Stress on the San Andreas at Cajon Pass is the highest in 1,000 years

Tectonic stress along the southern San Andreas and San Jacinto fault systems has reached, and in places exceeded, the highest levels recorded in the past 1,000 years, according to a study led by researchers at the University of Hawai’i at Mānoa. The junction where the two fault systems meet, Cajon Pass, sits at the center of the finding: more than 160 years have passed since the region’s last major rupture, and the study’s authors describe the system there as “critically loaded.”

The research, built from a physics-based computer model rather than a live monitoring alert, is not a forecast of when the next large earthquake will strike Southern California. It is instead a measurement of how much stored energy has accumulated along a stretch of fault that runs through the outskirts of Los Angeles, San Bernardino, Riverside and the Coachella Valley.

A 1,000-year earthquake record built from sediment and tree rings

Lead author Liliane Burkhard, a research affiliate at the Hawai’i Institute of Geophysics and Planetology and a scientist at the University of Bern, and her co-authors constructed 4D earthquake-cycle simulations spanning a millennium of the fault’s history, according to the University of Hawai’i’s account of the work. They fed that history, reconstructed from radiocarbon dating of displaced sediment layers and tree-ring records, into a model that tracks how stress builds and releases along the fault over time.

“Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems,” Burkhard said. The study was published in the peer-reviewed Journal of Geophysical Research: Solid Earth on June 3, 2026, as Statewide California Earthquake Center Contribution #15025, and lists co-authors from Northern Arizona University, the University of Bern, the U.S. Geological Survey’s Earthquake Science Center and the Scripps Institution of Oceanography at UC San Diego.

Cajon Pass acts as an “earthquake gate”

Cajon Pass, the mountain gap northeast of San Bernardino where the San Andreas and San Jacinto systems intersect, does not behave as a fixed barrier in the model. “Cajon Pass may act as an ‘earthquake gate’: sometimes blocking large ruptures from crossing between the faults, and sometimes allowing them to pass through and involve both systems in a single event,” Burkhard said, adding that whether the gate opens or stays shut appears to depend on how closely stress levels on the two fault systems align at the moment a rupture begins.

Two 19th-century earthquakes show both outcomes actually happening, according to a University of Bern summary of the research. The magnitude 7.9 Fort Tejon earthquake of 1857 stopped at Cajon Pass without rupturing the San Jacinto Fault, while the Wrightwood earthquake of 1812 crossed the junction and tore through both fault systems in a single event. Stress has continued to accumulate along these segments in the more than 160 years since Fort Tejon, the longest quiet interval in the model’s 1,000-year record.

That distinction matters because a joint rupture spanning both fault systems would be a markedly different event than a single-fault earthquake. The researchers found that when the stress gap between the San Jacinto Bernardino segment and the Mojave South segment narrows, the model shows the faults rupturing jointly rather than independently, a scenario the study’s authors say could be significantly more damaging than either fault failing on its own.

The numbers behind “critically loaded”

The Statewide California Earthquake Center’s published abstract puts figures on the buildup. By 2025, the model estimates Coulomb stress at 2.8 megapascals on the Mojave South segment of the San Andreas, 1.8 megapascals on the North San Bernardino segment, and 3.6 megapascals on the San Jacinto Bernardino segment of the San Jacinto Fault. Each segment has its own estimated failure threshold: 1.2 to 2.7 megapascals for Mojave South, 0.4 to 1.6 megapascals for North San Bernardino, and 1.2 to 2.9 megapascals for San Jacinto Bernardino, meaning several segments have already climbed into, or past, the range where the model expects past ruptures to have occurred.

Stress accumulates fastest north of Cajon Pass, at roughly 1.8 megapascals per century, compared with 1.0 to 1.5 megapascals per century to the south, a difference the researchers attribute to higher slip rates on that side of the junction. Because the Mojave South and San Jacinto Bernardino segments are both running high and close to each other in absolute terms, the University of Bern’s own release on the study describes that alignment as a configuration that has historically preceded ruptures crossing both faults at once.

What the study does not claim

Burkhard was explicit about the limits of the finding. “This is not a prediction of when an earthquake will happen,” she said. “However, studies like this are important contributions to national and global earthquake hazard research in that we are using rigorous, quantitative science to better understand the risk facing millions of people.” She described the modeling as a tool for refining seismic hazard assessments, informing building codes and guiding infrastructure planning rather than a countdown clock.

The research team is now working to adapt the same modeling framework to other complex fault junctions worldwide, treating Cajon Pass as a proof of concept for a method that could be applied anywhere two major faults meet. For Southern California, the immediate value of the work lies less in a specific warning than in a clearer picture of what a joint San Andreas-San Jacinto rupture could look like, and how the physics of stress transfer at a single mountain pass might determine whether that scenario stays contained to one fault or spreads across both.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview