California has a relatively high number of earthquakes due to its position on both the Pacific and North American Plates. These plates are in constant motion, and earthquakes occur when their sides slip against one another suddenly.
The San Andreas Fault is the boundary line between the two plates, and it runs roughly from north to south along much of the length of the state. There are hundreds of other smaller faults in the state, with about 200 of them considered potentially dangerous due to their slip rates. An estimated 70 percent of the state's population lives within 30 miles of a fault, where noticeable levels of ground shaking can occur.
California generally experiences two or three, large-scale, earthquakes per year of a magnitude of 5.5 or greater, the level at which moderate damage to structures can occur. Small earthquakes occur on an almost daily basis throughout the state, but most are too small to be felt. The state experienced over 2,900 earthquakes in total in 2014.
Though California is one of the regions of the world most prone to earthquakes, it does not have the most or the largest quakes. Within the United States, Alaska has both more and larger earthquakes.