A team led by Dr. Liliane Burkhard at the University of Bern built a physics-based, four-dimensional earthquake cycle model for the southern San Andreas and San Jacinto fault systems. The model incorporated a 1,000-year record of past earthquakes assembled from radiocarbon dating, tree-ring patterns, and historical accounts of surface ruptures. The results, published in the Journal of Geophysical Research: Solid Earth, indicate that tectonic stresses in parts of the system have reached, and in some places surpassed, the highest values seen anywhere in the model's 1,000-year history.
The study describes Cajon Pass, northeast of Los Angeles, as an "earthquake gate." The junction does not act as a simple barrier: depending on the stress state of surrounding faults, a rupture may stop there or continue from one fault system into the other. Past events show both possibilities. The 1857 Fort Tejon earthquake stopped at Cajon Pass, while the 1812 Wrightwood earthquake passed through the junction and ruptured both systems in a single event.
According to the model, the San Jacinto-Bernardino section currently carries 3.6 megapascals of stress, higher than any value reached elsewhere in the simulation. The nearby Mojave South section of the San Andreas fault is at 2.8 MPa. Because both segments are unusually highly stressed at the same time, the researchers say conditions may be more favorable for a rupture to cross Cajon Pass and continue through both systems. The study does not predict when an earthquake will happen.