Kim Blisniuk, professor of geology at San Jose State University, examines Sanborn County Park in Saratoga on August 6. Blisniuk investigates the terrain and rock formations within the park, where the San Andreas Fault runs directly below. Santiago Mejía/SF Chronicle According to new research, a large earthquake is more likely to occur just south of the Bay Area than previously thought. The speed at which the northern Santa Cruz Mountains portion of the San Andreas Fault, also known as the slip rate, is moving is faster than previous estimates suggested, according to research by Professor Kim Blisniuk, a seismic geologist at San Jose State University. Faster faults generally have more frequent earthquakes than equivalent slower-moving faults. According to Blisniuk, previous research on landslide rates in the area spanned a period of up to 1,000 years. His study, which has not yet been published, although he hopes to present the findings at a conference in October, covered the last 10,000 years. Article continues below this advertisement. Northern California’s portion of the San Andreas Fault is divided into three distinct segments, Blisniuk says: the north coast section, located north of San Francisco; the Peninsula section, from south of San Francisco to San José; and the section of the Santa Cruz Mountains further south. The entire fault, California’s longest, lies at the intersection of the Pacific and North American tectonic plates. Kim Blisniuk, a professor of geology at San Jose State University, points out a stream that has been displaced by earthquakes in the past at Sanborn County Park in Saratoga on August 6. Santiago Mejía/SF Chronicle Scientists previously theorized that the rate of slip on the northern San Andreas would slow further south along the fault, with one hypothesis being that “the slip then transfers onto other faults” along the way, such as the Calaveras or Hayward faults, Blisniuk said. But their research found that the slip rate (and therefore the probability of an earthquake) did not decrease, but actually stayed the same from the north coast section to the Santa Cruz Mountains section. The U.S. Geological Survey declined to comment on Blisniuk’s findings because they have not yet been published. Article continues below this advertisement. The slip rate of a fault is considered one of the most influential and important factors when geologists calculate the probability of earthquakes. Sanborn County Park in Saratoga is seen on August 6. Santiago Mejía/SF Chronicle “In general, faults with higher slip rates will have more frequent earthquakes and a probability of one,” Blisniuk said by email. “Faults with high slip rates generally have larger magnitude earthquakes due to a potentially larger buildup of stress that is released during an earthquake.” However, other factors such as the depth of the energy released and the length of the rupture are also important factors, he said, so “there is no direct correlation in terms of how we calculate the magnitude.” To calculate the slip rate of the San Andreas in the Santa Cruz Mountains, Blisniuk and his students headed to Sanborn County Park in Saratoga, about 20 miles northwest of the epicenter of the 1989 Loma Prieta earthquake. (He said the scientific community is “split” over whether the Loma Prieta earthquake occurred along the San Andreas fault or an adjacent fault; it is considered the last major earthquake along northern San Andreas was the San Francisco earthquake of 1906). Along the park’s San Andreas Trail, Blisniuk looks for areas where tectonic movement is evident, such as where the path of a seasonal stream makes two 90-degree turns in rapid succession, evidence of at least two earthquakes in the distant past. Article continues below this advertisement. His team uses drones that take thousands of photographs and send pulses of laser light to measure the extent to which previous earthquakes have displaced the current. After obtaining the distance data, Blisniuk and his students take samples from nearby rocks; At Sanborn, the rocks are usually sandstone boulders, composed primarily of quartz. The samples are then taken to San Jose State, Stanford University and Lawrence Livermore National Laboratory to determine their age in a process called dating. Kim Blisniuk, a geology professor at San Jose State University, places her hand on a rock at Sanborn County Park in Saratoga on August 6. She and her team take samples of rocks in the park and calculate their age by measuring the amount of beryllium-10. Santiago Mejía/SF Chronicle When rocks are exposed to air, the sandstone is 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 estimate the age of the rock. After determining the extent to which previous earthquakes have shifted the current and the age of the surrounding relief, Blisniuk can calculate the average annual rate at which the fault has moved over the past few thousand years. the fault or the magnitude it could have, since it depends on the amount of energy that is released and the depth of the seismic activity. “We can’t prevent earthquakes from happening. What we can do is prepare the community, our neighbors and the people… for this possible earthquake,” Blisniuk said, noting that people should have an earthquake kit with enough food, water, flashlights and candles, and prepare for cell service to go out. A sign at Sanborn County Park in Saratoga notes the proximity of the San Andreas Fault. Santiago Mejía/SF Chronicle He said the public can see the San Andreas Fault for themselves. Article continues below this advertisement “The San Andreas is in (people’s) backyard, and they can go see it, walk around and see the landforms,” Blisniuk said.
