Study Finds Southern California Faults Under Highest Stress in a Millennium

A new study released on June 3 indicates that tectonic stress on Southern California’s San Andreas and San Jacinto fault networks has risen to its highest level in roughly a millennium. Using a three-dimensional earthquake-cycle model combined with geological records such as radiocarbon dates, tree-ring anomalies and historic rupture data, the researchers mapped stress accumulation over the past 1,000 years.

The analysis identified Cajon Pass – where the two fault systems converge near Los Angeles, the Inland Empire and the Coachella Valley – as a prominent stress hotspot. Some fault sections there have reached, and in a few cases exceeded, stress levels not recorded in the past thousand years. The model estimated stress of about 3.6 MPa on the San Jacinto-San Bernardino segment and 2.8 MPa on the Mojave South portion of the San Andreas, a combination that could increase the likelihood of a rupture propagating across both fault systems.

"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," said lead author Liliane Burkhard.

Burkhard added that the findings provide a physics-based picture of the current stress state and a framework that could be applied to other complex fault junctions worldwide. While the study does not predict a specific timing for a future quake, the authors suggest that more precise modeling can aid planners, engineers and emergency officials in preparing infrastructure and response strategies for worst-case scenarios.

The researchers caution that the stress conditions at Cajon Pass may act as an "earthquake gate," potentially allowing a rupture to spread across both fault systems rather than remaining confined to one. They emphasize that the results are intended to improve hazard assessment rather than serve as a countdown to an event.