San Andreas and San Jacinto stress tops 1,000 years

New research from the University of Hawaii at Manoa says stress along the San Andreas and San Jacinto faults in Southern California has reached the highest levels in 1,000 years, with scientists pointing to the Cajon Pass as a potential “earthquake gate” that

For people living across Southern California, the ground beneath them is no longer just “moving”—it’s building pressure.

A new study from the University of Hawaii at Manoa says stress along the San Andreas and San Jacinto faults has reached the highest levels in 1. 000 years. The work focuses on two of the region’s major fault systems and what that combined stress could mean for seismic hazard planning in an area packed with homes. roads. and critical infrastructure.

Scientist Liliane Burkhard said the region “may be capable of a large through-going rupture involving both fault systems.” In the same research. she also described a key feature along the fault network: researchers found that Cajon Pass may act as an “earthquake gate.” Depending on conditions. she said. it can sometimes block large ruptures from crossing between the faults—and sometimes allow them to pass through and involve both systems in a single event.

To reach those conclusions, researchers built a physics-based model simulating stress build up on the two fault systems, including at Cajon Pass. They then fed the model 1,000 years of earthquake history from the region to estimate how much stress has accumulated.

Burkhard said the current state is unusually tense: “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’s findings track a pattern that is especially stark for a region used to studying seismic risk in terms of probabilities. Researchers found that stress normally released in large earthquakes has continued to grow—rising to levels they describe as unprecedented. They also found that Cajon Pass could contribute to a joint rupture of both faults simultaneously.

The potential consequences, the study says, would reach well beyond the fault traces themselves. Densely populated areas that could be significantly affected include Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

The San Andreas Fault is the primary boundary between the Pacific and North American plates. It runs up the Peninsula and stretches about 650 miles, with other faults, including the Hayward Fault, branching off it.

Between the vast length of the San Andreas system and the possibility of a combined event with the San Jacinto. the central question becomes less about whether stress exists—and more about how it finally releases. The study places Cajon Pass at the center of that decision point. describing it as a gate that can either keep ruptures separate or help connect them into something larger.

No one has to live on a fault line to feel the impact of what the ground stores. But according to this research, the region may be carrying more of that stored pressure than at any time in a millennium—while the last major rupture is now more than 160 years in the past.

San Andreas Fault San Jacinto Fault Cajon Pass earthquake stress 1 000 years study Southern California seismic risk Liliane Burkhard Los Angeles earthquake hazard San Bernardino Riverside Coachella Valley