Complex Evolution of Friction During Seismic Slip: New Experimental Results

We present initial high-speed friction experiments that indicate there are important compositionally controlled differences in the evolution of the frictional responses of faults at seismic slip velocities. Recent experimental studies show that rock friction undergoes a substantial evolution at slip rates of the order of centimeters per second and higher. A rapid decrease in the coefficient has been interpreted in terms of a number of mechanisms, including micro- and microscopic melting and formation of new amorphous phases such as silica gel. Existing experimental data covers a range of shear velocities from less than a millimeter per second up to several tens of centimeters per second. We conducted a new series of measurements of the dynamic coefficient of friction of granodiorite at slip rates ranging from 0.14 to 3 m/s (i.e., well within the seismic slip and melt generation range), and at normal stresses between 0.5 and 1.5 MPa. Experiments were carried out on a rotary shear machine. We used ring samples with outer-to-inner radii ratio of 1.5 to minimize variations in slip rate across the sample interface. Efforts were made to restrict gouge ejection from the shear zone in order to maintain the effective contact area. Our results reveal that granodiorite and diabase samples experience different evolutionary paths during frictional breakdown at high velocities. The quartz/feldspar-dominated lithology has a dynamic coefficient of friction of 0.3- 0.4 that overlaps with the high velocity end member values (between 0.14 and 0.2 m/s) recently obtain by Goldsby and Tullis (2002), Hirose and Shimamoto (2005), for quartz-dominated lithologies. However, our higher velocity data suggest the previously identified monotonic drop in frictional strength as velocities exceed ~0.1 m/s does not continue above 0.2m/s. Instead, the coefficient of friction is nearly constant in the velocity interval between ~20 cm/s and ~0.65 to 1 m/s and experiences a second weakening phase (friction drops to as low as 0.1-0.2) as velocities exceed 1m/s. However dramatic oscillations in frictional strength (coefficients jump between ~0.1 and ~0.5) also occur at the highest velocities. We attribute such jumps in strength to slip plane instability and rapid oscillations between melt and cataclastic flow dominated rheologies. In contrast, the feldspar dominated mafic lithologies experience a more monotonic weakening (coefficients drop from 0.3 to 0.2 as slip velocities exceed 0.14 m/s before they begin to melt as speeds rise above 0.65m/s. More complex frictional responses are seen during melting. The compositionally related differences in frictional evolution at high speed suggest there are complex thermo- mechanical processes occurring within the gouge layer that are impacted by variations mineralogical properties. This new experimental data suggest that the evolution of friction during seismic slip is complex, and a better theoretical understanding of the underlying physics is warranted.