Earthquakes occur when the friction that prevents a fault from slipping is overcome by the gradual accumulation of stress acting across the fault and starts the material on the two sides of the fault slipping past each other. As slip occurs, frictional resistance drops, and the resulting rapid and unstable slip generates seismic waves that radiate outward in all directions from the initiation point, or hypocenter. These seismic waves are recorded by seismographs deployed on the Earth's surface, and their arrival times are the information that is used to determine the hypocenter. The distribution of earthquake hypocenters is one of the principal tools that seismologists use to understand earthquake behavior. Thus, it is important to minimize the uncertainties in determining earthquake hypocenters. Because networks of seismographs are set up to locate and catalog earthquakes automatically, refinement of earthquake locations is often referred to as relocation.