Critically loaded faults heighten California’s ‘Big One’ stakes

critically loaded – A new study finds the San Andreas and San Jacinto fault systems are under the highest stress levels seen in 1,000 years. Researchers warn the Cajon Pass junction could allow a joint rupture—potentially far more damaging than a single-fault event—while emphasiz

For Californians, the fear doesn’t come from a distant headline—it comes from the ground beneath daily life. Maps and models are now adding sharper edges to that dread. pointing to fault systems described as “critically loaded” and warning that the next major rupture could be worse than a single fault event.

The stakes are rooted in history. California’s most significant recorded seismic event was a magnitude 7.9 earthquake in 1857 that ruptured about 225 miles on the San Andreas Fault. Dubbed the last “Big One,” it killed two people and produced shaking that lasted between one and three minutes.

Since then, California’s population has expanded dramatically, and large earthquakes have continued to rattle the state. Researchers have also maintained a more persistent message: another massive earthquake in Southern California is likely—but the timing remains unknown.

A recent study reports that warning signs have continued to grow in 2026. The San Andreas and San Jacinto fault systems have reached the highest stress levels seen in the past 1,000 years, according to a study conducted at the University of Hawaiʻi at Mānoa.

The San Andreas fault runs throughout the state, passing through San Francisco and San Bernardino. The San Jacinto fault is in Southern California, running through Riverside, San Diego and Imperial Counties.

The danger isn’t just that a rupture could come—it’s how much stress has built up. Faults rupture on the San Andreas Fault approximately every 150 years, according to UCLA professor Jonathan Stewart, who studies earthquake engineering. But it has been over 300 years since the last “large rupture” occurred south of the 1857 Fort Tejon earthquake. Stewart said, “As far we know it didn’t produce a large rupture since around 1690. To the present that’s a lot more than 150 years, so there’s a lot of build-up. That doesn’t mean it’s going to happen immediately, it just means there is more stress built-up.”.

That additional stress matters because it can translate into greater damage when shaking finally arrives. One of Stewart’s major concerns is the state’s water systems.

“An earthquake like this would rupture most, if not all of the major aqueducts bringing water into Southern California,” he said. “Most people will not be in a collapsed structure after this earthquake, but everybody’s going to be affected by water problems.”

Researchers say the new work helps explain not only where risk sits, but how it may interact across fault lines. To investigate the probability of an earthquake occurring. the study used a physics-based simulation and fed it the earthquake history from the region. The goal was to estimate how much stress has built up along the southern San Andreas and San Jacinto fault systems.

The findings point to an ominous window of vulnerability. Lead author Liliane Burkhard 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.”

More troubling still is a specific geographic junction: the Cajon Pass, where the San Andreas and San Jacinto faults meet. The study suggests that this location could facilitate a joint rupture of the two faults. In that scenario, the study says it would probably be “significantly more damaging” than a single-fault event.

What makes those maps so consequential is what they’re designed to measure. Seismic hazard assessments are critical for millions of Californians living in densely populated areas that are also earthquake-prone.

Seismic Hazard maps, the U.S. Geological Survey explains. show the relative hazard associated with earthquakes using information on past faults. the behavior of seismic waves and the near-surface conditions of specific locations. Larger values correspond to stronger shaking. A peak ground velocity of 269 cm/sec would correspond to extreme shaking and heavy damage.

For a reference point, the largest ground velocity recorded in Taiwan’s 1999 Chi-Chi earthquake reached 318 cm/sec with a magnitude of 6.7. That earthquake killed more than 2,000 people and incurred approximately $14 billion in damage.

Understanding how these faults move also clarifies why the consequences could spread quickly through the built environment. The San Andreas and San Jacinto faults are strike-slip faults, which typically cause horizontal displacement. Together, both faults mark the geologic boundary between the Pacific Plate and the North American Plate.

In a major earthquake on the San Andreas fault, surface rupture is likely—meaning a fault breaks through to the surface. Most earthquakes, however, do not produce surface rupture, according to the USGS.

Even without surface rupture, strong shaking can still be devastating. The danger is greatest near the fault and in areas built on soft or water-saturated soils, where shaking can be amplified. Rupture can also directly offset roads, buildings and other structures that span the fault trace.

The research may sharpen the public understanding of risk, but it comes with a boundary the scientists are careful to keep in place: the work is not a calendar. Burkhard emphasized it shouldn’t be treated as a forecast.

“This is not a prediction of when an earthquake will happen,” she said. “However, studies like this are important contributions to national and global earthquake hazard research in that we are using rigorous, quantitative science to better understand the risk facing millions of people.”

For Californians, that distinction—risk versus prediction—isn’t reassurance so much as it is a reality check. The facts point to something already measurable: stress at historically high levels. more time than the usual rupture rhythm. and a fault junction that could complicate the pattern of what breaks when it finally does break.

California earthquake risk San Andreas Fault San Jacinto Fault critically loaded faults Cajon Pass joint rupture University of Hawaiʻi at Mānoa study seismic hazard maps U.S. Geological Survey peak ground velocity