Most earthquakes occur along plate boundaries, as the constant movement of the plates causes faults to slip. The map above shows all earthquakes above magnitude 4 recorded in the world for the year 1996. The plate boundaries are shown as thin black lines. Below is a map of Southern California with all earthquakes that occurred between 1996 and 1997. The plate boundary between the North American plate and Pacific plate lies along the San Andreas fault but we can see that considerable earthquake activity occurs along the San Jacinto and Elsinore faults as well.
Earthquakes provide a good way to locate plate boundaries.
These recorders show only the vertical component of the seismic wave that travels past it. The PC map display shows the location of earthquakes that have occurred in Southern California in the past few days.