Southern California's San Andreas Fault Reaches Highest Stress Levels in 1,000 Years

Researchers have determined that Southern California's San Andreas and San Jacinto faults are under more stress than any point in the last 1,000 years, significantly increasing the probability of a major earthquake. Scientists warn that a rupture along these fault systems could cascade through a geological junction called the Cajon Pass, potentially spreading damage across a vast region including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

The Stress Buildup

Using geological evidence including tree-ring records and sediment samples, researchers created computer models to estimate how pressure accumulates along faults over centuries. The study reveals that stress has been gradually building since the last major earthquake in 1857, one of California's largest seismic events on record. Kate Scharer, a geologist with the U.S. Geological Survey and a co-author of the study, noted that because such a long time has passed since the last big earthquake on the Southern San Andreas or San Jacinto faults, substantial stress has accumulated.

According to the research, the chances exceed 50 percent that an earthquake measuring 6.7-magnitude or higher will occur along the southern stretch of the San Andreas fault in upcoming decades. The accumulated pressure operates like a coiled spring with nowhere to move, as tectonic forces continue moving the Earth's crust a few centimeters each year while certain zones remain locked in place.

The Earthquake Gate

A critical element in this seismic threat is the Cajon Pass, a narrow corridor between the San Gabriel and San Bernardino mountains located less than 60 miles northeast of downtown Los Angeles. Liliane Burkhard, the lead author of the study, described this pass as an "earthquake gate"—a junction that can either stop or transmit large ruptures between the San Andreas and San Jacinto fault systems depending on stress conditions. In 1812, researchers suspect that a 7.5-magnitude earthquake crossed the Cajon Pass and rippled along both fault systems. If this geological junction were to transmit a future rupture, the seismic waves could spread damage far beyond what a single fault rupture would cause, affecting millions across multiple counties simultaneously.