Southern California fault systems under highest stress in a millennium
A University of Hawaiʻi study finds the San Andreas and San Jacinto faults are at stress levels not seen in the past 1,000 years, increasing the potential for a large multi-fault earthquake.
Researchers from the University of Hawaiʻi at Mānoa have determined that the San Andreas and San Jacinto fault systems in Southern California are experiencing stress levels unprecedented in the last millennium. The study, published in the Journal of Geophysical Research: Solid Earth, describes the region as "critically loaded," meaning that accumulated strain across multiple fault segments could eventually link into a single, larger rupture.
The analysis highlights the Cajón Pass area as a potential "earthquake gate" where the two fault systems intersect, possibly either blocking or facilitating the transfer of rupture between them. Lead author Liliane Burkhard, a research affiliate at the university’s Institute of Geophysics and Planetology, noted that more than 160 years have passed since the last major rupture, leaving the system highly stressed.
"Right now, with stress at historically high levels across the region," Burkhard said.
While the study does not indicate an imminent quake, it warns that a combined rupture could produce stronger, longer shaking for densely populated communities such as Los Angeles, San Bernardino, Riverside and the Coachella Valley. Experts emphasize that the San Andreas is a strike-slip boundary, causing horizontal movement rather than a split that would drop California into the ocean.
California’s Earthquake Early Warning system and standard safety measures—dropping, covering, and holding on—remain the primary tools for reducing injury when shaking occurs.