A new scientific study finds the critically stressed San Andreas and San Jacinto faults in Southern California are carrying more stress than at any point in the last 1,000 years—suggesting a major earthquake is more likely now than at any time in that period.
The ground may be quiet, but beneath Southern California, pressure has been building for centuries—and a new study says it’s reached a point the region hasn’t seen in a thousand years.
Researchers say a major earthquake is more likely now than at any point in the last 1. 000 years along the “critically stressed” San Andreas and San Jacinto faults. The basic idea is simple and unsettling: as stress accumulates on a fault over long stretches of time. it has to be released. And scientists now estimate that the San Andreas and San Jacinto faults are under more stress than at any point in the last 1. 000 years—meaning a massive earthquake could be on the way.
“Because it’s been quite a long time since the Southern San Andreas or the San Jacinto have had a large earthquake, we’ve accumulated a lot of stress,” said Kate Scharer, a co-author of the study and a geologist with the U.S. Geological Survey.
The study’s evidence is not a hunch. Using geological records—including tree-ring records and sediment samples—a team of scientists built a computer model to show how pressure accumulates along faults over time. They then ran that model forward to estimate how much stress is building beneath the region today.
Their conclusion points back to 1857, when one of California’s largest seismic events on record struck. The scientists found that pressure has been gradually building since that “last Big One in 1857.”
Harold Tobin. director of the Pacific Northwest Seismic Network and a professor at the University of Washington who was not involved in the study. described the work as a step forward in how clearly the risk can be quantified. “The idea that all of those segments of the fault could have enough stress for an imminent future earthquake was already there. ” he said. “This [study] puts it on more of a quantitative, rigorous scientific basis.”.
One place drawing special attention is Cajon Pass, the narrow corridor between the San Gabriel and San Bernardino mountains. Liliane Burkhard. the lead author of the study and a research affiliate in the Hawaiʻi Institute of Geophysics and Planetology. said the pass could act as an “earthquake gate.” In her account. it would function like a junction that either stops or transmits large ruptures between the San Andreas and San Jacinto faults depending on stress conditions.
That detail matters because it shapes how far an earthquake’s damage could spread. Burkhard said a major earthquake could jump from one fault system to another through Cajon Pass. letting a rupture potentially spread farther across Southern California and affect millions more people across the Coachella Valley and San Bernardino County.
She also said she hopes to expand the work by studying other earthquake-prone regions where several fault systems interact—areas where the risks can be harder to predict.
Between the science and daily life, the message that keeps landing is preparation. Kate Scharer of the USGS framed the finding as a reminder of what many residents already feel but don’t always plan for. “This study was a great reminder that in Southern California. where we have parts of the most densely populated regions in the country. we are living on a multi-strand fault system. ” said Ahmed Elbanna of USC.
For households, the basics are practical: have an emergency kit with at least 72 hours of food, water and medications. If cellphone networks fail immediately after a major earthquake, the guidance is to have a communication and reunification plan. People are also urged to know their evacuation routes.
Scharer also recommends resources from the Earthquake Country Alliance.
Ahmed Elbanna. director of the Statewide California Earthquake Center and a professor at USC who was not involved in the study. stressed that the question isn’t whether another major earthquake will happen—it’s when. “It could happen today, tomorrow, or in 10 years, or in 30 years,” he said. “On geological time scales, these are all very short.”.
“It’s a question of when, not if,” Scharer added.
“We should certainly expect to experience large earthquakes in our lifetimes,” Scharer said.
For now, that’s the tension the study leaves behind: the timing can’t be pinned down to a date on a calendar, but the stress—measured through evidence, modeled through centuries—has never looked more urgent.
San Andreas fault San Jacinto fault earthquake study Southern California USGS Cajon Pass earthquake preparation 1857