California’s San Andreas Fault, the state’s most famous and closely watched seismic feature, is under greater strain than at any point in the past thousand years, according to two independent scientific studies released this year that examine opposite ends of the fault system.
The findings, which emerged separately over the summer, suggest that earthquake risk for the roughly 30 million people living within reach of the fault’s rupture zones may be higher than existing hazard models have assumed.
What did the new studies find?
One study, led by geophysicist Liliane Burkhard of the University of Bern and published in June in the Journal of Geophysical Research: Solid Earth, modelled a thousand years of earthquake history along the southern San Andreas and the neighbouring San Jacinto Fault Zone. It found that stress at the San Jacinto-San Bernardino section now exceeds any level recorded in that entire simulated period, with the adjacent Mojave South section of the San Andreas also running high. The second study, from San José State University geologist Kim Blisniuk, has not yet been peer-reviewed but was reported by the San Francisco Chronicle in August. It indicates that a stretch of the northern San Andreas beneath the Santa Cruz Mountains, south of San Jose, has been slipping faster over the past 10,000 years than long-standing models assumed.
Why is Cajon Pass considered an “earthquake gate”?
Burkhard’s team focused particular attention on Cajon Pass, a mountain gap northeast of Los Angeles where the San Andreas and San Jacinto faults converge to within roughly 1.5 kilometres of one another. Researchers describe such junctions as “earthquake gates,” since a rupture starting on one fault can sometimes cross through the gate and trigger a joint rupture on both. Historical records show the gate has behaved differently in the past: the 1857 Fort Tejon earthquake stopped at Cajon Pass, while the 1812 Wrightwood earthquake is believed to have crossed it, rupturing both fault systems together. “Not only is it concerning that the stresses are reaching historic highs,” Burkhard said, “but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously.”
What is happening in the Santa Cruz Mountains?
Blisniuk’s team used a dating method known as beryllium-10 cosmogenic exposure dating to measure how long displaced rock formations at Sanborn County Park, near Saratoga, had sat exposed at the surface, allowing them to calculate the fault’s slip rate over roughly ten millennia. Her findings challenge a long-held assumption that plate motion tapers off along this section of the fault as it moves south, instead suggesting the section has carried a broadly consistent load throughout that period. Blisniuk described the result as a data update rather than new movement, telling reporters that the fault “is really late in the earthquake cycle and an earthquake will rupture in our lifetime.”
How significant would a worst-case rupture be?
A widely cited 2008 US Geological Survey scenario estimated that a magnitude 7.8 earthquake on the southern San Andreas could cause around 1,800 deaths and 213 billion US dollars in damage. Burkhard’s team says a joint rupture crossing Cajon Pass could reach a magnitude of between 7.4 and 7.8 while affecting a considerably larger geographic area than a single-fault event, given the corridor’s role as home to Interstate 15, major rail lines and regional energy and telecommunications infrastructure.
What are officials advising residents to do?
Neither research team claims to know when a major earthquake will occur, and both stress that their work informs long-term hazard planning rather than short-term prediction. California’s Governor’s Office of Emergency Services continues to recommend standard precautions, including securing heavy furniture and water heaters, storing at least three days of water and non-perishable food per household member, knowing the location of gas shut-off valves, and maintaining a family communication plan that does not rely on mobile networks remaining operational. Residents can check their proximity to mapped fault zones through the state’s EQ Zapp portal, run by the California Geological Survey.
