Southern California rests upon one of the most thoroughly monitored and geologically active fault networks in the United States. Earthquakes in this seismically volatile region typically occur where immense sections of the Earth’s crust grind against one another, become temporarily locked in place, and continuously accumulate intense mechanical stress over decades and centuries. When that heavily stored tectonic energy is finally overcome and violently released, the ground ruptures, resulting in a potentially catastrophic earthquake.
Two of the most dominant and critical fault systems driving this tectonic activity in Southern California are the San Andreas and the San Jacinto faults. Together, these massive geological structures accommodate a significant portion of the region’s relentless tectonic motion. Northeast of the sprawling metropolitan expanse of Los Angeles, the two fault systems converge closely at Cajon Pass—a complex and geologically intricate junction where a seismic rupture traveling along one fault corridor may, under the right physical conditions, leap across and continue onto the other.
This possibility carries profound implications for millions of residents because the broader Los Angeles region has not experienced a comparable major earthquake since the devastating magnitude 7.9 Fort Tejon earthquake struck in 1857. Throughout this unusually long quiet period spanning nearly two centuries, continuous tectonic stress has kept building silently along various segments of the fault system, fueling longstanding questions among scientists and emergency planners regarding how a future large rupture might unfold when the system finally gives way.
California Fault Stress Reaches Extreme Levels
A comprehensive new study led by Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern has now taken a much broader, millennium-long view of that escalating seismic risk. An international team of researchers successfully modeled roughly 1,000 years of historical and simulated earthquake activity along both the southern San Andreas and San Jacinto fault systems to estimate the exact amount of mechanical stress currently concentrated around the pivotal Cajon Pass junction.
The collaborative research effort also involved distinguished scientists from the University of Hawaiʻi at Mānoa, the U.S. Geological Survey Earthquake Science Center located in Pasadena, and the Scripps Institution of Oceanography at UC San Diego.
Their findings suggest that tectonic stresses in localized parts of the system have officially reached, and in certain specific locations surpassed, the absolute highest values recorded anywhere in the model’s entire 1,000-year simulation history.
Furthermore, the researchers formally describe Cajon Pass as an "earthquake gate"—a critical fault junction that essentially acts as a switch, helping to determine whether a large earthquake remains confined to a single fault or successfully propagates across both major fault systems simultaneously. The groundbreaking study has just been published in the peer-reviewed Journal of Geophysical Research: Solid Earth.
Modeling 1,000 Years of Earthquakes
To accurately reconstruct how mechanical stress shifted, concentrated, and dissipated across centuries, the research team developed an advanced, physics-based, four-dimensional earthquake cycle model. In practical application, this sophisticated model simulates complex tectonic mechanics across three dimensions of geographic space while continuously tracking how the subterranean fault system evolves dynamically through time.
The team then fed the model a meticulous 1,000-year record of past earthquakes compiled from a wide array of geological and historical evidence. This baseline data included precise radiocarbon dating of ancient fault displacements, unusual growth patterns preserved within ancient tree rings, and detailed historical accounts describing surface-rupturing earthquakes that permanently altered the landscape.
"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, tectonic stresses across the entire regional system are now sitting at their highest overall level observed across the past millennium.
Why Cajon Pass Acts Like an "Earthquake Gate"
One of the study’s most critical central findings revolves around the complex dynamics of what happens when a moving seismic rupture finally reaches the Cajon Pass junction.
Rather than behaving like a simple physical barrier that automatically halts a fault rupture, the junction functions much more conditionally. Depending entirely on the prevailing state of mechanical stress across the surrounding fault networks, Cajon Pass may either abruptly stop a propagating rupture in its tracks or allow it to seamlessly cross over from one fault system into the other, effectively doubling the scale of the event.
Historical earthquakes in the region vividly illustrate both of these distinct possibilities. During the massive Fort Tejon earthquake of 1857, the surface rupture famously stopped at Cajon Pass and failed to continue onto the neighboring San Jacinto Fault. Conversely, the destructive Wrightwood earthquake of 1812 successfully pushed through the junction, violently rupturing both major systems in a single, continuous through-going event.
"The earthquake gate concept captures something important about how fault junctions work," Dr. Burkhard explains. "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 sheer magnitude of stress measured on any single fault is only part of the overarching geological puzzle.
The researchers discovered that the relative relationship between the stress levels accumulated on both interacting fault systems may be an even more decisive factor. If both faults happen to become intensely stressed at the exact same time, environmental conditions within the Earth’s crust become substantially more favorable for a rupture to successfully cross Cajon Pass and cascade through both fault systems consecutively.
Conversely, if stress levels along the two corridors rise asynchronously at different times, an advancing earthquake rupture is statistically far more likely to stall out and terminate at the junction.
Currently, the advanced model places the mechanical stress on the San Jacinto-Bernardino section at an alarming 3.6 megapascals (MPa), a numerical unit scientists use to measure internal pressure and mechanical stress. This figure stands higher than any value reached anywhere else during the entire 1,000-year simulation. Meanwhile, on the nearby Mojave South section of the San Andreas fault, modeled stress has climbed to 2.8 MPa.
This means both vital fault segments are currently carrying unusually high and relatively comparable levels of accumulated tectonic stress. According to the research team, this specific configuration closely mirrors the physical conditions observed in the model right before historical ruptures that successfully crossed both fault systems simultaneously.
"So not only is it concerning that the stresses are reaching historic highs," Dr. Burkhard notes, "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 multi-fault rupture that successfully crosses Cajon Pass and involves both the San Andreas and San Jacinto faults would inevitably impact a vastly larger geographic area than an earthquake arbitrarily confined to just one individual fault.
The surrounding region encompasses some of the most densely populated, economically vital, and infrastructure-dependent urban centers in the United States. Major population zones and economic hubs that could experience severe shaking and structural damage include the greater Los Angeles metropolitan area, San Bernardino, Riverside, and the heavily populated Coachella Valley.
Cajon Pass itself serves as a vital transportation, economic, and infrastructure corridor for the American Southwest. Major interstate highways, heavily traveled freight and passenger rail lines, and critical energy pipelines all run directly through the area, rendering the junction exceptionally important not only from a purely academic geological perspective but also for regional emergency planning and disaster mitigation.
"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 says. "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 these historic stress measurements, the study’s authors strongly emphasize that their findings must not be misinterpreted as a definitive forecast or prediction of an upcoming earthquake date.
Even when underground faults are known to be critically stressed, modern seismology still lacks the capability to use that information to determine the precise day, month, or year when a sudden rupture will finally trigger. Instead, the physics-based model serves a different vital purpose: it helps researchers determine which specific earthquake scenarios are physically plausible given the precise current state of the regional fault system.
Dr. Burkhard reiterates this crucial distinction: "The study is not a prediction of when an earthquake will occur. 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."
