New geological research reveals that the Santa Cruz Mountains segment of the northern San Andreas Fault is slipping faster than previously estimated, raising the probability of seismic activity. Concurrently, separate studies indicate that stress levels on the fault’s southern stretches near Cajon Pass have reached highs over a 1,000-year span, amplifying concerns about major earthquakes across California.
Faster Fault Movement in the Santa Cruz Mountains
A section of the San Andreas Fault running through the Santa Cruz Mountains south of San Francisco is moving faster than earlier estimates suggested. According to research conducted by Kim Blisniuk, an earthquake geologist and geology professor at San Jose State University, the slip rate across this northern portion of the fault did not decrease as scientists previously hypothesized, but instead remained consistent from the North Coast section down to the Santa Cruz Mountains.
Previous estimates of slip rates in the region typically spanned windows of up to 1,000 years, while Blisniuk’s unreleased study extends the timeline across the last 10,000 years. Faster-moving faults generally accumulate stress more rapidly, which leads to more frequent earthquakes and potentially higher magnitude events when that stored energy finally releases.
To calculate these slip rates, Blisniuk and her students worked along the San Andreas Trail in Sanborn County Park, situated roughly 20 miles northwest of the epicenter of the 1989 Loma Prieta earthquake. The research team utilized drones equipped with laser pulses to map stream paths displaced by historical tectonic shifts. They then gathered sandstone boulder samples and analyzed them at San Jose State University, Stanford University, and Lawrence Livermore National Laboratory.
By measuring the concentration of beryllium-10—an isotope created when cosmic rays bombard quartz-bearing rocks exposed to the air—the team determined the age of the rock formations and calculated how far past earthquakes had shifted the terrain over millennia. The United States Geological Survey declined to comment on the findings because the research is not yet published.
Stress Levels Along Southern Fault Segments
While northern sections face questions over slip rates, researchers studying the southern portion of the fault system have identified extreme subterranean pressures. In June, scientists from the University of Hawaii published findings showing that the southern segment of the San Andreas Fault, located northeast of Los Angeles, is experiencing stress levels not seen in 1,000 years.
Liliane Burkhard, a geophysicist at the University of Bern, noted the severity of these localized pressures during recent evaluations of the fault network.
“What we’ve found now is that, you know… stresses are naturally high, and they’re actually in some places, the highest that we’ve seen in all of these 1,000 years.”
Liliane Burkhard, geophysicist at the University of Bern
This accumulation of stress is particularly concerning around the Cajon Pass, where the San Andreas and San Jacinto faults intersect. Neither the San Francisco Bay Area nor the Cajon Pass region has experienced a major seismic event in generations—the Bay Area has avoided a major quake since the 1989 Loma Prieta event and the catastrophic 1906 San Francisco earthquake, while the Cajon Pass stretch has remained quiet for 160 years.
Infrastructure Vulnerabilities and Simulation Projections
The convergence of higher slip rates in the north and millennial-high stress in the south has renewed focus on statewide infrastructure vulnerabilities. Researchers warn that a significant earthquake near critical corridors like the Cajon Pass could disrupt essential lifelines across multiple cities.
Aqueducts actually cross the San Andreas Fault, and if something happens, you might not have water for a while,
Burkhard explained regarding the potential impact on regional water distribution networks.
These real-world vulnerabilities mirror comprehensive disaster scenarios developed by seismologists. In 2008, a scientific simulation modeled a magnitude 7.8 rupture along the fault line, projecting nearly two minutes of sustained shaking, hundreds of deaths, and billions of dollars in damages.
“It is the longest fault in California and therefore capable of the biggest earthquakes.”
Dr. Lucy Jones, seismologist at Caltech
Dr. Jones compared the mechanics of the San Andreas Fault to the transform fault system that triggered a powerful earthquake in Venezuela in June, noting that a rupture beginning away from urban centers and progressing toward populated areas can focus seismic energy directly into metropolitan regions.
Preparedness and Economic Disruption Risks
Despite decades of active safety updates, mandatory building retrofits, and revised construction codes throughout California, experts emphasize that physical damage is only part of the threat. Dr. Jones cautioned that the broader societal fallout is frequently overlooked.
We underestimate the social disruption and the economic disruption,
Dr. Jones said. We are so focused on our fear of dying, that we forget to be afraid of being bankrupted.
Because exact earthquake timing remains scientifically unpredictable—with faults capable of remaining dormant for another century or rupturing without warning—specialists continue to urge residents to maintain emergency supplies, secure structural reinforcements, and establish clear family communication protocols.
When shaking does begin, immediate protective physical actions remain the most effective defense against injury. If you use those few seconds to get under a strong table, you now can at least get dug back out,
Dr. Jones said. So we say drop, cover, hold on, and it really is the safest thing to do.
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