New research from San Jose State University indicates that a major earthquake just south of the Bay Area is more likely than earlier thought. Investigators found that the Santa Cruz Mountains portion of the northern San Andreas Fault moves at a slip rate faster than earlier estimates suggested.
New Research Shows Santa Cruz Mountains Section of San Andreas Fault Moves Faster Than Previously Believed
The study was conducted by Kim Blisniuk, a professor of geology and earthquake geologist at San Francisco Chronicle, alongside her students. The research stretched over a 10,000-year window, documenting how landforms preserve previous earthquakes.
Methods Used at Sanborn County Park in Saratoga
To calculate the slip rate of the fault in the Santa Cruz Mountains, researchers focused on Sanborn County Park in Saratoga, which sits about 20 miles northwest of the epicenter of the 1989 Loma Prieta earthquake and directly over the fault line.
Along the park’s San Andreas Trail, Blisniuk examined areas showing apparent tectonic movement, including a seasonal stream path making two 90-degree turns in quick succession as evidence of past earthquakes. The team also took rock samples from the park and calculated their age by measuring beryllium-10 amounts. This chemical element is formed when sandstone is exposed to air and bombarded by high-energy cosmic rays from outer space.
According to Blisniuk, the Northern California portion of the San Andreas Fault is partitioned into three distinct segments:
- The North Coast section, located north of San Francisco
- The Peninsula section, stretching from south of San Francisco to San Jose
- The Santa Cruz Mountains section farther south
Implications for Earthquake Frequency and Preparedness
And the rate is similar to the rate that we get for the San Andreas Fault to the north in the north coast section,
Blisniuk said. And 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.
Faults that move faster generally produce earthquakes more frequently. In general faults with higher slip rates will have more frequent earthquakes and a probability of one,
Blisniuk stated, adding that such faults also generally have higher magnitude earthquakes due to potentially higher stress accumulation being released.
Historically, major seismic events like the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake occurred along the San Andreas Fault zone. While the research has not yet been published or peer-reviewed—though Blisniuk hopes to present the findings at a conference in October—experts emphasize ongoing readiness.
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,
Blisniuk said, recommending that residents maintain an earthquake preparedness kit.