Southern California’s earthquake faults have reached their most stressed state in a millennium and solar farms blanketing the region could face consequences no renewable energy planner has fully modeled
Beneath Southern California, two of the country’s most consequential fault systems have been quietly loading for generations. The San Andreas and San Jacinto faults together absorb much of the tectonic motion pushing through the region — and the last comparable major earthquake near Los Angeles was the magnitude 7.9 Fort Tejon event in 1857, nearly 170 years ago. A new physics-based simulation now suggests that stress along both systems has reached its highest point in 1,000 years of modeled history — with significant implications for the infrastructure blanketing the region.
A 1,000-year simulation with a striking result
The team behind this finding is international. Dr. Liliane Burkhard of the University of Bern led the research, with collaborators from the University of Hawaiʻi at Mānoa, the U.S. Geological Survey Earthquake Science Center in Pasadena, and the Scripps Institution of Oceanography at UC San Diego.
Their tool was a physics-based, four-dimensional earthquake cycle model — one that tracks fault behavior across three dimensions of space while following how the system evolves through time. To anchor it in real history, the team assembled a 1,000-year record of past earthquakes drawn from radiocarbon dating, tree-ring anomalies, and historical ground-rupture records.
Cajon Pass doesn’t just block or redirect ruptures — it responds dynamically to the stress state of the faults around it, and that response can cut either way.
The model traces how each earthquake reshuffles stress onto neighboring fault segments, how stress accumulates during quiet intervals, and how the deeper crust slowly relaxes after large ruptures. Run forward to the present, it produces a clear result: stress levels across parts of the system have reached — and in some places exceeded — the highest values seen anywhere in the entire 1,000-year run.
What makes Cajon Pass an ‘earthquake gate’
Cajon Pass sits northeast of Los Angeles, where the San Andreas and San Jacinto fault systems draw close together in a geometrically complicated zone. It’s already a well-known landmark for infrastructure planners — highways, rail lines, and energy corridors all thread through it. This study reframes it as something geologically significant in its own right.
The researchers call it an “earthquake gate.” That label captures something a simple barrier analogy misses. Cajon Pass doesn’t just block or redirect ruptures — it responds dynamically to the stress state of the faults around it, and that response can cut either way.
History illustrates both outcomes. The 1857 Fort Tejon earthquake stopped at Cajon Pass and never crossed onto the San Jacinto Fault. The 1812 Wrightwood earthquake did the opposite, passing through the junction and rupturing both systems in a single through-going event. The difference, the researchers suggest, came down to the stress conditions present at the time.
Why both faults being stressed together changes the picture
Stress on a single fault tells only part of the story. What the model reveals is that the relationship between stress levels on the two systems matters just as much — perhaps more.
Right now, the San Jacinto-Bernardino section carries modeled stress of 3.6 MPa, the highest value recorded anywhere across the entire 1,000-year simulation. The adjacent Mojave South section of the San Andreas fault sits at 2.8 MPa. Both segments are simultaneously carrying unusually elevated loads.
That simultaneous loading pattern is what concerns the researchers most. When both faults are highly stressed at the same time, the model shows conditions becoming more favorable for a rupture to cross Cajon Pass and pull both systems into a single event. The current configuration resembles what the model shows just before past through-going ruptures — a point Burkhard describes as especially concerning.
What a joint rupture could mean for the region
A rupture crossing both fault systems would affect a far larger area than any single-fault earthquake. Greater Los Angeles, San Bernardino, Riverside, and the Coachella Valley all fall within the potential impact zone — some of the most densely populated and infrastructure-dependent terrain in the country.
Cajon Pass compounds that concern. Highways, rail lines, and energy systems converge there, so damage to the junction itself could ripple outward well beyond the immediate shaking zone. Emergency planners have long treated the pass as a logistical chokepoint, and this study gives them a physics-based reason to think carefully about what a through-going rupture would mean for those systems.
The researchers also note that their modeling framework isn’t limited to California. Complex fault junctions exist worldwide, and the same approach could help assess stress states and rupture scenarios in other high-risk regions.
High stress is not the same as an imminent earthquake
The study’s authors are direct on this point: nothing in their findings predicts when an earthquake will occur. Current science can’t extract a date or a timeline from stress measurements alone, and the researchers don’t claim otherwise.
What the model provides is a clearer picture of which rupture scenarios are physically plausible right now — information aimed at hazard assessment, infrastructure planning, and emergency preparedness rather than public alarm.
Burkhard puts it plainly: the system is critically stressed, and physics-based models like this one define the range of scenarios that planners and engineers should be ready for. As that modeling framework improves, the next step will be translating these stress portraits into concrete guidance — for building codes, emergency corridors, and the expanding solar installations that Southern California is counting on for its future.
All the information is available through the complete version of the study: Liliane M. L. Burkhard, Bridget R. Smith‐Konter, Katherine M. Scharer, David T. Sandwell. Cajon Pass and the Southern San Andreas Fault System: Earthquake Cycle Stress Accumulation and Present‐Day Loading. Journal of Geophysical Research: Solid Earth, 2026; 131 (6) DOI: 10.1029/2025JB033213
Daniel García is an Editor-in-Chief with strong expertise in structural work and engineering principles. He combines this technical foundation with deep knowledge of energy, spatial design, and emerging technologies, bringing a forward-thinking and analytical approach to editorial leadership.
