Tectonic fault cores are formed substantially of clay minerals. Even a slight change in mineral composition or in water saturation can result in a significant alteration of the sliding regime on the fault. We present results of laboratory experiments on a slider model set-up that was used to study the regularities of slip behavior in a model fault filled with gouge. The gouge consisted of quartz sand and clays of different types (bentonite

illite and kaolinite). The slip behavior essentially depended on gouge mineralogy. The accumulated stress could release via both fast and slow slips. The scaled kinetic energy for fast slips was 10–10

while that for the slowest slips was 10–10. Fast stick-slip is characteristic of model faults filled with quartz sand in dry and moistened conditions. A gradual transformation from stick-slip to stable sliding was observed for quartz sand/clay gouge as the clay content approached 20%. Under moistening clay

mineralogy played a key role. If the illite clay content was 5%

the moistening led to an increase in peak velocity by more than an order of magnitude; if the bentonite clay was 5%

it led to stabilization of sliding. While alteration in friction coefficient after moistening remained relatively small

the scaled kinetic energy could vary by several orders of magnitude.