Frictional Aging of Single‐Asperity Nanoindentation Contacts in Quartz and Calcite

The evolution of fault friction during the interseismic period affects the mechanics of a future earthquake on the same fault patch. Frictional aging has been previously tied to time‐dependent contact area growth through observations made on rock analogs. However, our understanding of the processes that control frictional aging is limited and is dependent on experiments that explore only numerous mechanisms. We conduct slide‐hold‐slide experiments with a dual‐axis nanoindenter on single‐crystal surfaces of quartz and calcite. Our results show that frictional aging in diamond‐quartz contacts is independent of time and contact area, in stark contradiction to past experiments done on quartz‐quartz contacts in rocks. Diamond‐calcite contacts show modest frictional aging, but still well below previous reported values from calcite‐calcite contacts. These results suggest that frictional aging of like‐on‐like minerals may be of chemical origin, as suggested in recent studies with atomic force microscopy and molecular dynamics simulations. Plain Language Summary Frictional aging is the process of re‐strengthening of frictional contacts, either by an increase in contact area or by an increase in shear strength of the contact. Aging is important in the earthquake cycle where it affects the mechanical response of a future earthquake. Frictional aging has been studied extensively in the past through slide‐hold‐slide (SHS) experiments in rocks where the contact is slid at a constant velocity, then held stationary for a predetermined ‘hold time’, after which sliding resumes. These experiments show that the static friction measured at the end of the hold, as sliding commences, scales with the logarithm of hold time. This result is commonly interpreted as being due to contact area growth as observed in rock‐analogue materials (such as polymers). We conducted SHS experiments with a nanoindenter capable of inducing frictional sliding of a diamond tip over a substrate while continuously monitoring contact area. Experiments performed on single crystals of quartz and calcite show no or minimal frictional aging even though contact area grows with the logarithm of hold time. Our results suggest that frictional aging of these mineral contacts is due to a chemical mechanism and not explicitly due to contact area growth. Key Points Diamond‐quartz and diamond‐calcite contacts show time‐dependent contact area growth Quartz surfaces do not exhibit frictional aging, suggesting that agin