San Andreas fault reaches highest stress level in 1,000 years, study say

Scientists are raising the alarm after a new study found the San Andreas and San Jacinto faults have reached their highest tectonic stress level in 1,000 years, causing concerns about “an imminent large earthquake” in Southern California. The study — which was led by University of Hawaii at Mānoa Earth scientists and published in Journal of Geophysical Research: Solid Earth” — found that in some areas, tectonic stress has exceeded the highest levels observed in the past 1,000 years. The study points out that the faults have not produced a major earthquake in the Los Angeles area in over 100 years, and says that during that time, tectonic stress has continued to build. That, the study says, has increased the likelihood of a “large future rupture.” Researchers say the findings highlight “direct implications for seismic hazard assessments in one of the most densely populated and infrastructure-critical corridors in the U.S.,” according to a release the university published this month. “Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems,” said lead author Liliane Burkhard. Burkhard said researchers found that Cajon Pass, a mountain pass located about 63 miles from LA between the San Bernardino Mountains and the San Gabriel Mountains, could “act as an ‘earthquake gate’: sometimes blocking large ruptures from crossing between the faults, and sometimes allowing them to pass through and involve both systems in a single event.” Cajon Pass is a critical junction between the two fault systems, the researchers explain. How did researchers reach their findings? To conduct the research, the scientists built a physics-based computer model, which simulates how stress builds up and releases along the southern San Andreas and San Jacinto fault systems — including at Cajon Pass. They then presented it with a 1,000-year record of earthquake history of the region, which was reconstructed from geological evidence such as radiocarbon dating of displaced sediments and tree-ring records, the university’s release states. Running the simulation forward to the present day, researchers estimated how much stress has built up, the release states. Burkhard said the conditions that “determine whether the ‘earthquake gate’ at Cajon Pass opens or stays closed “appear to be related to how closely the stress levels on the two fault systems are aligned with each other at the time of rupture.” “Right now, with stress at historically high levels across the region and more than 160 years elapsed since the last major rupture, the system is in a critically loaded state,” she said. In its release, the university explains that the study’s results “suggest the stress that would normally be released in large earthquakes has continued to accumulate and is now at unprecedented levels.” But more importantly than that, the university said, the research indicates that Cajon Pass “could facilitate a joint rupture of both the San Andreas and San Jacinto faults simultaneously.” That would present a scenario that could be “significantly more damaging than a single-fault event, and one that affects densely populated areas." Those areas include Los Angeles, San Bernardino, Riverside and the Coachella Valley. The university says the physics-based stress modeling done in the research project “can help refine seismic hazard assessments and inform infrastructure planning, emergency preparedness, and building codes in the region,” but it would also be applicable to other complex fault junctions around the world. Burkhard makes it clear that the research findings are “not a prediction of when an earthquake will happen,” but she said the study and those like it are “important contributions to national and global earthquake hazard research in that we are using rigorous, quantitative science to better understand the risk facing millions of people.” The university notes that researchers from Northern Arizona University, University of Bern, U.S. Geological Survey and University of California, San Diego were also authors of the study.