New geological research reveals that the Santa Cruz Mountains segment of the northern San Andreas Fault moves faster than previously estimated, potentially increasing earthquake frequency. Simultaneously, experts warn that stress levels along parts of the fault system in Southern California have reached highs not seen in a millennium.
Slip Rates in the Santa Cruz Mountains
California’s Bay Area has not experienced a major earthquake in 120 years, following the historic 1906 event. However, research by San Jose State University professor Kim Blisniuk indicates that the northern San Andreas Fault section running through the Santa Cruz Mountains moves faster than earlier scientific models suggested.
Faster-moving faults generally produce more frequent earthquakes due to quicker stress accumulation. While previous regional studies measured slip rates over a 1,000-year window, the soon-to-be-published research analyzed data spanning the last 10,000 years.
To calculate these rates, Blisniuk and her students visited Sanborn County Park in Saratoga, examining stream beds shifted by historical tectonic activity. The team used drones and lasers to measure displacement, then collected sandstone boulders for dating at San Jose State, Stanford University, and Lawrence Livermore National Laboratory. By measuring the accumulation of beryllium-10—a chemical created when cosmic rays strike rocks exposed to air—the team determined the age of the shifted terrain.
The findings challenge previous theories that fault slip rates slowed down further south as energy transferred to adjacent faults like the Calaveras or Hayward systems. Instead, the research found that the slip rate remained steady from the North Coast section down to the Santa Cruz Mountains.
“In general faults with higher slip rates will have more frequent earthquakes and a probability of one.”
Kim Blisniuk, professor of geology at San Jose State University
The United States Geological Survey declined to comment on the findings because the research remains unpublished. Blisniuk plans to present the data at a conference in October.
Southern California Stress Levels and Infrastructure Risks
While northern segments undergo scrutiny, research published by scientists from the University of Hawaii indicates that the southern portion of the San Andreas Fault northeast of Los Angeles exhibits stress levels unseen in a thousand years.
Liliane Burkhard, a geophysicist at the University of Bern, noted that stresses are naturally high across several areas of the southern fault system. This regional vulnerability includes the Cajon Pass, where the San Andreas and San Jacinto faults intersect and have not produced a major earthquake in 160 years.
“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
A significant rupture near the Cajon Pass threatens critical regional infrastructure. Researchers point out that major aqueducts cross the fault line directly, creating the potential for extended water service disruptions across multiple cities following a major seismic event.
Modeling ‘The Big One’ and Societal Preparedness
Seismologists continue to evaluate potential worst-case scenarios. A simulation developed in 2008 modeled a magnitude 7.8 rupture along the San Andreas Fault, projecting roughly two minutes of intense shaking, hundreds of deaths, and billions of dollars in damage.
“It is the longest fault in California and therefore capable of the biggest earthquakes.”
Dr. Lucy Jones, seismologist at Caltech
Jones compared the modeled scenario to the transform fault earthquake that struck Venezuela in June, where a rupture began west of Caracas and propagated toward the city, concentrating seismic energy into heavily populated areas. Seismologists expect a similar dynamic during a major San Andreas event.
Despite decades of building retrofits and updated safety codes, experts caution that physical damage represents only one facet of a major disaster. Beyond the physical destruction, researchers emphasize the severe economic and social fallout that accompanies large-scale tectonic events.
“We underestimate the social disruption and the economic disruption. We are so focused on our fear of dying, that we forget to be afraid of being bankrupted.”
Dr. Lucy Jones, seismologist at Caltech
While earthquake timing remains unpredictable—ranging from decades away to happening at any moment—authorities continue urging residents to maintain emergency supplies, secure structural retrofits, and follow standard protective guidance such as dropping, covering, and holding on during shaking.
