A new study finds that tectonic stress along Southern California's San Andreas and San Jacinto faults has reached levels unseen in the past millennium. Researchers identified Cajon Pass as an earthquake gate that could allow a rupture to spread across both systems. The findings, based on a physics-based model, highlight conditions that historically preceded major multi-fault events.
The study, led by Dr. Liliane Burkhard at the University of Bern, reconstructed 1,000 years of earthquake activity using geological records. It shows current stress at 3.6 MPa on the San Jacinto-Bernardino section and 2.8 MPa on the Mojave South section of the San Andreas Fault. These similar high levels match patterns seen before large ruptures that crossed the junction.
Cajon Pass functions as an earthquake gate, where stress conditions determine whether a quake stays on one fault or propagates through both. Historical examples include the 1857 Fort Tejon event, which stopped at the pass, and the 1812 Wrightwood quake, which crossed it.
Burkhard emphasized that the work is not a prediction of timing. Instead, it supports hazard assessment, infrastructure planning, and emergency preparedness across the Los Angeles region and surrounding areas.
