When Was the San Andreas Fault Formed?

Plate Tectonics

• Scientists think the Earth's crust is made of huge, moving plates. As they move, the plates tear huge cracks in the Earth's surface called faults. Earthquakes occur when pressure built up by the plates is released along the faults.

San Andreas Fault

• The San Andreas Fault formed 15 million to 30 million years ago along the boundary between the Pacific and North American tectonic plates. It is the main fault in a series that extends from northwestern California to the Gulf of California.

Features

• The San Andreas is a strike-slip fault, in which sections of the crust move parallel to each other. Because it marks the location where two plates are changing, it is also called a transformation fault. The movement of the plates sliding past each other causes thousands of tiny earthquakes along the fault, but major earthquakes are much more rare.

1906 San Francisco Earthquake and Fire

• An estimated 8.3 on the Richter scale, the 1906 San Francisco earthquake was the largest ever recorded along the San Andreas. It lasted almost 45 seconds and caused the ground along the western side of the fault to move 21 feet.