Tectonic stress levels along Southern California’s San Andreas and San Jacinto fault systems have reached their highest point in 1,000 years, with some areas exceeding this threshold, according to new research led by scientists at the University of Hawaiʻi at Mānoa. While the system is not showing immediate signs of rupture, it is under unusually high stress within a long-term seismic cycle that could lead to significant earthquakes, potentially involving multiple faults.
Critically Loaded State
The study, published in the Journal of Geophysical Research: Solid Earth, suggests the region is in a “critically loaded state,” with stress accumulating across several fault segments. Researchers focused on Cajon Pass, a junction between the two fault systems, which may function as an “earthquake gate.” This gate could either impede ruptures from moving between faults or facilitate their connection into a larger, single event.
Liliane Burkhard, the lead author and a research affiliate at the University of Hawaiʻi’s Institute of Geophysics and Planetology and scientist at the University of Bern, stated that the system is highly stressed due to the more than 160 years that have passed since the last major rupture. “Right now, with stress at historically high levels across the region and more than 160 years elapsed since the last major rupture, the system is in a critically loaded state,” Burkhard said.
Potential for Damaging Earthquakes
A rupture involving both the San Andreas and San Jacinto fault systems could be substantially more damaging than a single-fault earthquake. This increased risk is due to the potential size of such an event and its proximity to major population centers, including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
The San Andreas Fault is described as a strike-slip plate boundary, where the Pacific Plate and North American Plate slide horizontally past each other, rather than pulling apart. In a large earthquake, this results in sideways movement along the fault, not a breakup of the landmass. While significant shifts can occur, both sides of the fault remain part of the same crustal system.
Surface rupture, a clear physical expression of an earthquake where movement along a fault reaches the surface, can cause permanent deformation. Strike-slip faults like the San Andreas typically produce horizontal displacement. In a major rupture scenario, intense shaking could last from tens of seconds to over a minute. Severe damage is expected near the fault and in areas with soft or water-saturated soils, which can amplify shaking and increase liquefaction risk. Infrastructure built across active faults is particularly vulnerable to direct displacement from surface rupture.
California is prone to earthquakes because it lies along the boundary between the Pacific and North American tectonic plates. The state has over 500 active faults, with the San Andreas Fault being the most prominent, stretching approximately 800 miles and marking the primary boundary between the two plates.
Steve Lopez is the Editorial Page Editor for News Raise. He covers Health. He has won more than a dozen national journalism awards for his reporting and column writing at seven newspapers and four news magazines.
