What is a fault?

A fault is a fracture in the Earth's crust whose sides have moved in relation to each other.

What is the San Andreas fault?

The San Andreas fault is a lateral fault.  Lateral movement occurs when rock on one side of the fault slides horizontally past rock on the other side, with very little vertical motion.  The San Andreas is a right-lateral fault.  Geologists use the term right-lateral to describe the direction the fault moves.  If you were standing on one side of a right-lateral fault and facing the fault, a person standing on the opposite side would move to your right during an earthquake.

The San Andreas fault is also the boundary between the Pacific and North American tectonic plates.  The fault is about 700 miles (1100 km) long, stretching from Cape Mendocino to the Salton Sea.  On average, the Pacific side is moving horizontally past the North American side at a rate of 1.3 inches (34 mm) per year -- about as fast as a fingernail grows.  But the fault here is not moving every single minute -- it only moves during large earthquakes, which happen once every few hundred years.  At Wallace Creek, it moved 30 feet (9 m) on January 9, 1857.