A physics-based 4D simulation reveals that the San Andreas and San Jacinto fault systems have reached historically unprecedented levels of tectonic stress, increasing the risk of a massive, multi-fault megaquake.
For over a century, the southern San Andreas Fault System has remained unsettlingly quiet. However, beneath the surface of Southern California, tectonic forces have been steadily building. A groundbreaking study published in the Journal of Geophysical Research: Solid Earth warns that the region is now critically stressed, with tectonic stress levels hitting a 1,000-year high.
The Cajon Pass “Earthquake Gate”
At the heart of this seismic threat lies the Cajon Pass—a complex geological junction where the San Jacinto Fault splits from the main trace of the San Andreas Fault. Scientists refer to this junction as an “earthquake gate.” Depending on the mechanical conditions at the time of a rupture, this gate can either halt a propagating earthquake or allow it to pass through, linking multiple faults into a single catastrophe.
Historically, the gate behaves inconsistently:
- The 1857 Fort Tejon Earthquake (Mw 7.9): Initiated in the north but stopped dead at Cajon Pass (“Gate-Closed”).
- The 1812 Wrightwood Earthquake (Mw sim. 7.5): Successfully breached the junction, tearing through both fault systems (“Gate-Open”).
Aligning Stress Fields
Led by Liliane Burkhard, a planetary geologist at the University of Bern and the University of Hawaiʻi at Mānoa, the research team built a 4D earthquake cycle simulation replicating the last millennium of major seismic history. By utilizing paleoseismic data like sediment displacement and tree-ring records, the model tracked how stress accumulates, transfers, and releases.
The model revealed a critical physical rule: when the stress disparity between the San Jacinto and the San Andreas segments narrows, the faults are highly likely to rupture jointly.
Right now, that disparity is dangerously low. Estimated Coulomb stress has reached 2.8 MPa on the Mojave South segment and an unprecedented 3.6 MPa on the San Jacinto Bernardino segment. Because both systems are highly, comparably loaded, the “earthquake gate” is effectively unlocked.
Two Looming Scenarios
The current stress configuration could unleash two types of complex ruptures through Cajon Pass:
- Joint Rupture: An earthquake that links and breaks the two main sub-segments of the San Andreas Fault across the pass.
- Tripartite Rupture: A severe chain-reaction event involving both branches of the San Andreas Fault and the San Jacinto Fault simultaneously.
A tripartite event would unleash significantly more destruction than any single-fault earthquake, threatening heavily populated areas including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
Preparing for the Unpredictable
While physics-based models cannot predict the exact calendar date of the next quake, they confirm the system is at a breaking point.
“What we can say is that the system is critically stressed,” Burkhard noted, emphasizing that this quantitative data is vital for infrastructure planning, regional hazard assessments, and emergency preparedness. With millions of lives relying on the infrastructure crossing Cajon Pass, understanding this 1,000-year stress peak is no longer just a geological curiosity—it is a matter of public safety.
Sources:
Burkhard, L. M. L., Smith‐Konter, B. R., Scharer, K. M., & Sandwell, D. T. (2025). Cajon Pass and the Southern San Andreas Fault System: Earthquake cycle stress Accumulation and Present-Day Loading. Journal of Geophysical Research Solid Earth, 131(6). https://doi.org/10.1029/2025jb033213
