A new scientific study has found that parts of Southern California's most active fault systems are carrying some of the highest levels of accumulated stress recorded over the past thousand years, highlighting the region's ongoing risk of a powerful earthquake.

The research, published on June 3, analyzed how tectonic stress has developed over the last millennium along three sections of the San Andreas and San Jacinto faults near the San Bernardino Mountains. Using advanced computer modeling alongside geological evidence, researchers concluded that two of the three fault segments have reached—or even surpassed—the highest stress levels identified during the study period.

Although the findings do not indicate that a major earthquake is imminent, scientists say they reinforce the long-standing understanding that Southern California remains vulnerable to a large seismic event.

Ahmed Elbanna, director of the Statewide California Earthquake Center and a professor of Earth sciences and engineering at the University of Southern California, explained that the amount of accumulated stress would likely require an earthquake measuring magnitude 7 or greater to be released.

For comparison, a magnitude 7 earthquake would release more than 100 times the energy of the recent magnitude 5.6 earthquake that struck Mendocino County. That event activated approximately 657,000 alerts through California's MyShake early-warning system, temporarily cut electricity to around 8,000 Pacific Gas & Electric customers, caused several injuries, and resulted in property damage as merchandise fell from store shelves.

According to the study, a magnitude 7 or stronger earthquake could affect nearly 24 million residents across Southern California, including the greater Los Angeles metropolitan area and the Inland Empire.

Researchers emphasized that the latest findings align with seismologists' long-held beliefs. Southern California's fault network is already under significant stress, but current science cannot determine exactly when the next major rupture will occur. It could happen in the near future or many years from now.

To reconstruct the region's seismic history, the research team combined evidence from radiocarbon-dated sediments, tree-ring data, and historical earthquake records spanning roughly 1,000 years.

Particular attention was given to Cajon Pass in San Bernardino County, where the San Andreas and San Jacinto fault systems intersect. Lead researcher Liliane Burkhard of the University of Bern noted that the location is not only geologically important but also serves as a vital transportation and infrastructure corridor.

Major highways, railway lines, and energy networks pass through Cajon Pass, meaning a significant earthquake there could create widespread disruptions extending far beyond the immediate area affected by ground shaking.

Scientists often refer to the area as an "earthquake gate" because it can influence whether a rupture continues through the fault system or stops. If an earthquake were to propagate through this region, it could expose larger populations to intense shaking while increasing the likelihood of disruptions to transportation, utilities, and other essential services.

Despite these concerns, experts stress that California's modern building regulations are designed to improve safety during major earthquakes. While strong shaking can still cause cosmetic damage, broken utilities, and falling objects, newer structures are generally engineered to reduce the risk of catastrophic collapse.

Researchers conclude that although the timing of the next major Southern California earthquake remains impossible to predict, continued investment in earthquake preparedness, resilient infrastructure and early-warning technology remains essential to reducing future risks.