Is the big earthquake imminent? More tectonic stress has accumulated in Southern California’s subsurface than at any time in the last thousand years, new analysis reveals. There is a risk of a strong earthquake in the Los Angeles area and surrounding metropolitan areas. Also threatening: an “earthquake gate” near Los Angeles could cause the southern part of the San Andreas Trench and the neighboring San Jacinto Fault to rupture at the same time.
California is in a tectonic ejection seat: several earth’s plates move sideways past each other and form a complex structure of interconnected faults. In Southern California, most of this plate movement occurs along the San Andreas Trench and the San Jacinto Fault. If they get caught, tension builds up in these faults until the rock gives way and breaks – an earthquake is the result.
The next strong earthquake is long overdue
However, such an earthquake is long overdue in Southern California. Historical earthquake data suggests that a strong earthquake occurs on the San Andreas and San Jacinto faults approximately every hundred years. But since the magnitude 7.9 Fort Tejon earthquake in 1857, there has been an ominous calm on the southern San Andreas fault. Since then, there have been smaller earthquakes on side branches, but not on the two main faults in Southern California.
“This prolonged calm raises concerns that the next event in this region could be particularly strong and complex,” explain Liliane Burkhard from the University of Bern and her colleagues. They therefore analyzed the seismological-tectonic situation along the San Andreas and San Jacinto faults in more detail. To do this, the researchers evaluated earthquake data for Southern California going back thousands of years and used them to feed a geophysical model that reconstructs the processes three-dimensionally and over time.
“The model shows how each earthquake changes the stress on neighboring fault segments and how the stress builds up again in the calm intervals between events,” explains Burkhard. “By simulating Southern California’s earthquake history, we can estimate how much stress the fault system is under today.”
Highest values of the last thousand years
The evaluations confirm the danger: More tension has built up in the southern California faults today than at any time in the last thousand years, as Burkhard and her colleagues determined. This confirms that Los Angles and the neighboring metropolitan areas are sitting on a seismological “time bomb” – a new strong earthquake is brewing underground. “The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geosciences,” emphasizes Burkhard.
One area turns out to be a seismologically critical point: the Cajon Pass northeast of Los Angeles. “While there is no visible connection between the San Andreas and San Jacinto faults at Cajon Pass, this area is considered the critical link between the two,” the team explains. According to their analysis, this contact point acts as an “earthquake gateway”: the conditions at Cajon Pass determine whether an earthquake spreads across both faults or is confined to just one.
“In the last thousand years, a gate open event has occurred six times,” report Burkhard and her colleagues. Only twice did the earthquake gate remain closed and stronger quakes were confined to one of the faults.
Crucial “earthquake gate” at Cajon Pass
But which variant will occur in the next big earthquake in Southern California? Are both major faults rupturing together or just one of them? The seismological and geophysical data also provided new insights. There is therefore some evidence to suggest that the “earthquake gate” at Cajon Pass could be wide open during the next big earthquake. If one of the two faults breaks, the other one will also take it with it.
The reason for this: Whether the “earthquake gate” blocks or connects the San Andreas and San Jacinto faults depends on the tension between the two trenches, as the researchers explain. If the stress in both faults increases to the same extent and is in a similarly high range, this favors a seismological connection and thus a common earthquake.
This is exactly the case at the moment: According to the analyses, the San Jacinto section just before the Cajon Pass has accumulated a stress of 3.6 megapascals, and in the neighboring Mojave South section of the San Andreas fault it is 2.8 megapascals. This is similar enough to open the “earthquake gate” in a future quake, according to Burkhard and her team.
Is the “Big One” coming?
“What concerns us is not only that stresses are reaching historic highs, but that the relative stress conditions between the two fault systems are now approaching the range where an earthquake can cross both systems,” says Burkhard. A joint earthquake on the San Andreas and San Jacinto faults would be a much more serious event for the region.
The region affected by such a “Big One” includes some of the most densely populated areas in the United States, including the Los Angeles metropolitan area, San Bernardino, Riverside and the Coachella Valley. Important highways, railway lines and energy infrastructure also run through the Cajon Pass.
However: “Our study does not provide a prediction of when such an earthquake will occur. What we can say is that the system is critically stressed and that physics-based models like ours reveal more about the scenarios we should be prepared for,” emphasizes Burkhard. “This information is important for hazard assessment, infrastructure planning and emergency preparedness.”
Source: Liliane Burkhard (University of Bern) et al., Journal of Geophysical Research: Solid Earth, 2026; doi: 10.1029/2025JB033213