Southern California Faults Under Highest Stress in a Millennium, Study Finds

A study published June 3 in the *Journal of Geophysical Research, Solid Earth* finds that tectonic stresses on Southern California’s San Andreas and San Jacinto faults have reached the highest levels in the past 1,000 years. The last quake, the 1857 Fort Tejon M 7.9 event, occurred over 160 years ago. The analysis highlights the Cajon Pass junction, where the faults converge, as a zone where stress could permit a large-scale rupture spanning both systems.

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The researchers built a three-dimensional physics-based earthquake-cycle model simulating a millennium of large-earthquake activity. Input data—radiocarbon dates, tree-ring anomalies, historic ground-rupture records—let the model track stress accumulation on fault segments over time and assess how strain on one segment influences neighbors.

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Simulations show a peak stress of 3.6 MPa on the San Jacinto-Bernardino segment and 2.8 MPa on the neighboring Mojave South segment of the San Andreas fault—the highest values in the 1,000-year run. The study calls Cajon Pass an “earthquake gate” that can block or channel ruptures depending on the contemporaneous stress of both faults.

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Cajon Pass lies near Los Angeles, San Bernardino, Riverside and the Coachella Valley, home to millions. A joint rupture could amplify damage beyond isolated events, raising the region’s seismic hazard. The authors stress that communities should prepare for a worst-case scenario despite the unknown timing of the next major quake.

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Lead author Liliane Burkhard, a geophysicist and planetary geologist at the University of Bern, Switzerland, emphasizes that the analysis does not predict when the next major earthquake will occur; it only shows the fault system is presently in a critically stressed state. She and her colleagues argue the new hazard-assessment framework can help California and other seismically active regions develop mitigation strategies, update building codes, and refine risk-mapping.

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• “The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience,” — Liliane Burkhard, Lead Author

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• “Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide.” — Liliane Burkhard

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• “Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries.” — Liliane Burkhard

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• “The earthquake gate concept captures something important about how fault junctions work,” — Liliane Burkhard

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The authors hope the stress-mapping method will be incorporated into state-level seismic risk assessments. They recommend emergency-management agencies update scenario planning, engineers consider joint-fault rupture potential in design standards, and public-education campaigns stress household-level earthquake readiness.

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