This research focuses on understanding fundamental processes responsible for causing earthquakes. Crustal earthquakes occur due to rapid sliding on faults. The sliding occurs suddenly because the rocks can slide more easily once sliding begins to speed up, but it is not known why faster sliding makes rocks weaker. Thus, the investigators are trying to understand why sliding is easier when it occurs faster, and why faults grow stronger as they sit quietly between earthquakes. The laboratory experiments are designed to test some of the fundamental explanations that have been offered. If we can understand the processes, the increased understanding of the earthquake process might show ways to predict earthquakes and to understand the factors responsible for their damaging strong ground motions.