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 -