California Earthquake Risk at Highest Level in 1,000 Years, Study Finds

Tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has reached the highest levels seen in the past millennium, according to a landmark study published in the Journal of Geophysical Research: Solid Earth. The research reveals that a critical junction known as Cajon Pass — located just 50 miles from downtown Los Angeles — may act as an “earthquake gate” capable of linking both faults in a single, catastrophic rupture.

While scientists emphasize this is not a prediction of when an earthquake will occur, the findings have significant implications for seismic hazard assessments, infrastructure planning, and emergency preparedness across one of the most densely populated regions in the United States.

A 1,000-Year Stress Record

Led by Dr. Liliane Burkhard of the University of Hawaiʻi at Mānoa and the University of Bern, an international research team constructed a physics-based computer model that simulated earthquake cycles along the southern San Andreas and San Jacinto fault systems over a millennium. By feeding the model a 1,000-year record of earthquake history reconstructed from geological evidence — including radiocarbon dating of displaced sediments, tree-ring records, and historical documentation — the researchers were able to estimate present-day stress levels with unprecedented precision.

According to the University of Hawaiʻi, the results show that modeled stress has reached 3.6 MPa on the San Jacinto-Bernardino section — exceeding the highest value seen anywhere in the 1,000-year simulation — and 2.8 MPa on the Mojave South section of the San Andreas fault. Both segments are highly and similarly stressed, a configuration that historically has preceded joint ruptures crossing both fault systems.

“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 in a statement.

The ‘Earthquake Gate’ at Cajon Pass

A central finding of the study is the role of Cajon Pass, a tectonically complex junction northeast of Los Angeles where the San Andreas and San Jacinto faults converge. The researchers describe it as an “earthquake gate” — sometimes blocking ruptures from crossing between faults, and sometimes allowing them to link into a single, larger event.

Historical examples illustrate both behaviors. The 1857 Fort Tejon earthquake (magnitude 7.9), the last “Big One” in Southern California, ruptured approximately 225 miles along the San Andreas Fault but terminated at Cajon Pass — the gate was “closed.” In contrast, the 1812 Wrightwood earthquake ruptured through the junction, crossing both fault systems in a single through-going event — the gate was “open.”

Currently, the University of Bern reports that stress conditions suggest the gate may be “open” for a joint rupture. “So not only is it concerning that the stresses are reaching historic highs,” Burkhard explained, “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.”

A Critically Loaded System

It has been over 160 years since the last major rupture in the region. According to UCLA professor Jonathan Stewart, who studies earthquake engineering, faults on the San Andreas system typically rupture approximately every 150 years. The southernmost section of the fault has not produced a large rupture since around 1690 — more than 300 years ago.

“To the present that’s a lot more than 150 years, so there’s a lot of build-up,” Stewart told USA Today. “That doesn’t mean it’s going to happen immediately, it just means there is more stress built-up.”

Burkhard described the current state of the fault system as “critically loaded,” noting that the stress that would normally be released in large earthquakes has continued to accumulate and is now at unprecedented levels.

Infrastructure at Risk

A joint rupture of both fault systems could have devastating consequences for densely populated areas including Los Angeles, San Bernardino, Riverside, and the Coachella Valley. Major highways, railroads, and energy infrastructure running through Cajon Pass itself would be directly affected.

Perhaps the most concerning vulnerability is Southern California’s water supply. Stewart warned that a major earthquake could rupture most, if not all, of the major aqueducts bringing water into the region. “Most people will not be in a collapsed structure after this earthquake, but everybody’s going to be affected by water problems,” he said.

The last major devastating earthquake to hit California was the 1994 Northridge earthquake (magnitude 6.7) in Los Angeles, which destroyed 87,000 homes and businesses and killed more than 60 people, as The Guardian reported.

Preparedness and Next Steps

California has invested significantly in earthquake preparedness. The state’s Earthquake Early Warning system — launched in 2019 — provides vital seconds of notice through the MyShake app, Wireless Emergency Alerts, and Android Earthquake Alerts. Governor Gavin Newsom’s office has highlighted more than $33.6 million in state investments for early warning system build-out and maintenance.

Caroline Thomas Jacobs, director of the California Governor’s Office of Emergency Services (Cal OES), emphasized that while earthquakes cannot be predicted, readiness can be controlled. “Our advanced technology Earthquake Early Warning system is providing vital moments of notice for people to take lifesaving action,” she said.

Burkhard stressed that the study’s value lies not in prediction but in preparation. “What we can say is that the system is critically stressed, and that physics-based models like this one give us a clearer picture of the range of scenarios we should be prepared for,” she said. “That information matters for hazard assessments, infrastructure planning, and emergency preparedness.”

The modeling framework developed for this study is also applicable to other complex fault junctions globally, potentially improving hazard assessments worldwide. For now, the message for millions of Californians is clear: the science has never been clearer about the risk — even if it cannot say exactly when the next big one will arrive.