Earthquakes in California are common occurrences since the state is located on the San Andreas Fault, which cuts acrossCalifornia and forms the tectonic plate boundary between the Pacific and the North American Plate. There are many thousands of small earthquakes per year, most of them so small that they are not felt. California's complex landscape can be attributed to the network of faults that run through the state. The earliest reported earthquake in California was felt in 1769 by the Portola expedition about 30 miles southeast of Los Angeles.
California has numerous active faults throughout the state which are known to produce large earthquakes. The most active of these is the San Jacinto Fault Zone in Southern California, which has produced large events on a regular basis throughout recent history. The Mendocino Triple Junction located offshore of Northern California is also very active, producing many earthquakes above M6 throughout history.[3] Northern California is also subject tomegathrust earthquakes on the Cascadia subduction zone (extending north from Mendocino), such as the 1700 Cascadia earthquake, magnitude of approximately 9. The town of Parkfield in central California is located on a section of the San Andreas Fault that produces an earthquake of about M6 every 20–30 years on average in 1857, 1881, 1901, 1922, 1934, 1966 and 2004.
Information copy From wikipedia