This is a worrying discovery from the latest research on earthquakes in Southern California: tectonic stresses at Cajon Pass, a major geological crossroads, have never been this high in a thousand years. This critical point, where two of the region's most active faults brush against each other, could be the site of a large-magnitude earthquake.
Cajon Pass, located northeast of Los Angeles, is a place where the San Andreas Fault and the San Jacinto Fault dangerously converge. Since the last major earthquake in this area, the Fort Tejon quake of 1857, stresses have been building relentlessly. Scientists have long been concerned about this apparent calm, as it could precede a major rupture.
Artistic illustration of a fault cutting a city in two.
An international team of researchers, led by Liliane Burkhard from the University of Bern, has reconstructed a thousand years of seismic history along these faults. To do so, they used a four-dimensional computer model (three spatial dimensions plus time) that simulates the earthquake cycle. By integrating multiple data sets, they were able to calculate current stresses.
The results are unequivocal: today, tectonic stress levels have reached or even exceeded the maximums of the past ten centuries. On the San Jacinto-Bernardino section, the modeled stress stands at 3.6 MPa, an absolute record. On the neighboring Mojave South segment, on the San Andreas Fault, it reaches 2.8 MPa. Both faults are therefore highly stressed and to a comparable degree.
The researchers also introduced an interesting concept: Cajon Pass acts as a "seismic gate." Depending on the stress state, this intersection can either block a rupture or allow it to pass from one fault to the other. In 1857, the gate was closed: the earthquake stopped at the pass. In 1812, it was open: a rupture crossed both systems simultaneously.
Extent of seismic ruptures passing or stopping at Cajon Pass, based on the maximum rupture model of Scharer and Yule (2020) and modeling by Rodríguez Padilla et al. (2021). Colors indicate earthquakes with the same rupture extent, with the mean age (EC). Small arrows indicate the extension of ruptures beyond the map excerpt. Left inset: chronology of "open gate" or "closed gate" events at Cajon Pass, i.e., ruptures stopping/passing at the intersection of MOS, northern San Bernardino, and San Jacinto Bernardino.
Credit: Journal of Geophysical Research: Solid Earth (2026). DOI: 10.1029/2025jb033213
The decisive factor appears to be the alignment of stresses on both faults. When they rise together to high levels, conditions favor a joint rupture. That is exactly what is happening today. Liliane Burkhard specifies that this is not a prediction, but an indication that the system is under critical tension, which is useful for risk preparedness.
A joint rupture of both faults would have far more serious consequences than an earthquake on a single fault. The affected region includes Los Angeles, San Bernardino, Riverside, and the Coachella Valley — densely populated areas with vital infrastructure. Highways, railways, and energy networks cross Cajon Pass itself. These results help to better define the scenarios to consider.