Cajon Pass fault stress at 1,000-year high, physics-based model finds

Tectonic stress along the southern San Andreas and San Jacinto fault systems has reached its highest level in a 1,000-year historical simulation, according to a physics-based model developed by an international research team and published in the Journal of Geophysical Research: Solid Earth. The study focuses on Cajon Pass—a vital transit and infrastructure corridor northeast of Los Angeles—where scientists warn that the junction acts as an “earthquake gate” capable of either stopping a rupture or allowing a major seismic event to tear through both fault systems simultaneously.

Fault Stress Reaches Historical Highs at Cajon Pass

Southern California sits atop a complex tectonic network where sections of the Earth’s crust move against one another, lock, and accumulate stress over centuries. The broader Los Angeles region has not experienced a comparable major earthquake since the magnitude 7.9 Fort Tejon earthquake in 1857. During that long quiet period, stress has continued to build along the San Andreas and San Jacinto fault systems. To evaluate current risks, an international team led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences at the University of Bern modeled roughly 1,000 years of earthquake activity. The research also involved scientists from the University of Hawaiʻi at Mānoa, the U.S. Geological Survey (USGS) Earthquake Science Center in Pasadena, and the Scripps Institution of Oceanography at UC San Diego.

The research team built a physics-based, four-dimensional earthquake cycle model that simulates spatial movement across three dimensions while tracking temporal changes. They calibrated this simulation using a 1,000-year historical record compiled from radiocarbon dating, tree-ring patterns, and documented ground-surface ruptures. According to the model, tectonic stresses in parts of the system have reached, and in some locations surpassed, the highest values found anywhere in the simulation’s history. Specifically, the model places stress on the San Jacinto-Bernardino section at 3.6 MPa, marking a peak across the 1,000-year timeline.

How the Earthquake Gate Controls Multi-Fault Ruptures

Cajon Pass serves as a critical geological junction where the San Andreas and San Jacinto faults come into close proximity. Rather than acting as a simple physical barrier, the pass functions as an “earthquake gate” that responds dynamically to surrounding stress conditions. Historical events demonstrate two distinct behaviors at this junction. During the 1857 Fort Tejon earthquake, the rupture stopped at Cajon Pass and did not jump to the San Jacinto Fault. Conversely, the 1812 Wrightwood earthquake pushed through the junction, rupturing both systems in a single through-going event.

“Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries,” Burkhard explained. The study indicates that the relative stress levels on both faults determine whether a rupture crosses the pass. When both fault systems become highly stressed simultaneously, conditions favor a multi-fault rupture. If stress levels rise at disparate times, a rupture is more likely to arrest at the junction. “We are primed to have another one in our generation,” Burkhard told National Geographic.

Infrastructure Vulnerabilities Threaten Southern California Lifelines

The concentration of stress carries severe implications for the critical infrastructure packed into the Cajon Pass corridor, which connects Southern California to the rest of the region through the San Bernardino and San Gabriel Mountains. An average of approximately 160,000 vehicles travel through the pass each day, alongside major rail lines, power transmission towers, fiber-optic cables, aqueducts, and high-pressure natural gas pipelines. Ken Hudnut, who helped the USGS develop seismic resiliency initiatives for California, told National Geographic, “We all know the San Andreas fault is loaded. It’s ready to go. For years the scientific community has been saying so. Cajon Pass keeps me up at night.”

Previous earthquake modeling, including the 2008 ShakeOut Scenario, projects that a magnitude 7.8 rupture along the southern San Andreas fault could result in more than 1,800 deaths, 50,000 injuries, and roughly $200 billion in damage. Such an event could sever fiber-optic connections, rupture gas infrastructure, and bring down transmission towers via earthquake-triggered landslides, with an estimated 75,000 cubic meters of debris blocking roadway traffic. Former USGS seismologist Lucy Jones warned that “any sort of lifeline crossing the fault will be cut off.” In response, the California Governor’s Office of Emergency Services established a task force to coordinate restoration efforts with the private sector, while utilities such as Southern California Edison reported investing roughly $300 million in seismic hardening.

Does the New Study Predict the Next Earthquake?

• Does the new study predict when the next major earthquake will occur in Southern California?
  No. The research findings do not predict the exact timing of the next big earthquake, according to findings reported by the New York Post.
• What specific geological evidence was used to build the 1,000-year earthquake model?
  The international team used a 1,000-year record compiled from radiocarbon dating, unusual growth patterns recorded in tree rings, and historical documentation of past ground-surface ruptures.
• How much stress is currently concentrated on the San Jacinto-Bernardino section?
  The physics-based simulation places stress on the San Jacinto-Bernardino section at 3.6 MPa, which represents the highest value recorded anywhere in the model’s 1,000-year history.