New research published on June 3 reveals that stress levels along two segments of the San Andreas and San Jacinto faults in California have reached their highest point in at least 1,000 years. The study utilized computer simulations to analyze stress accumulation over the past millennium in areas near the San Bernardino Mountains.
Researchers found that these two fault segments are either at or have surpassed the peak stress levels observed during the 1,000-year period. While the findings do not suggest an imminent major earthquake, they underscore the long-standing warnings from seismologists that Southern California remains highly susceptible to a powerful seismic event.
Ahmed Elbanna, director of the Statewide California Earthquake Center and a professor at the University of Southern California, noted that the region has been fortunate to avoid a major urban earthquake since the 1994 Northridge event. To release the accumulated stress identified in the study, a magnitude 7 or larger earthquake would be necessary, Elbanna explained.
A quake of such magnitude would be significantly more powerful than recent tremors, such as the magnitude 5.6 earthquake in Mendocino County that triggered hundreds of thousands of early-warning alerts. That tremor caused power outages, injuries, and property damage.
Scientists estimate that an earthquake of magnitude 7 or greater could affect approximately 24 million people across Southern California, encompassing the greater Los Angeles area and the Inland Empire. Even before this latest study, experts generally agreed that the region would eventually experience another major earthquake.
"The authors themselves, and the community, do not find the findings of the study surprising," Elbanna stated, adding that the crust in Southern California is "critically stressed and ready to rupture in a large-sized event. Whether that event would happen tomorrow, or in 10 years, that’s the part that we don’t know."
The study reconstructed 1,000 years of earthquake history by examining radiocarbon-dated sediments, tree-ring records, and historical accounts. Special attention was given to Cajon Pass in San Bernardino County, a location where the San Andreas and San Jacinto faults intersect.
Liliane Burkhard, lead author of the study and a professor at the University of Bern in Switzerland, described the Cajon Pass as more than a geological feature. "It is a chokepoint for major highways, rail lines and energy infrastructure, so disruption there would have consequences well beyond the immediate shaking," she told The Sacramento Bee. "It could affect transportation and energy corridors that the broader region depends on."
Scientists refer to this area as an "earthquake gate," as it can dictate whether a fault rupture continues or ceases. Elbanna elaborated that a quake propagating through Cajon Pass would expose a larger population to shaking and increase the potential for cascading infrastructure failures.
Despite these findings, experts emphasize that California's stringent building codes are designed to withstand major seismic activity. Angie Lux, a seismologist at the Berkeley Seismology Laboratory, stated that while structural damage like cracks or falling objects may occur, building structures themselves are engineered not to collapse. Scientists continue to work on improving earthquake preparedness and warning technologies, with continued investment deemed critical.