The world’s largest tectonic fault has reached its highest stress level in 1,000 years: experts fear a “major rupture”
A new study suggests two major fault systems are under unusually high strain, but researchers stress it is not a prediction of earthquake strike.
The San Andreas Fault and the San Jacinto Fault in Southern California have reached their highest level of tectonic stress in 1,000 years, according to a recent study by scientists at the University of Hawaiʻi at Mānoa.
Published on June 10 in the Journal of Geophysical Research: Solid Earth, the research has direct implications for seismic hazard assessments in one of the most densely populated and infrastructure-rich regions of the United States.
The danger of the seismic gateway
The study, led by Dr. Liliane Burkhard of the University of Bern, used a four-dimensional physical model to reconstruct a millennium of seismic activity using geological evidence, including tree rings and radiocarbon dating.
The researchers found that the greatest concern lies in the interaction between the San Andreas and San Jacinto faults. The two fault systems converge near a strategic location known as the Cajon Pass, northeast of Los Angeles, which scientists describe as a “seismic gateway.”
When stress levels differ between the two faults, an earthquake rupture is more likely to stop when it reaches the gateway. “The Cajon Pass may act as a ‘seismic gate,’ sometimes blocking large ruptures from crossing between the faults and other times allowing them to pass through and involve both fault systems in a single event,” Burkhard explained.
However, the study found that both faults are now under exceptionally high – and nearly identical – levels of stress. “At present, with historically high stress levels across the region and more than 160 years having passed since the last major rupture, the system is in a critically loaded state,” the University of Bern researcher said.
Is there a risk of a double earthquake?
The study concludes that the Cajon Pass could allow the San Andreas and San Jacinto faults to rupture together in a single event – a scenario that could be significantly more destructive than an earthquake involving just one fault.Such an event could affect densely populated areas including Los Angeles, San Bernardino, Riverside and the Coachella Valley. But does that mean a double earthquake is likely?
As seismologist Dr. Lucy Jones recently told CBS, “You have to remember that, to a geologist, imminent means something that will happen in the next century.”
In other words, the study does not indicate when the next major earthquake will occur. Instead, it provides new insight into the long-term seismic behavior of the region and helps researchers develop plausible future scenarios.
As Burkhard emphasized, the study “does not predict when an earthquake will occur. What we can say is that the system is under critical stress, and physics-based models like ours provide a clearer picture of the range of scenarios we need to be prepared for. This information is important for risk assessment, infrastructure planning and emergency preparedness.”
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