Scientists have discovered that a major earthquake zone in Southern California may be carrying more built-up stress than at any time in the past 1,000 years.
While this does not mean a large earthquake is about to happen, researchers say the findings highlight the importance of being prepared.
Earthquakes occur when stress builds up along cracks in Earth’s crust called faults. In Southern California, two of the most important faults are the San Andreas Fault and the San Jacinto Fault.
These faults help accommodate the movement of large tectonic plates beneath the Earth’s surface.
A particularly important area lies northeast of Los Angeles at a place called Cajon Pass.
Here, the two fault systems come close together, creating a complex junction. Scientists believe this location can determine whether a future earthquake stays on one fault or spreads across both fault systems, creating a much larger event.
In a new study, researchers from the University of Bern and several U.S. institutions created a detailed computer model to reconstruct 1,000 years of earthquake activity in the region.
They used evidence from geological records, including radiocarbon dating, tree-ring data, and historical reports of past earthquakes.
The model allowed the team to estimate how stress has built up and changed over time. It also tracked how previous earthquakes affected nearby fault sections and how the Earth’s crust slowly recovered after major events.
According to the study, stress levels in the region have now reached some of the highest values seen during the entire 1,000-year period examined.
One of the most important findings is the idea of Cajon Pass acting as an “earthquake gate.” Rather than simply stopping or allowing earthquake ruptures, the junction appears to respond to changing stress conditions over long periods of time.
Researchers found that the key factor is not just the amount of stress on a single fault. Instead, what matters is whether both fault systems are becoming stressed at similar rates.
When stress levels on both faults rise together and reach similarly high levels, the chance of a rupture spreading across both systems increases.
Historical earthquakes show that both scenarios are possible. The massive Fort Tejon earthquake of 1857 stopped at Cajon Pass and did not continue onto the San Jacinto Fault. However, the Wrightwood earthquake of 1812 appears to have crossed the junction and involved both fault systems.
Today, the stress levels on key sections of both faults are unusually high and relatively similar. According to the researchers, this is the type of pattern that has preceded larger, connected ruptures in the past.
A joint rupture involving both faults could have serious consequences. The affected area includes the greater Los Angeles region, San Bernardino, Riverside, and the Coachella Valley. Major transportation routes, rail lines, and energy infrastructure also pass through Cajon Pass.
The researchers stress that their study is not a prediction of when an earthquake will occur. Instead, it provides a clearer picture of current conditions and helps scientists better understand the range of possible future scenarios.
They hope the findings will support earthquake planning, infrastructure protection, and emergency preparedness efforts in one of America’s most heavily populated regions.
