The San Andreas Fault is a continental transform boundary running roughly 1,200 kilometres through California, marking the line where the Pacific Plate slides horizontally past the North American Plate. The Pacific side creeps northwest at about 3 to 5 centimetres per year, so the two blocks grind sideways rather than one diving beneath the other.
As a right-lateral strike-slip fault, its motion is horizontal: features on the far side appear shifted to the right of an observer. The system splits into northern, central and southern segments, and the central portion near Parkfield famously creeps steadily while locked northern and southern sections store strain for major ruptures.
The fault produced the catastrophic 1906 San Francisco earthquake (magnitude ~7.9), which ruptured some 480 kilometres of the northern segment and devastated the city with shaking and fire. The 1857 Fort Tejon quake broke the southern section, and seismologists warn the long-quiet southern San Andreas is overdue for another great event.