New research indicates that tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has climbed to, and in some places surpassed, the highest levels seen in the last 1,000 years. The findings, led by Earth scientists at the University of Hawaiʻi at Mānoa and published in the Journal of Geophysical Research: Solid Earth, do not suggest an earthquake is about to happen, but they do show that the region is in a critically loaded state with elevated long-term seismic risk.
The study focuses on how stress has accumulated across multiple fault segments in Southern California after more than 160 years since the last major rupture in the area. Researchers say the fault network is not uniformly primed for failure, but it is under unusually high pressure within the natural earthquake cycle. That buildup could support a major event in the future, including an earthquake that involves more than one fault system.
A central area of concern is Cajon Pass, where the San Andreas and San Jacinto faults meet. Scientists describe it as a possible “earthquake gate,” meaning it could either stop a rupture from spreading from one fault to the other or allow a larger, linked earthquake to occur. If both systems rupture together, the resulting quake could be much more destructive than a single-fault event because of the greater size of the rupture and the large population centers nearby, including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
The article also explains that the San Andreas Fault cannot “split California apart.” It is a strike-slip fault, which means the Pacific Plate and North American Plate slide horizontally past each other rather than pulling away from one another. In a major earthquake, the ground would move sideways along the fault, sometimes by several feet or even tens of feet, but the land would not break away into separate pieces. Over millions of years, however, continued plate motion could shift the relative positions of California cities.
If the San Andreas Fault were to rupture, the main dangers would include strong shaking, surface rupture, and damage to infrastructure that crosses the fault trace. Surface rupture happens when movement underground reaches the Earth’s surface, creating visible offsets in roads, buildings, and the ground itself. The most severe impacts would likely be felt near the fault and in areas built on soft or water-saturated soil, where shaking can intensify and increase the risk of liquefaction.
The article also reviews why California experiences so many earthquakes. The state lies on the boundary between the Pacific and North American plates and contains more than 500 active faults. Earthquakes occur when stress builds along faults until friction is overcome and energy is released as seismic waves. Scientists stress that earthquakes cannot be predicted with precision, but California does operate an earthquake early warning system designed to give residents a few seconds of notice before the strongest shaking arrives.
