Relationship between textures and sliding motion of experimentally deformed fault gouge: Application to fault zone behavior

The run products of many triaxial friction experiments on clay-rich and quartzofeldspathic gouges have been examined petro- graphically, to see if any physical differences between stable and stick-slip motion can be found that will help to identify the cause of earthquakes and/or be useful for paleoseismology. For both gouge types the transition from stable sliding to stick-slip is correlated with (1) a change from pervasive deformation of the gouge layer to localized slip in subsidiary shears, and (2) an increase in the angle between the crosscutting (Riedel) and boundary-parallel shears. A possible hypothesis to explain earthquakes based on these differences is that the localization of slip creates the potential for large stress build-ups, which are generated where the Riedel and boundary shears meet at relatively high angles. This hypothesis will be tested further experimentally, with emphasis on the Riedel shears to identify the controls on their orientation. Evidence for distinguishing physical characteristics will also be sought along locked and creeping portions of the San Andreas fault.