Southern California fault systems show stress levels unseen in a millennium, study finds
Research by University of Hawaiʻi scientists indicates that the San Andreas and San Jacobi faults are under the highest tectonic stress recorded in the past 1,000 years, though no imminent rupture is evident.
Tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has reached, and in some sections exceeded, the highest levels documented over the last millennium, according to a study published in the Journal of Geophysical Research: Solid Earth. The research team, led by University of Hawaiʻi at Mānoa geophysicist Liliane Burkhard, said the region is operating in a "critically loaded state" after more than 160 years since the last major rupture.
The analysis suggests that stress is building across multiple fault segments, with particular attention to Cajon Pass, a junction that could either block or facilitate rupture propagation between the two systems. While the scientists do not see signs of an imminent earthquake, they warn that a combined rupture involving both fault networks could produce larger ground shaking and affect densely populated areas such as Los Angeles, San Bernardino, Riverside and the Coachella Valley.
The San Andreas Fault is a strike-slip boundary where the Pacific and North American plates slide past each other horizontally. Even in a major event, movement would be lateral rather than a split of the landmass. However, the accumulated stress raises the potential for multi-fault earthquakes that could cause extensive damage, especially where infrastructure crosses active fault traces.
U.S. Geological Survey officials note that most earthquakes do not generate surface rupture, but when it occurs, it can offset roads, buildings and other structures. The state’s earthquake early-warning system, which uses seismic sensors and mobile alerts, aims to give residents a brief window to take protective actions before strong shaking arrives.