta mu(ss)/Delta ln V and parameter values, suggesti ng that a steady state condition is reached on the fault Surface. At V less than or equal to 10 mu m/s, Delta mu(ss)/Delta ln V = -0.004 +/- 0.001. At higher rates the response is sensitive to normal stress: At sigma(n) = 25 MPa granite shows a transition to effective velocity st rengthening (Delta mu(ss)/Delta ln V = 0.008 +/- 0.004) at the highest slip rates tested. At 10 MPa granite shows a less dramatic change to Delta mu(ss)/Delta ln V approximate to 0 at the highest rates. The max imum temperature measured in the granite is similar to 60 degrees C at 25 MPa and 10(3.5) mu m/s. Temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogene ous frictional heating over the sliding surface. The simplest interpre tation of our measurements of Delta mu(ss)/Delta ln V is that the gran ite is inherently velocity weakening (partial derivative mu(ss)/partia l derivative) ln V < 0) and temperature strengthening (partial derivat ive mu(ss)/partial derivative T-1 < 0) at all velocities. At high slip rates the response of CL to changes in temperature from shear heating may outweigh the response to changing velocity, such that the net eff ect Delta mu(ss)/Delta > 0 mimics velocity strengthening. These result s have implications for the frictional behavior of faults during earth quakes. High slip rates may cause a switch to effective velocity stren gthening which could limit peak coseismic slip rate and stress drop. F or fluid-saturated faults, strengthening by this mechanism may be part ly or fully offset by weakening due to thermal pressurization of a poo rly drained pore fluid.