Study Finds Southern California Faults Under Highest Stress in a Millennium

A new study published in the Journal of Geophysical Research, Solid Earth, reports that tectonic stress on Southern California’s San Andreas and San Jacinto fault systems is at its highest level in the last 1,000 years. Researchers used a three-dimensional physics-based earthquake cycle model, calibrated with radiocarbon dates, tree-ring data and historic rupture records, to estimate stress accumulation on fault segments over time.

Lead author Liliane Burkhard, a geophysicist at the University of Bern, said the results provide a clearer, physics-based picture of the current stress state and that the modeling framework could be applied to other complex fault junctions worldwide. > “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,” she said in a release.

The simulation shows stress levels 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, both approaching the upper range observed in the millennium-long model. The study highlights Cajón Pass, where the two faults converge, as a potential “earthquake gate” that could allow a rupture to propagate across both fault systems when stress conditions are similar.

While the authors caution that the research does not predict the timing of a future quake, they note that the elevated stress and the relative balance between the two fault systems increase the likelihood of a large joint rupture, which would have significant implications for densely populated communities in the Los Angeles, San Bernardino, Riverside and Coachella Valley areas. The authors hope their hazard-assessment framework will aid preparedness efforts in California and other seismically active regions.