California’s Faults Are Under the Biggest Stress in 100 years. Is a “Big One” on the Way?

For earthquakes it always pays to be prepared

In the mountains North of Los Angeles, the San Andreas meets the San Jacinto fault system. To commuters, the pass is a route through rugged country. To seismologists, it’s appears to be a key junction where earthquakes either fade out or cause a larger disaster.

A new study concludes that this pass (called Cajon Pass) is a kind of mechanical gate. Sometimes, in the geologic past, earthquakes appear to have stopped there. At least once, in 1812, a rupture may have broken through, linking strands of the San Andreas and San Jacinto systems in a broader event.

The difference, the researchers argue, may come down to stress; and right now, there’s more stress than at any time in the past millennium.

A Thousand Years of Strain

Earthquakes usually occur along fracture zones in the Earth’s crust. Most commonly, this happens along areas where tectonic plates move past one another and become locked. When they do this, they gather more and more stress, until this is finally released in the form of an earthquake.

Well, Southern California is right alongside such a system. The two great fault systems, the San Andreas and the San Jacinto, carry most of the plate motion between the Pacific and North American plates in the region. Together, they have hosted at least 36 earthquakes of magnitude 6.4 or larger over the past thousand years.

But the southern San Andreas has been quiet in historic time. The 1857 Fort Tejon earthquake, estimated at magnitude 7.9, tore through more than 330 kilometers of the San Andreas but stopped just north of Cajon Pass. The San Jacinto has produced moderate earthquakes, but not a region-spanning rupture near the pass.

Cajon Pass sits right at the fault intersection. The San Andreas Fault runs through it. The San Jacinto Fault approaches from the south. The pass also carries highways, rail lines and energy corridors that serve the greater Los Angeles area, making it a geological and infrastructure choke point.

In a new study led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern, an international research team has modeled 1,000 years of earthquake history to estimate the present-day stress loading at Cajon Pass. 

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Modelling Stress

In the model, faults are loaded slowly by plate motion, rupture during earthquakes, then reload again.

The numbers are striking. By 2025, the model estimates Coulomb stress — a measure of how close a fault is pushed toward slipping — at 2.8 megapascals on Mojave South, 1.8 megapascals on North San Bernardino and 3.6 megapascals on San Jacinto Bernardino. The San Jacinto Bernardino value exceeds the highest modeled value on that segment in the previous 1,000 years.

This doesn’t mean that an earthquake is coming; the study says nothing about when or how big the next earthquake will be. But it shows that rocks in the area have been gathering a lot of stress, stress that eventually has to be released.

“This simulation allows us to understand how stresses in the fault system build up over centuries,” exlains Burkhard. “By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today.” The researchers show that stresses in the region are currently at their highest level in the last 1,000 years. 

An Earthquake Gate

The researchers call ruptures that stop at Cajon Pass “Gate-Closed” events. Ruptures that continue through the junction are “Gate-Open.” A rupture that links all three nearby strands would be tripartite. It would also be likely more damaging.

That possibility makes Cajon Pass especially important: under the right stress conditions, it may allow an earthquake to grow larger and more complex.

“So not only is it concerning that the stresses are reaching historic highs,” says Burkhard, “but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously — and that is a scenario with much larger consequences for the region.”

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For people living in Southern California, the basic message remains the same. The region remains earthquake country, and preparation matters.

“The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience. Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide,” says Burkhard.

The researcher emphasized that the study is not a prediction and preparedness is essential for local communities.

“What we can say is that the system is critically stressed and that physics-based models like ours give a clearer picture of the range of scenarios we should be prepared for. This information is important for hazard assessment, infrastructure planning and emergency preparedness.” 

The study was published in JGR Solid Earth.