San Andreas Fault at highest stress in 1,000 years; study warns of potential simultaneous rupture
New research indicates that tectonic stress on Southern California's fault systems has reached levels not seen in a millennium, with models suggesting the San Andreas and San Jacinto faults could rupture together, posing a greater threat.
Scientists from the University of Hawai'i at Mānoa have published a study in the Journal of Geophysical Research: Solid Earth analyzing earthquake history in Southern California. Using a computer model based on roughly 1,000 years of geological data, including radiocarbon dating of displaced sediments and tree-ring records, the researchers estimated that stress on the San Andreas and San Jacinto fault systems has reached unprecedented levels.
On the San Jacinto-Bernardino section, stress measures 3.6 megapascals—equivalent to the pressure of two blue whales pressing on every square meter of that fault segment. Extrapolated across the entire area, the fault holds pressure equal to the weight of over 6 billion blue whales.
A key focus is Cajon Pass, where the two major fault systems intersect. This area can act like an "earthquake gate," sometimes blocking large quakes from jumping between faults and other times allowing passage. The model shows that under certain conditions, Cajon Pass could permit the San Andreas and San Jacinto faults to rupture simultaneously in the same earthquake, potentially far more destructive than a single-fault event. Such a scenario could affect millions of people in Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
The researchers emphasize that the study does not predict the timing of an earthquake, as that remains impossible. However, the findings could improve earthquake hazard analyses, guide infrastructure investments, update building codes, and strengthen emergency preparedness. The team also notes that the modeling approach can be applied to complex fault intersections worldwide to assess risks from multiple fault systems.
