The study was conducted by University of Hawaii at Mānoa Earth scientists. The findings assert that “direct implications for seismic hazard assessments in one of the most densely populated and infrastructure-critical corridors in the U.S.”
The Faults
Due to an “earthquake gate” at the Cajon Pass, these faults could rupture at the same time or separately, at the point where the two faults are divided from the main trace of the San Andreas fault. The Cajon Pass can stop or facilitate the movement of earthquakes between the faults, depending on the similarity of their stress levels at the time of rupture, according to researchers.
Currently, the San Andreas fault and the San Jacinto fault have similar stress levels. This could mean tectonic trouble for San Bernardino, Los Angeles, Coachella Valley, and Riverside.
Liliane Burkhard, a planetary geologist at the University of Bern in Switzerland and at the University of Hawaii at Manoa, says, “Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems.”
Over the last 1,000 years, the San Andreas and San Jacinto faults have caused 35 earthquakes of a magnitude 6.4 and greater. The last large quake in Southern California was a magnitude 7.9 in 1857. A 205-mile (330 km) segment of the San Andreas fault shifted horizontally between Parkfield and Cajon Pass. The earthquake did not propagate through Cajon Pass, but in 1812 a similar quake did. This strongly suggests it could happen again with much greater intensity and densely populated area, according to the study.
It is noted that the 1906 quake that killed 3,000 people and devastated the Bay Area did so on the northern San Andreas fault. When an earthquake occurs on one part of the fault, the pressure is not affected or relieved in the other areas where stress has built up.
The Risk at the San Andreas Fault Convergence
Burkhard and colleagues created a model replicating the major earthquakes over the last 1,000 years along the southern San Andreas and San Jacinto fault systems.
The team used observations from tree-ring records and age data from sediments displaced to reconstruct the earthquake history of the area. This information was fed into the model that simulated the accumulation, release, and propagation of tectonic stress in the faults.
The results of the study were published on June 3, 2026, in the Journal of Geophysical Research: Solid Earth. They suggest the faults are “primed” for an Earth-shattering rupture that could involve the earthquake gate at Cajon Pass and could unleash more destruction than one fault could cause on its own.
If a rupture were to occur at the point of convergence, it would create a joint breach, according to the study, and would constitute a tripartite quake. It is not clear what the chances actually are for each of these events to happen, but there is an understanding that the stress is building up within the system and could assist policymakers and planners in preparing for what is possible.
Burkhard says, “What we can say is that the system is critically stressed, and that physics-based models like this one give us a clearer picture of the range of scenarios we should be prepared for. That information matters for hazard assessment, infrastructure planning, and emergency preparedness.”
According to scientists, their model could be applied to other fault junctions and be a tool to assess hazards across the globe. She adds, “We are using rigorous, quantitative science to better understand the risk facing millions of people.”
Other Experts Speak on San Andreas Fault
Professor emeritus of geophysical and climate hazards at University College London, Bill McGuire, who was not part of the research study, told BBC Science Focus that large earthquakes “are now due for both northern (San Francisco and Bay Area) and southern (Los Angeles) California.”
He added, “The idea that such a junction at Cajon Pass acts as an ‘earthquake gate’ that controls whether one or more faults rupture during a quake is a neat one, which has important implications for building future quake scenarios.
“Also important, in terms of the timing of a future ‘big one’ in the LA region, is the finding that stress levels on these faults are at an historic high.”
Sources:
Live Science: ‘The system is critically stressed: San Andreas and San Jacinto faults scarily close to major earthquake, study finds
BBC Science Focus Magazine: Chance of mega earthquake hitting California now at a ‘historic high,’ experts warn
WYFF News 4: San Andreas fault reaches highest stress level in 1,000 years, study says
Featured Image Courtesy of Raymond Shobe’s Flickr Page – Creative Commons License
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