Energy is released because of rapid movement on a fault. A fracture on which movement has occurred. Rapid movement of 1-10 meters is typically necessary to generate a significant earthquake. The focus is the point on the fault surface where movement begins, the earthquake source. Seismic waves radiate outward from the focus.
Shallow earthquakes are the most common.
The epicenter is the geographic location of the point on the earth's surface directly above the focus. Earthquakes are named for the epicenter location, for example the 1994 Northridge earthquake occurred several kilometers below the city of Northridge in metropolitan Los Angeles. Seismic waves represent the energy released from the earthquake focus. There are two types of seismic waves. Undulations of the land surface during an earthquake are a representation of surface waves.