Check out this site run by the California Division of Mines and Geology.
Earthquakes occur when two pieces of the earth snap past each other along a fault.
That fault is actually a crack that extends deep into the earth. It depends on how you measure size. Magnitude wasn't used as a measure of size until the last 100 years or so. Check out this table prepared by the U.S. Geological Survey that lists significant California, Baja California, and Nevada earthquakes from 1769 until the present. Check out this site run by the U.S. Geological Survey.