California is prone to earthquakes because I lies on the San Andreas Fault - Wikipedia. Faults are areas where two tectonic plates come together. The San Andreas Fault is the boundary between the North American and Pacific Plates and was discovered in 1895 by Professor Andrew Lawson of UC Berkeley. read more
The most famous example is California's San Andreas Fault, which stretches some 600 miles (1,000 kilometers) from southern California to north of San Francisco. The sideways motion of the fault's branches is caused by the Pacific Ocean's crustal plate moving to the northwest under North America's continental crust. read more