Megapascals (MPa) measure pressure or mechanical stress. In geological terms, the values reported here are unusually high for this fault network.

What is an 'earthquake gate'?

Cajon Pass is the narrow zone where the San Andreas and San Jacinto faults run close together. Past events show two outcomes:

• The magnitude 7.9 Fort Tejon earthquake of 1857 ruptured the San Andreas but stopped at Cajon Pass.
• The Wrightwood earthquake of 1812 crossed the junction and broke both fault systems in a single through-going event.

"The earthquake gate concept captures something important about how fault junctions work," Burkhard said. "Cajon Pass doesn't simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries."

The model suggests Cajon Pass is more likely to let a rupture cross when both adjacent segments are heavily loaded at the same time.

Why would a joint rupture matter?

The two modeled stress readings — 3.6 MPa and 2.8 MPa — sit close together, and both are near or above the maximum values seen anywhere in the 1,000-year simulation. According to the researchers, that pattern resembles preconditions in the model for past through-going ruptures.

"So not only is it concerning that the stresses are reaching historic highs," Burkhard said, "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."

A rupture involving both faults could shake metropolitan Los Angeles, San Bernardino, Riverside, and the Coachella Valley at once. Cajon Pass itself carries major highway, rail, and energy infrastructure.

Does this mean an earthquake is coming soon?

No — and Burkhard is explicit about that point.

"The study is not a prediction of when an earthquake will occur," she said. "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 last comparable earthquake in the region was the 1857 Fort Tejon event, leaving more than 165 years of accumulated strain on parts of the system.

A broader framework

The collaboration also drew on scientists 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.

"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," Burkhard said. "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."