In Southern California, two of the region's most important fault systems are the San Andreas Fault and the San Jacinto Fault. Together, they accommodate most of the tectonic motion in the area. Northeast of Los Angeles, the two systems come close together at Cajon Pass, a geologically complex area where a rupture on one fault may be able to jump to the other. Since the magnitude 7.9 Fort Tejon earthquake struck in 1857, stress has continued to accumulate along these fault segments. That unusually long quiet period has fueled concerns among scientists about the possibility of a major future earthquake.

A new study led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern examined 1,000 years of earthquake activity along the southern San Andreas and San Jacinto fault systems to estimate current stress levels at Cajon Pass. The international research team included scientists from the University of Hawaiʻi at Mānoa, the U.S. Geological Survey Earthquake Science Center in Pasadena, and the Scripps Institution of Oceanography at UC San Diego.

Their findings indicate that tectonic stress in the region has reached, and in some locations surpassed, levels seen at any point during the past millennium. The researchers also introduce a new concept, describing Cajon Pass as an "earthquake gate," a key junction that may determine whether a large earthquake remains confined to a single fault or spreads across both fault systems at the same time. The study was published in Journal of Geophysical Research: Solid Earth.

Reconstructing 1,000 Years of Earthquake Activity

To understand how stress has changed over time along the San Andreas and San Jacinto faults, as well as at Cajon Pass, the team developed a physics-based four-dimensional earthquake cycle model. The model simulates fault behavior in three dimensions while also tracking changes over time.

Researchers supplied the model with a 1,000-year earthquake history reconstructed from geological evidence, including radiocarbon dating, tree ring records, and historical observations of ground ruptures.

"The model tracks how each earthquake changes stress on neighboring fault segments, how stress accumulates during the quiet intervals between events, and how the deeper layers of the crust slowly relax following large ruptures," explains Burkhard.

"This simulation allows us to understand how stresses in the fault system build up over centuries," continues Burkhard. "By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today."

According to the results, stress levels across the region are now higher than at any other point during the 1,000-year period examined by the model.

The Role of the "Earthquake Gate"

One of the study's most significant findings involves the role of Cajon Pass as an "earthquake gate." The researchers describe it as a fault junction that can influence whether a rupture stops on one fault or continues across both the San Andreas and San Jacinto systems.

Past earthquakes provide examples of both outcomes. The Fort Tejon earthquake of 1857 stopped at Cajon Pass and did not rupture the San Jacinto Fault. In contrast, the Wrightwood earthquake of 1812 crossed the junction and propagated through both fault systems as a single event.

"The earthquake gate concept captures something important about how fault junctions work," explains Burkhard. "Cajon Pass doesn't simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries."

The researchers found that the key factor is not just the amount of stress on an individual fault. Equally important is how closely the stress levels on the two fault systems match one another.

When stress builds to similarly high levels on both faults, conditions become more favorable for a large rupture that crosses the junction and spreads through both systems. When the stress levels differ significantly, ruptures are more likely to stop at Cajon Pass instead of continuing farther.

The model estimates that stress has reached 3.6 MPa on the San Jacinto-Bernardino section, exceeding any value recorded during the entire 1,000-year simulation. The neighboring Mojave South section of the San Andreas Fault currently stands at 2.8 MPa. Because both sections are experiencing high and relatively similar levels of stress, the fault system is in a configuration that has historically preceded multi-fault ruptures.

"So not only is it concerning that the stresses are reaching historic highs," says Burkhard, "but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously -- and that is a scenario with much larger consequences for the region."

What a Multi-Fault Earthquake Could Mean

An earthquake that ruptures both the San Andreas and San Jacinto faults through Cajon Pass would likely be far more severe than one limited to a single fault system.

Such an event could affect some of the nation's most densely populated and infrastructure-dependent regions, including the greater Los Angeles area, San Bernardino, Riverside, and the Coachella Valley. Cajon Pass itself contains major transportation corridors, rail lines, and energy infrastructure.

"The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience. Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide," says Burkhard.

At the same time, Burkhard stresses that the findings should not be interpreted as a prediction.

"The study is not a prediction of when an earthquake will occur. What we can say is that the system is critically stressed and that physics-based models like ours give a clearer picture of the range of scenarios we should be prepared for. This information is important for hazard assessment, infrastructure planning and emergency preparedness."