Portal:San Francisco Bay Area/Selected article/75

Radar generated 3-D view of the San Andreas Fault, at Crystal Springs Reservoir near San Mateo

The San Andreas Fault is a continental transform fault that extends roughly 810 miles (1,300 km) through California in the United States. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics, and a different degree of earthquake risk. Although the most significant (Southern) segment only dates back about 5 million years, the oldest sections were formed by the subduction of a spreading ridge 30 million years ago.

The fault was first identified in 1895 by Professor of geology Andrew Lawson from UC Berkeley who discovered the northern zone. It is named after a small lake which was formed in a valley between the two plates. Following the 1906 San Francisco earthquake, Lawson concluded that the fault extended all the way into southern California. In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the San Andreas Fault.

A project called the San Andreas Fault Observatory at Depth (SAFOD) is drilling into the fault to improve prediction and recording of future quakes. (more...)