Principal slip zone in kaolinite gouge: A seismic slip indicator?

Some fault gouges (e.g., kaolinite) exhibit velocity-strengthening behavior and slide stably at low, subseismic slip rates (e.g., 1-10 mu m/s), accompanied by the preferential development of foliation rather than a principal slip zone or a discrete narrow band of large shear strain. However, a remarkable observation from experimental gouges sheared at seismic slip rates (>0.1 m/s) is that all the gouges show a well-developed principal slip zone, irrespective of their frictional behaviors (velocity weakening or velocity strengthening) at subseismic slip rates. These observations imply that in some gouges, a principal slip zone develops only at seismic slip rates and can be used as a seismic slip indicator. To test this idea, we conducted shear tests on dry kaolinite powder and wet kaolinite paste at low (0.1-100 mu m/s), intermediate (similar to 1 mm/s), and high (1 m/s) slip rates and at a normal stress of 3 MPa. The dry powder specimen recovered after the experiments showed a principal slip zone at all slip rates. In contrast, the paste specimens showed a principal slip zone only at the seismic slip rate. They display foliation and R-shears at low and intermediate slip rates, indicating the occurrence of distributed shear. The micro-structure of the wet paste at the seismic slip rate is similar to that reported from a natural kaolinite-rich seismic slip zone. Considering the experimental results, the structures of the experimental and natural seismic slip zones, and the unlikely fault slip under dry conditions, we suggest that kaolinite gouges with principal slip zone(s) can be a seismic slip indicator. Further studies may be extended to other gouge materials showing similar velocity strengthening and stable sliding at subseismic slip rates to determine whether the same conclusion can be drawn.