A major new study reports that the junction of the San Andreas and San Jacinto fault systems—located just northeast of Los Angeles, California—has reached its highest tectonic stress levels in 1,000 years, raising concerns about the potential for a large earthquake in one of the nation’s most populated areas.

Researchers at the University of Hawaii at Mānoa, using advanced computer simulations and a millennium of earthquake records, found that both fault systems are now in what they call a “critically loaded state.” Lead scientist Liliane Burkhard explained that stress has accumulated to unprecedented levels since the last major earthquake over a century ago. She said, “Right now, with stress at historically high levels across the region and more than 160 years elapsed since the last major rupture, the system is in a critically loaded state.”

The study, published in the Journal of Geophysical Research: Solid Earth, highlights Cajon Pass as a critical junction where the two faults meet. This area, about 50 miles from downtown Los Angeles, acts as an “earthquake gate.” Researchers say the pass could sometimes block large ruptures or allow both faults to slip in a single, massive event. If a rupture crosses both faults, it could trigger a far larger and more damaging earthquake than if only one fault was involved, with possible impacts on Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

The research team did not predict when such an earthquake might happen, but emphasized that the findings are important for earthquake hazard planning. The results indicate that modern building codes may help lessen damage, but a joint rupture could still be catastrophic for southern California.

The study also underscores the need for ongoing seismic hazard assessments and preparedness in the region. Researchers hope that their physics-based modeling approach will help inform emergency planning and infrastructure decisions, and could be used to study other complex fault zones worldwide.