Shear-induced graphitization of carbonaceous materials during seismic fault motion: Experiments and possible implications for fault mechanics

Carbonaceous materials often concentrate in fault zones developed in pelitic rocks. Among carbonaceous minerals, graphite is known as a lubricant and possibly plays a key role in frictional properties of the fault. Graphite reported from slip localized zones suggests that graphitization can occur during seismogenic fault motion. Thus, we performed friction experiments on amorphous carbon and graphite to investigate how graphite forms in association with fault motion and how these carbonaceous minerals affect frictional properties of faults. Experiments were done at normal stresses of 0.5–2.8 MPa and slip rates of 50 μm/s to 1.3 m/s in atmospheres of air and N2 gas, using rotary-shear apparatuses. XRD and TEM analyses revealed that graphitization can indeed occur during seismogenic fault motion perhaps due to large shear strain, short-lived flash heating and stress concentration at asperity contacts, even at low temperatures and pressures under anoxic environments. We found large differences in steady-state friction coefficient μss between graphite (μss = 0.1) and amorphous carbon (μss = 0.54) at low slip rate. But amorphous carbon exhibits marked velocity weakening at slip rate above 10 mm/s, and its steady-state friction reduces to the same level as that of graphite at a slip rate of 1.3 m/s. Faults with amorphous carbon are not weak at low slip rates, but they can become dynamically weak to foster fault motion during the generation of large earthquakes. Enriched graphite in fault zones can lubricate at all slip rates even at great depths and should receive more attention.