In order to understand the risk that different areas of the U.S. face for earthquake hazards, we first need to know where the faults are and how they behave. We are aware of the existence of a fault only if it has produced an earthquake that we know about from modern seismic instrument recordings or historical written records or if it has left a recognizable mark on the earth’s surface that can be discovered by remote imagery. Once a fault has been identified, the next step is to determine how it behaves.