San Andreas Fault – Carrizo Plain to the Imperial Valley
The San Andreas Fault is probably the most famous of fault lines. It is the fault that people in Southern California refer to as THE Fault. Through Southern California, it stays a fair distance from major population centers until the Cajon Pass, where it passes right next to San Bernardino, Redlands, and Palm Springs. The fault began forming in the Miocene Epoch, about 30 million years ago, when the North American Plate finally overrode the remainder of the Farallon Plate. The former plate boundary between the Farallon and the Pacific Plates was a spreading center. Some sections still exist today, comprising part of the East Pacific Rise. When this plate boundary came onshore, there was a fair amount of volcanism, as demonstrated in a lot of Miocene rock in Southern California. It also changed this portion of the boundary to a transform fault. The San Andreas Fault is a transform boundary connecting the Cascadia Subduction Zone near Cape Mendocino to the Imperial Valley, where it reaches the Brawley Seismic Zone. Movement along the fault is right-lateral and strike-slip, which means each side is sliding by horizontally instead of vertically.
Since the formation of the fault, it has moved quite a bit. Over the last 30 million years, it has shifted over 150 miles. Extreme examples of this offset can be found in the rocks within Pinnacles National Park near Soledad, CA and the Neenach Volcanics near Gorman, CA. Evidence of the fault and its movement is also quite easy to come by. Large valleys and canyons line the San Andreas Rift Zone from the Carrizo Plain to the Imperial Valley. Some of the features, such as groundwater damming, aren’t as obvious from the ground. Aerial photography shows in great detail the changes in plant life on either side of the fault in the Palm Springs area.
In the Southern California, the fault shows itself in many ways. In the Carrizo Plain, it is evidenced by a large “gash” running through the valley. Streams and canyons there show a great deal of offset, allowing for some dating of the structures to take place. Tejon Pass, where I-5 and former US 99 cross the mountains, was originally cut by the fault. Erosion accelerated by the fault weakened rock is responsible for many of the smaller canyons along the fault. Others are the result of a process known as “sagging” where the land between two parallel faults will sag between, creating a small graben. Water may collect there creating a “sag pond”. These ponds are sometimes enlarged to create reservoirs for drinking water. One such sag pond, San Andreas Lake in the San Francisco area, is the namesake of the San Andreas Fault.
This website will show various sections of the fault, its features, and where they can be found. Popular culture has helped to create a lot of myths about the fault line. The biggest is that “California will fall into the sea.” Even if the fault were capable of it, which it is not, it would only be the area west of the fault. The state line runs much farther to the east. So, come and check out the world famous San Andreas Fault. Perhaps you may gain a certain respect for the most powerful force in California and learn why we built our cities where we did.
Return to the Geology page