A new scientific study has found that Southern California’s major fault system is carrying more stress than at any point in the last 1,000 years, raising concerns about the possibility of a powerful future earthquake. Researchers have also identified a critical geological junction that could determine whether a rupture remains limited to one fault or spreads across multiple fault systems, potentially creating a much larger disaster.
The study, led by scientists from the University of Bern and published in the Journal of Geophysical Research: Solid Earth examined a millennium of earthquake activity along the San Andreas and San Jacinto fault systems. Using a physics based model, researchers reconstructed how stress has accumulated over centuries and found that current levels are at historic highs.
Earthquakes occur when stress built up in the Earth’s crust is suddenly released along fractures known as faults. In Southern California, the San Andreas Fault and the San Jacinto Fault accommodate much of the region’s tectonic movement.
To better understand how these faults behave over long periods, researchers created a four dimensional earthquake cycle model that simulated fault activity across 1,000 years. The model incorporated geological evidence, including radiocarbon dating, tree-ring records and historical observations of ground ruptures.
The simulations revealed that tectonic stress across the southern San Andreas fault system has reached levels not seen during the entire period studied. Scientists say the long gap since the magnitude 7.9 Fort Tejon earthquake in 1857 has allowed stress to continue accumulating beneath the region.
One of the most significant findings of the study involves Cajon Pass, located northeast of Los Angeles, where the San Andreas and San Jacinto faults come close together.
Researchers describe the area as an “earthquake gate” because it may determine whether a future earthquake remains confined to a single fault or jumps across both fault systems.
Historical evidence suggests both scenarios have occurred in the past. The 1857 Fort Tejon earthquake stopped at Cajon Pass, while the 1812 Wrightwood earthquake is believed to have crossed the junction and ruptured both fault systems.
According to the study, the likelihood of a multi-fault rupture depends not only on how much stress exists on individual faults but also on how closely stress levels match between them. When both faults are highly stressed at similar levels, the chances of a rupture spreading across the junction increase significantly.
Researchers found that stress levels on key fault segments near Cajon Pass are now exceptionally high and approaching conditions associated with past large scale earthquakes.
A major earthquake involving both the San Andreas and San Jacinto faults could have far reaching consequences for Southern California. Densely populated regions such as Los Angeles, Riverside, San Bernardino and the Coachella Valley could experience stronger and more widespread shaking.
Critical infrastructure located around Cajon Pass, including transportation corridors, rail lines and energy networks, could also be affected.
Scientists emphasize that the study does not predict when an earthquake will occur. However, it provides a clearer picture of current fault conditions and highlights the importance of preparedness.
The researchers say their findings can help improve hazard assessments, infrastructure planning and emergency response strategies. They also note that the modelling framework could be applied to other complex fault systems around the world.
While no one can predict the timing of the next major earthquake, the study suggests that Southern California’s fault network is currently in one of the most stressed states seen in at least a millennium, making continued monitoring and preparedness more important than ever.