Southern California sits atop one of the most active, closely monitored, and complex fault networks in the United States. Earthquakes here, as in similar tectonic regions around the world, typically occur where massive sections of the Earth’s crust grind against one another, become frictionally locked, and slowly accumulate immense mechanical stress over decades and centuries. When that stored subterranean energy finally overcomes the frictional resistance and is violently released, the result can be a catastrophic earthquake that shakes millions of lives and reshapes the landscape.
Two of the most vital and active fault systems in this densely populated region are the San Andreas and the San Jacinto faults. Together, these two geological giants accommodate a substantial portion of the relentless tectonic motion taking place across Southern California. Northeast of Los Angeles, these two major systems converge at a critical juncture known as the Cajon Pass. This complex geological gateway represents a point where a seismic rupture traveling along one fault may, under the right physical conditions, leap across and continue onto the other, creating a multi-fault mega-earthquake.
That specific possibility carries profound implications for the region, largely because the broader Los Angeles metropolitan area has not experienced a comparable major earthquake along this northern segment since the massive magnitude 7.9 Fort Tejon earthquake shook the region in 1857. During the long, deceptive quiet period that has persisted in the nearly two centuries since, tectonic stress has continued to quietly accumulate along various segments of the fault system. This prolonged silence has long fueled intense scientific debate among seismologists and geologists about how a future large rupture might unfold when the system finally gives way.
California Fault Stress Reaches Extreme Levels
A new study led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences at the Physics Institute of the University of Bern has now taken a remarkably long-term view of that seismic risk. An international team of researchers modeled roughly 1,000 years of historical and prehistoric earthquake activity along both the southern San Andreas and the San Jacinto fault systems. Their primary objective was to estimate the precise amount of mechanical stress currently concentrated around the pivotal Cajon Pass.
The collaborative research effort also involved scientists from the University of Hawaiʻi at Mānoa, the U.S. Geological Survey Earthquake Science Center in Pasadena, and the Scripps Institution of Oceanography at the University of California, San Diego. By pooling their expertise in geodesy, geology, and advanced computational modeling, the team was able to peer deep into the mechanical behavior of the subterranean crust.
The results of their extensive analysis suggest that tectonic stresses in certain critical parts of the system have reached, and in some specific locations even surpassed, the absolute highest values found anywhere throughout the model’s simulated 1,000-year history.
Furthermore, the researchers formally describe the Cajon Pass as an "earthquake gate"—a dynamic fault junction that acts as a gatekeeper, helping to determine whether a large earthquake remains confined to a single fault or successfully bridges the gap to rupture both fault systems simultaneously. The findings of this important study have just been published in the peer-reviewed Journal of Geophysical Research: Solid Earth.
Modeling 1,000 Years of Earthquakes
To accurately reconstruct how stress has evolved over centuries of tectonic inactivity and seismic release, the research team developed an advanced, physics-based, four-dimensional earthquake cycle model. In practical terms, this sophisticated computer model simulates complex physical processes across three dimensions of space while rigorously tracking how the entire fault network changes and responds through time.
The research team then fed the model a comprehensive 1,000-year record of past earthquakes compiled from multiple independent lines of geological and historical evidence. These vital inputs included precise radiocarbon dating of displaced sediment layers, unusual growth patterns recorded in ancient tree rings that experienced sudden ground deformation, and detailed historical accounts describing past ruptures that broke the Earth’s surface.
"The model tracks how each earthquake changes stress on neighboring fault segments, how stress accumulates during the quiet intervals between events, and how the deeper layers of the crust slowly relax following large ruptures," explains Dr. Burkhard, lead author of the study. "This simulation allows us to understand how stresses in the fault system build up over centuries. By running the earthquake history of Southern California as a simulation, we can estimate the extent to which the fault system is already under stress today."
According to the output generated by the model, the accumulated stresses across the Southern California region are now sitting at their highest overall level in the past millennium.
Why Cajon Pass Acts Like an "Earthquake Gate"
One of the study’s most critical central findings revolves around the complex physical mechanics of what happens when a seismic rupture finally reaches the Cajon Pass.
Rather than behaving like a simple physical barrier that permanently halts the propagation of an earthquake, the junction is far more versatile. Depending heavily on the shifting state of mechanical stress across the surrounding network of faults, the pass may either arrest a moving rupture and bring it to a halt, or it may facilitate its continuation, allowing the seismic energy to spill over from one fault system directly into the other.
Historical earthquakes in the region provide clear precedents for both potential outcomes. During the famous magnitude 7.9 Fort Tejon earthquake of 1857, the massive rupture traveled down the San Andreas fault but ultimately stopped at Cajon Pass, failing to jump across to the San Jacinto Fault. In stark contrast, the destructive Wrightwood earthquake of 1812 successfully breached the junction, cutting through both fault systems in a single, continuous, through-going event.
"The earthquake gate concept captures something important about how fault junctions work," Dr. Burkhard notes. "Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries."
Two Faults Are Becoming Stressed Together
The absolute amount of mechanical stress measured on an individual fault is only part of the complex puzzle. The researchers discovered that the relationship and balance between the stress levels on the two adjacent fault systems may be an even more crucial factor in determining future seismic hazards.
According to the study’s findings, if both major faults become highly stressed at the same time, physical conditions within the Earth’s crust become substantially more favorable for a rupture to successfully cross the Cajon Pass and propagate through both systems concurrently. Conversely, if stress levels on the two faults rise at different, uncoordinated times, an earthquake rupture is statistically and physically more likely to stall and terminate at the junction.
The computer model currently places the mechanical stress on the San Jacinto-Bernardino section at an estimated 3.6 megapascals (MPa), which is higher than any value recorded anywhere else during the entire 1,000-year simulation. Megapascals are the standard unit of measurement scientists use to quantify internal pressure or mechanical stress within rocks.
Meanwhile, on the nearby Mojave South section of the San Andreas fault, modeled stress has reached 2.8 MPa.
This means that both major fault segments are currently carrying unusually high and relatively comparable levels of mechanical stress. According to the research team, this particular configuration closely mirrors the physical conditions observed in the model prior to historical ruptures that successfully crossed both fault systems simultaneously.
"So not only is it concerning that the stresses are reaching historic highs," Dr. Burkhard explains, "but also that the relative stress conditions between the two fault systems are approaching the range we associate with major ruptures crossing both faults simultaneously—and that is a scenario with much larger consequences for the region."
Why a Joint Rupture Could Have Wider Consequences
A seismic rupture that successfully crosses the Cajon Pass to involve both the San Andreas fault and the San Jacinto fault would inherently affect a vastly larger geographic area and release significantly more energy than an earthquake restricted to just a single fault.
The surrounding region encompasses some of the most densely populated, economically vital, and infrastructure-dependent urban centers in the United States. Areas that would find themselves directly in the path of such a combined event include the greater Los Angeles metropolitan area, San Bernardino, Riverside, and the heavily populated Coachella Valley.
Furthermore, the Cajon Pass itself is a vital lifeline for Southern California, serving as a primary transportation, logistics, and infrastructure corridor. Major interstate highways, heavily trafficked rail lines, and critical energy pipelines all traverse the narrow mountain pass, making this geological junction critically important not only from an academic geoscience perspective, but also for regional emergency planning, disaster response, and national supply chain resilience.
"The question of when and how the next major earthquake will occur in this region is one of the most pressing problems in applied geoscience," Dr. Burkhard emphasizes. "Our results provide a clearer, physics-based picture of the current stress state of the fault system, and the framework we developed is not just applicable to California, but also for other complex fault junctions worldwide."
High Stress Does Not Mean an Earthquake Is Imminent
Despite the alarming nature of finding historic stress levels within the fault network, the researchers strongly emphasize that their new study should not be interpreted as a deterministic forecast or a prediction of an exact earthquake date.
Even when faults are known to be critically stressed, modern seismology still cannot use that information alone to pinpoint the exact day, month, or year when a rupture will initiate. Instead, advanced physics-based models serve a different, highly valuable purpose: they help scientists reveal which specific earthquake scenarios are physically plausible given the precise current state of the subterranean fault system.
"The study is not a prediction of when an earthquake will occur," Dr. Burkhard reiterates. "What we can say is that the system is critically stressed and that physics-based models like ours give a clearer picture of the range of scenarios we should be prepared for. This information is important for hazard assessment, infrastructure planning and emergency preparedness."
