SAN JOSE, CA - A major earthquake just south of the Bay Area is more likely than previously thought, according to new research.

The speed at which the Santa Cruz Mountains portion of the northern San Andreas Fault moves - also known as the fault's slip rate - is faster than earlier estimates suggested, according to research by San Jose State University professor Kim Blisniuk, an earthquake geologist. Faster faults generally have more frequent earthquakes than equivalent slower-moving faults.

Previous research into slip rates in the area spanned a time period of up to 1,000 years, according to Blisniuk. Her study - which is not yet published, though she hopes to present the findings at a conference in October - stretched over the last 10,000 years.

The Northern California portion of the San Andreas Fault is partitioned into three distinct segments, Blisniuk says: the North Coast section, located north of San Francisco; the Peninsula section, from the south of San Francisco to San Jose; and the Santa Cruz Mountains section further south. The full fault, the longest in California, lies at the intersection of the Pacific and the North American tectonic plates.

Scientists previously theorized that the slip rate of the northern San Andreas would slow down further south along the fault, with one hypothesis being that "slip is then being transferred onto other faults" along the way, such as the Calaveras or Hayward faults, Blisniuk said. But her research found that the slip rate - and therefore the likelihood of an earthquake - did not decrease but actually stayed the same from the North Coast section to the Santa Cruz Mountains section.

The slip rate of a fault is considered one of the most influential and important factors when geologists calculate the likelihood of earthquakes.

"In general faults with higher slip rates will have more frequent earthquakes and a probability of one," Blisniuk said by email. "High slip rates faults generally have higher magnitude earthquakes due to potentially higher stress accumulation being released during an earthquake." However, other factors such as the depth of energy released and rupture length are also important factors, she said, so "there is no direct correlation in terms of how we calculate magnitude."

When the rocks are exposed to air, the sandstone gets bombarded by cosmic rays - high-energy particles from outer space - that create a form of the chemical element beryllium called beryllium-10. By measuring the relative amount of beryllium-10 in a sample, scientists can calculate the rock's age.

Impacts of a major earthquake

Blisniuk said it's "hard to say" what locations would be impacted if there were an earthquake along the Santa Cruz Mountains segment of the fault or what magnitude it could be since it depends on how much energy is released and the depth of the seismic activity.

"We can't prevent earthquakes from happening. What we can do is prepare the community and our neighbors and people … for this potential earthquake," Blisniuk said, noting that people should have an earthquake kit with sufficient food, water, flashlights and candles - and prepare for cell service to be down. (Source: The San Francisco Chronicle)

Story Date: August 20, 2026