Southern California Faults Reach Highest Stress Levels in Millennia, Study Finds

Researchers from the University of Hawaiʻi at Mānoa and collaborators have reported that tectonic stress on the southern San Andreas and San Jacinto fault systems is now at or above levels recorded over the past millennium. The findings, published in the Journal of Geophysical Research: Solid Earth, are based on a physics-based computer model that incorporated a 1,000-year geological record derived from radiocarbon dating, displaced sediments and tree-ring data.

Lead author Liliane Burkhard said the results show the region is in a "critically loaded state" with stress levels at historically high points and more than 160 years since the last major rupture. The study highlights the role of Cajon Pass, a mountain corridor about 63 miles east of Los Angeles, which may act as an "earthquake gate" that can either block or permit a combined rupture of both fault systems.

The authors suggest that a joint rupture involving the two faults could affect densely populated areas including Los Angeles, San Bernardino, Riverside and the Coachella Valley, and would likely be more damaging than a single-fault event. While the research does not predict when an earthquake will occur, the authors argue that the stress modeling can improve seismic hazard assessments, infrastructure planning, emergency preparedness and building codes in Southern California and similar fault junctions worldwide.

"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," Burkhard said.

"The conditions that determine whether the ‘earthquake gate’ at Cajon Pass opens or stays closed appear to be related to how closely the stress levels on the two fault systems are aligned with each other at the time of rupture," she added.