Study Finds Southern California Faults Under Highest Stress in a Millennium

A new study led by scientists at the University of Hawaii at Mānoa indicates that tectonic stress on the southern segments of the San Andreas and San Jacinto fault systems is at its highest level in the last millennium. The research, published in the Journal of Geophysical Research: Solid Earth, used a physics-based computer model calibrated with a 1,000-year record of geological evidence, including radiocarbon-dated sediments and tree-ring data.

"Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems," said lead author Liliane Burkhard.

The model suggests that the Cajón Pass, a mountain corridor about 63 miles east of Los Angeles, could act as an "earthquake gate," sometimes preventing, and at other times allowing, ruptures to propagate between the two fault systems. With more than 160 years since the last major rupture in the Los Angeles area, the authors describe the fault system as being in a "critically loaded state."

The findings have direct implications for seismic hazard assessments in a densely populated corridor that includes Los Angeles, San Bernardino, Riverside and the Coachella Valley. Researchers say the stress modeling approach can inform infrastructure planning, emergency preparedness and building-code updates, and may be applicable to other complex fault junctions worldwide.

The study involved collaborators from Northern Arizona University, the University of Bern, the U.S. Geological Survey and the University of California, San Diego. While the authors caution that the work does not predict the timing of a future earthquake, they emphasize its value for improving quantitative risk assessments for millions of residents.