Tuesday, 28 July 2026, 13:18
New research reveals that stress along two of California's most dangerous fault systems has reached levels not seen in the past 1,000 years, increasing the likelihood of a major earthquake in Southern California. The study, published June 3 in the Journal of Geophysical Research: Solid Earth, focuses on the southern San Andreas and San Jacinto faults, which converge near Cajon Pass in San Bernardino County.
California's seismic risk is once again in the spotlight after a new study found that stress along the southern San Andreas and San Jacinto faults is at its highest point in a millennium. The findings, which reinforce what many seismologists already suspected, suggest the region is primed for a large earthquake that could impact millions of people across Southern California.
Scientists led by Dr. Liliane Burkhard of the University of Bern built a physics-based simulation modeling stress accumulation over the past 1,000 years. The model incorporated data from radiocarbon dating, tree-ring anomalies, and historical accounts of ground ruptures. The results show that stress on two of three fault segments studied has surpassed historical levels, indicating that the crust is "critically stressed and ready to rupture in a large-sized event," according to Ahmed Elbanna, director of the Statewide California Earthquake Center. "We have been lucky in California not to experience a large urban earthquake since 1994 on Northridge," Elbanna told The Sacramento Bee. "In order to release the stresses, the stress levels that we are talking about in this study, we need a magnitude 7 or larger earthquake." A quake of that size would be more than 125 times stronger than last week's magnitude 5.6 earthquake in Mendocino County.
Cajon Pass, where the San Andreas and San Jacinto faults meet in San Bernardino County, is described as an "earthquake gate." This junction can either stop a rupture or allow it to propagate across both fault systems, amplifying the damage. Past earthquakes illustrate both scenarios: the 1857 Fort Tejon earthquake stopped at Cajon Pass, while the 1812 Wrightwood earthquake ruptured both faults. A rupture that passes through Cajon Pass could expose more people to shaking and disrupt critical transportation and energy corridors. Lead author Liliane Burkhard emphasized that the pass is more than a geological hotspot: "It is a chokepoint for major highways, rail lines and energy infrastructure, so disruption there would have consequences well beyond the immediate shaking. It could affect transportation and energy corridors that the broader region depends on."
While the study focuses on Southern California, the entire state faces seismic threats. A magnitude 5.6 earthquake in Mendocino County last week served as a reminder, triggering nearly 657,000 early-warning alerts via the MyShake app. That quake injured several people, knocked out power to about 8,000 customers, and caused thousands of dollars in damage. The new research underscores that a much larger event—potentially magnitude 7 or higher—could threaten nearly 24 million people in areas like greater Los Angeles and the Inland Empire.
The study provides a numerical estimate of stress levels, confirming that Southern California's faults are overdue for a major earthquake. While scientists cannot predict exactly when a quake will occur, the research highlights the importance of preparedness. Despite the warning, experts emphasized that California's strict building codes are designed to withstand major earthquakes. "You might see cracks or have things falling down, but the structures themselves should not fail," Angie Lux, a seismologist at the Berkeley Seismology Laboratory, told The Bee. Residents are urged to review earthquake safety plans and ensure their homes and workplaces are retrofitted to withstand strong shaking.
Scott Brandenberg, a professor of civil and environmental engineering at UCLA, noted that the region's tectonic activity is part of why California is so scenic. "Part of why we have mountains and it's such a beautiful place to live, is because we have a plate boundary, and there are earthquakes," he said. "We just need to know about that and be ready." Jonathan Stewart, the Sabol-Scott Term Chair in Civil and Environmental Engineering at the UCLA Samueli School of Engineering, added that the study helps people understand how faults work but does not mean people are less safe than before. The UC Seismic Safety Program is working with campuses to identify and address vulnerable buildings, though cost remains a major barrier. The LA City Council passed a seismic retrofit law in 2015, mandating improvements for older structures. Strengthening buildings also has sustainability implications, as tearing down and rebuilding consumes resources. Limited state funding has been a challenge, with estimated construction costs for seismic improvement across all UC campuses at $6.7 billion. UCLA's off-campus buildings, especially smaller medical clinics, have less earthquake support. Alton Wu, a rising fourth-year civil engineering student, said he is not concerned because he has seen firms fortify vulnerable buildings, but he noted that some firms are reactive rather than proactive. Experts also emphasized the importance of preparedness: having enough drinking water, securing furniture, and storing canned food. "Studies like this one that show that the stresses are actually pretty high based on the modeling … and maybe bring earthquake safety to the forefront again," Brandenberg said.
