The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. The primary boundary between these two plates is the San Andreas Fault.
The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. In California there are two plates - the Pacific Plate and the North American Plate. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. An earthquake is caused by a sudden slip on a fault.