Scientists find an “earthquake gate” as California fault stress hits a 1,000-year high

Two of the most important fault systems in Southern California are the San Andreas and San Jacinto faults. Together, they accommodate much of the tectonic motion in the region. Northeast of Los Angeles, the two systems come close together at Cajon Pass - a complicated geological junction where a rupture traveling along one fault may, under the right conditions, continue onto the other.

That possibility matters because 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, tectonic stress has continued to build along parts of the fault system, raising longstanding questions about how a future large rupture might unfold.

California fault stress reaches extreme levels

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 has now taken a much longer view of that risk. An international team modeled roughly 1,000 years of earthquake activity along the southern San Andreas and San Jacinto fault systems to estimate the amount of stress currently concentrated around Cajon Pass.

The research also involved 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 results suggest that tectonic stresses in parts of the system have reached, and in some locations surpassed, the highest values found anywhere in the model's 1,000-year history.

The researchers also describe Cajon Pass as an "earthquake gate": a fault junction that may help determine whether a large earthquake remains limited to one fault or continues across both fault systems. The study has just been published in Journal of Geophysical Research: Solid Earth.

Modeling 1,000 years of earthquakes

To reconstruct how stress changed over centuries, the researchers developed a physics-based, four-dimensional earthquake cycle model. In practical terms, the model simulates what happens across three dimensions of space while also tracking how the fault system changes through time.

The team then incorporated a 1,000-year record of past earthquakes assembled from several types of evidence. These included radiocarbon dating, unusual patterns recorded in tree rings, and historical records describing ruptures that broke the ground surface.

"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 model, stresses across the region are now at their highest level in the past 1,000 years.

Why Cajon Pass acts like an "earthquake gate"

One of the study's central findings concerns what happens when a rupture reaches Cajon Pass.

The junction does not behave like a simple barrier. Depending on the state of stress across the surrounding faults, it may either stop a rupture or allow it to continue from one fault system into the other.

Past earthquakes illustrate both possibilities. During the Fort Tejon earthquake of 1857, the rupture stopped at Cajon Pass and did not continue onto the San Jacinto Fault. By contrast, the Wrightwood earthquake of 1812 passed through the junction and ruptured both systems in a single through-going 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."

Two faults are becoming stressed together

The amount of stress on an individual fault is only part of the story.

The researchers found that the relationship between the stress levels on the two fault systems may be especially important. If both faults become highly stressed at the same time, conditions can become more favorable for a rupture to cross Cajon Pass and continue through both systems.

If the stress levels rise at different times instead, an earthquake rupture may be more likely to stop at the junction.

The model currently places stress on the San Jacinto-Bernardino section at 3.6 MPa, which is higher than any value reached elsewhere during the 1,000-year simulation. MPa stands for megapascals, a unit scientists use to measure pressure or mechanical stress.

On the nearby Mojave South section of the San Andreas fault, modeled stress has reached 2.8 MPa.

That means both fault segments are carrying unusually high and relatively similar levels of stress. According to the researchers, that configuration resembles conditions seen in the model before past ruptures that crossed both fault systems.

"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."

Why a joint rupture could have wider consequences

A rupture that crossed Cajon Pass and involved both the San Andreas fault and the San Jacinto fault could affect a much larger area than an earthquake limited to only one fault.

The surrounding region includes some of the most densely populated and infrastructure-dependent parts of the country. Areas that could be affected include greater Los Angeles, San Bernardino, Riverside and the Coachella Valley.

Cajon Pass itself is also a major transportation and infrastructure corridor. Highways, rail lines and energy infrastructure all pass through the area, making the junction important not only from a geological perspective but also for emergency planning.

"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.

High stress does not mean an earthquake is imminent

The researchers emphasize that their findings should not be interpreted as a forecast of an earthquake date.

Even when faults are highly stressed, scientists cannot use that information to determine exactly when a rupture will occur. Instead, the model helps reveal which earthquake scenarios may be physically plausible given the current state of the fault system.

Burkhard emphasizes: "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."