New geological research from San Jose State University reveals that a section of the San Andreas Fault in the Santa Cruz Mountains is slipping faster than previously estimated. The findings, which span a 10,000-year window, suggest the fault maintains a constant slip rate comparable to northern sections of the fault system.
A major earthquake just south of the San Francisco Bay Area may be more probable than earlier models suggested, according to recent fieldwork along the Santa Cruz Mountains portion of the northern San Andreas Fault. The research, conducted by San Jose State University geology professor Kim Blisniuk and her students, indicates that the fault is slipping faster than previously thought, altering long-standing assumptions about how seismic stress builds and releases across the region.
Tracking Ten Thousand Years of Tectonic Movement at Sanborn County Park
To calculate the revised slip rate, researchers focused on Sanborn County Park in Saratoga, located approximately 20 miles northwest of the epicenter of the 1989 Loma Prieta earthquake. Previous geological studies in the area typically examined time periods spanning up to 1,000 years. Blisniuk’s team expanded that scope to cover the past 10,000 years by examining landforms that preserve the record of prehistoric seismic events.
“And so the great aspect of this work is that it is a rate of which the landform preserves previous earthquakes along the fault, And the rate is similar to the rate that we get for the San Andreas Fault to the north in the north coast section. But prior to the work that my colleagues and I did at Sanborn County Park, the rate appears to not decrease along the San Andreas Fault, but it appears to be constant over this 10,000-year window that we were able to document.”
Kim Blisniuk, Professor of Geology at San Jose State University, via Abc7news
Along the park’s San Andreas Trail, the research team inspected seasonal streams bent at distinct 90-degree angles by past tectonic displacement. Using drones equipped with cameras and pulses of laser light, the researchers measured how far prehistoric earthquakes had offset the water channels. They then collected sandstone boulder samples from the site and analyzed them at San Jose State, Stanford University, and Lawrence Livermore National Laboratory. By measuring beryllium-10 isotopes created when quartz-rich rocks are bombarded by cosmic rays, the team determined the exposure age of the displaced terrain.
Challenging Assumptions About Slip Rate Distribution in Northern California
The findings challenge a prior scientific hypothesis that the northern segment of the San Andreas Fault slowed down farther south as tectonic energy supposedly transferred onto adjacent parallel structures like the Calaveras or Hayward faults. Instead, the data show the slip rate remains steady from the North Coast section down through the Santa Cruz Mountains.
Slip rate serves as one of the primary variables geologists use to calculate seismic probability. Faster-moving faults accumulate stress more rapidly, which typically translates to more frequent and higher-magnitude earthquakes when that stress ultimately releases.
“In general faults with higher slip rates will have more frequent earthquakes and a probability of one, High slip rates faults generally have higher magnitude earthquakes due to potentially higher stress accumulation being released during an earthquake.”
Kim Blisniuk, Earthquake Geologist, via The Inertia
The northern portion of the fault is partitioned into three segments: the North Coast section north of San Francisco, the Peninsula section stretching from south of San Francisco to San Jose, and the Santa Cruz Mountains section. While the region has not experienced a major earthquake in decades—dating back to the catastrophic 1906 San Francisco earthquake estimated at a 7.8 magnitude—historical precedents loom large. Both the 1906 disaster and the 7.1 Loma Prieta earthquake in 1989 occurred within this fault zone.
Peer Review Status and Statewide Seismicity Context
The new research has not yet been published or peer-reviewed, though the author plans to present the findings at a scientific conference in October. Because the work remains under review, the United States Geological Survey declined to comment on the specific conclusions.
The San Jose State study arrives alongside broader statewide research into California’s fault systems. In June, scientists from the University of Hawaii published findings indicating that the southern portion of the San Andreas Fault near Cajon Pass is experiencing stress levels unseen in 1,000 years, having gone 160 years without a major rupture.
“We know that we live on a plate boundary, and we are always surprised when we hear, ‘Oh my gosh, an earthquake occurred in Japan or Colombia,’ but we are going to have an earthquake in California, and we should be prepared.”
Kim Blisniuk, Professor of Geology at San Jose State University, via Abc7news
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