The San Andreas Fault
San Andreas fault in Lone Pine Canyon, Cajon Pass
The Defining Fault of Southern California
The San Andreas Fault is the dominant geologic structure of Southern California and one of the most studied faults in the world. More than a single crack in the ground, it is the boundary between two enormous tectonic plates: the Pacific Plate to the southwest and the North American Plate to the northeast. These plates have been sliding past one another for roughly 28 million years, slowly reshaping the landscape of California.Within the Mojave Desert and surrounding mountain ranges, the San Andreas Fault is more than an earthquake source. It is the framework upon which much of the region's geography has developed. Mountain ranges, valleys, passes, springs, and even transportation corridors owe their existence, directly or indirectly, to the movement of this remarkable fault system.
From the Chocolate Mountains to Frazier Park, the fault passes through nearly every major landscape represented on Digital-Desert.com, creating a continuous geologic story that links the Colorado Desert, the Mojave Desert, the San Bernardino Mountains, the San Gabriel Mountains, Antelope Valley, and the western Transverse Ranges.
A Plate Boundary
Unlike most faults that form from local stresses within Earth's crust, the San Andreas is a plate boundary fault. The Pacific Plate moves northwest relative to the North American Plate at approximately 50 millimeters (about two inches) per year. Most of this movement is accommodated along the San Andreas Fault and a network of associated faults throughout California.The San Andreas is classified as a right-lateral strike-slip fault. If two people stood on opposite sides of the fault, each would see the other moving to the right as the plates slowly slide past one another. Although the movement is only a few inches each year, over millions of years it has displaced rocks hundreds of miles from where they originally formed.
The Southern Section
The southern San Andreas begins near the Salton Sea, where the spreading center of the Gulf of California transitions into continental transform faulting. Near the Chocolate Mountains, the boundary between the Pacific and North American plates becomes a distinct land-based fault.Here the crust is relatively thin and active. Numerous faults share the movement between the two plates, making this one of California's most seismically active regions.
The Coachella Valley
Northwest of the Salton Sea, the San Andreas follows the northeastern edge of the Coachella Valley. This is one of the clearest expressions of the fault anywhere in California.Linear mountain fronts, offset stream channels, sag ponds, pressure ridges, and fault scarps all reveal the location of the fault. Geologists consider this section especially hazardous because it has not experienced a major surface-rupturing earthquake since approximately the late seventeenth century, allowing stress to accumulate.
San Gorgonio Pass
Near San Gorgonio Pass, the fault changes dramatically.Rather than remaining a single, well-defined fracture, it divides into multiple branches that interact with reverse and thrust faults. The compression created here helps lift both the San Bernardino and San Gabriel Mountains.
This area is among the most structurally complex portions of the entire San Andreas system. It demonstrates that plate motion is not always accommodated by simple sideways movement. Compression, uplift, and folding also become important.
Cajon Pass
At Cajon Pass, the San Andreas passes between the San Bernardino Mountains and the San Gabriel Mountains.
Many familiar Mojave landmarks occur along or near this section of the fault, including Mormon Rocks, Lost Lake, Crowder Canyon, and the historic transportation corridor used by Native peoples, Spanish explorers, emigrants, railroads, Route 66, and Interstate 15.
The steep mountains bordering the pass owe much of their elevation to repeated uplift associated with movement along the San Andreas and nearby faults. Erosion then exploits these weakened zones, carving natural passages through otherwise rugged terrain.
The Mojave Desert
Northwest of Cajon Pass, the San Andreas enters the Mojave Desert along the southern edge of Antelope Valley.
The fault passes near Palmdale, Littlerock, and Elizabeth Lake before continuing toward Gorman. Throughout this region, offset washes, shutter ridges, sag ponds, fault scarps, and linear valleys clearly record thousands of years of movement.
This section ruptured during the great Fort Tejon earthquake of January 9, 1857. The earthquake, estimated at magnitude 7.9, produced nearly 225 miles of surface rupture and remains one of the largest historic earthquakes in California. Horizontal displacement reached as much as 30 feet in some locations.
The Big Bend
Near Gorman and Frazier Park, the San Andreas reaches one of its most important features: the Big Bend.Instead of continuing northwest, the fault bends westward by approximately 15 to 20 degrees. Although this change appears modest on a map, it has profound geologic consequences.
The Pacific Plate continues attempting to move northwest, but the bend restricts that movement. The resulting compression shortens and thickens Earth's crust, forcing rocks upward rather than allowing them to slide freely past one another.
This restraining bend is responsible for the uplift of much of the western Transverse Ranges.
Frazier Park
Around Frazier Park, the effects of the Big Bend reach their maximum.The landscape is characterized by folded sedimentary rocks, active thrust faults, landslides, fractured crystalline rocks, and rapidly rising mountains. Nearby Mount Pinos, Frazier Mountain, and surrounding highlands continue to rise because of compression generated by the bend in the fault.
This region marks the transition between the Mojave Desert and California's east-west trending Transverse Ranges, a mountain system whose unusual orientation exists largely because of the geometry of the San Andreas Fault.
A Landscape Shaped by Motion
The San Andreas Fault is often described only as an earthquake fault, but earthquakes represent only brief moments in a much longer story.
Over millions of years, the fault has controlled where mountains rise, where valleys form, where rivers change course, where springs emerge, and where people travel. Cajon Pass, San Gorgonio Pass, Elizabeth Lake, the Antelope Valley, and the mountain front bordering the Mojave all reflect the influence of plate movement.
Many of Southern California's major landscapes are not simply crossed by the San Andreas Fault; they exist because of it.
For visitors exploring the Mojave Desert, the fault can be seen in straight valleys, offset stream channels, steep mountain fronts, unusual rock formations, and broad passes that have guided both wildlife and human travel for thousands of years. Although the fault is usually quiet, it remains active today, continuing to reshape the region one small movement at a time.
References
Atwater, T. (1970). Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America. Geological Society of America Bulletin.
Norris, R. M., and Webb, R. W. Geology of California.
Schulz, S. S., and Wallace, R. E. The San Andreas Fault.
U.S. Geological Survey. The San Andreas Fault System.
U.S. Geological Survey. Quaternary Fault and Fold Database.
Powell, R. E. Balanced Geological Cross Sections Across Southern California.
Crowell, J. C. The San Andreas Fault System Through Time.
San Andreas fault in Lone Pine Canyon, Cajon Pass
The San Andreas Fault - Introduction
next >
by Sandra S. Schulz and Robert E. Wallace
The presence of the San Andreas fault was brought dramatically to world attention on April 18, 1906, when sudden displacement along the fault produced the great San Francisco earthquake and fire. This earthquake, however, was but one of many that have resulted from episodic displacement along the fault throughout its life of about 15-20 million years.
IntroductionAbout the San Andreas Fault
Location of the Fault
Surface Features
Movement Along the Fault
Earthquakes Along the Fault
Forecasting the Next Earthquake
