No region in the US is at greater risk for a catastrophic earthquake than Southern California. The danger comes mainly from two volatile faults, the San Andreas and the San Jacinto. According to new research, stresses along these faults are the highest they have been in the last 1,000 years.
The study, published The June 3 issue of the Journal of Geophysical Research, Solid Earth, highlights scientists’ concerns that this fault system is primed to produce a large earthquake, although the authors emphasize that it does not predict when such an event will occur. The findings also suggest that a critical fault northeast of Los Angeles could play a key role in the magnitude of the next big earthquake.
“The question of when and how the next large earthquake will occur in this region is one of the most pressing problems in applied geoscience,” said lead author Liliane Burkhard, a geophysicist and planetary geologist at the University of Bern in Switzerland. saying in a statement. “Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not only applicable to California, but also to other complex fault junctions around the world.”
Highest fault stress in a millennium
Video showing animation of the stress accumulation time series for the southern San Andreas fault system, spanning the years 1100 to 2025.
Earthquakes typically occur along boundaries between tectonic plates. These fracture zones, or faults, build up stress as plates slide over each other and lock. Eventually, so much tension builds up that the plates suddenly slide, releasing all the stored energy in the form of an earthquake. The more time has passed since the last large earthquake, the more stress may have built up along the fault.
The last major earthquake to affect the Los Angeles region was the Fort Tejón earthquake of 1857. With a magnitude of 7.9, remains one of the largest earthquakes ever recorded in California. More than a century has passed since this event, so scientists are concerned that the San Andreas fault system could produce another large earthquake at any time.
To gain a clearer understanding of the current stress along the San Andreas and San Jacinto fault zones, Burkhard and his colleagues used computer simulations of the last 1,000 years of major seismic activity to estimate how stress builds up on the fault segments over time and how this affects neighboring segments.
Their physics-based seismic cycle model simulates this process in three spatial dimensions over time. The researchers fed it geological evidence from past earthquakes, such as radiocarbon dating, tree ring anomalies, and historical records of ground ruptures. When they ran it, the results indicated that tectonic stresses along the San Andreas and San Jacinto fault zones had reached and, in some cases, exceeded the highest levels of the last millennium.
The fundamental role of Cajon Pass
Cajon Pass is the tectonically complex junction where the San Andreas and San Jacinto faults converge. It is located near several densely populated communities, including Los Angeles, San Bernardino, Riverside, and Coachella Valley. According to the study, this crossing may pose a greater danger to these areas than scientists thought until now.
Burkhard and his colleagues determined that Cajon Pass may act as a so-called “seismic gate,” controlling whether large ruptures remain confined to a single fault or cross both fault systems. “The seismic gate concept captures something important about how fault junctions work,” he said in the statement. “Cajon Pass doesn’t simply block or channel earthquakes: it responds to stress conditions, and those conditions change over centuries.”
According to the study, when stress on both faults builds simultaneously toward equally high levels, a large joint rupture is more likely to traverse both systems. The model showed that stress reached 3.6 megapascals (MPa) in the San Jacinto-Bernadino section, the highest level observed in the 1,000-year simulation. Meanwhile, the neighboring South Mojave section of the San Andreas Fault has accumulated 2.8 MPa of stress, indicating that both segments are under equally high stress levels.
“So it’s not only concerning that stresses are reaching historic highs, but also that the relative stress conditions between the two fault systems are approaching the range that we associate with major ruptures crossing both faults simultaneously, and that is a scenario with much greater consequences for the region,” Burkhard explained.
Again, this study does not predict when the next big earthquake might hit Southern California, nor does it show that such an event is necessarily imminent. Rather, it offers a clearer picture of the region’s seismic hazard. This critically stressed fault system could break at any time, so communities must prepare for the worst-case scenario. Burkhard hopes that the hazard assessment framework she and her colleagues built will help California and other seismically active parts of the world protect against the next big earthquake.
