Fault pressure in California reaches levels not seen in a millennium, and researchers say the region is primed for a major quake

Scientists say pressure on Southern California’s San Andreas and San Jacinto faults has climbed to among the highest levels shown in a 1,000-year computer model. The two faults carry heavy strain at the same moment, which makes the Cajon Pass junction more likely to break into a rupture that could span both systems.

Dr. Liliane Burkhard of the Division of Space Research and Planetary Sciences (WP) at the Physics Institute of the University of Bern led the research. 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 UC San Diego took part in the work. The team modeled about 1,000 years of earthquake activity along the southern San Andreas and San Jacinto fault systems to estimate how much stress is currently built up around Cajon Pass.

The Stress Model

The researchers built a physics-based, four-dimensional earthquake cycle model to reconstruct how stress changed over centuries. This model simulates events across three dimensions of space while also following how the fault system changes through time. A 1,000-year record of past earthquakes, drawn from several kinds of evidence, was then added into the model. That record includes:

1. Radiocarbon dating
2. Unusual patterns recorded in tree rings
3. Historical accounts of ruptures that broke the ground 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 Burkhard. “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, the researchers estimate the extent to which the fault system is already under stress today.

The model shows that pressures throughout the area have reached a peak not seen for the past 1,000 years.

What Cajon Pass Does

Instead of acting as a mere obstacle, the junction’s performance varies according to the condition of strain across the faults around it. It can halt a rupture entirely, or it can permit the disturbance to spread from one fault system into another. Historical earthquakes show examples of both outcomes.

The rupture of the Fort Tejon earthquake of 1857 ended at Cajon Pass, without extending onto the San Jacinto Fault. The Wrightwood earthquake of 1812, however, crossed the junction and broke both fault systems together in a single continuous event. That difference is explained by “The earthquake gate concept captures something important about how fault junctions work,”. The interval between these two earthquakes was “Cajon Pass doesn’t simply block or channel earthquakes: It responds to stress conditions, and those conditions change over centuries.”.

Stress on Both Faults Together

What matters isn’t just how much pressure builds up on any single fault. It’s also about how those pressures relate to one another across two separate fault systems. When both systems reach high stress at the same moment, the odds rise that a rupture could pass through Cajon Pass and keep going across both. But if the stress peaks at different points in time, an earthquake may be more likely to end at the point where the two systems meet.

During the 1,000-year simulation, the San Jacinto-Bernardino section is under the greatest stress within the model, at 3.6 MPa, a reading that exceeds the pressure recorded on any other part of the system. That measurement is expressed in megapascals (MPa), a scientific unit for gauging force or strain. The adjacent Mojave South portion of the San Andreas fault has seen modeled stress rise to 2.8 MPa.

Both fault segments are now holding unusually high and closely matched amounts of strain, and the researchers say that arrangement mirrors what their model showed just before earlier ruptures that spanned both fault systems.

The 1857 Comparison

One of the most closely watched fault networks in the United States lies beneath Southern California. Where sections of Earth’s crust move against one another, become stuck, and slowly build up strain, earthquakes tend to occur. The release of that stored energy produces the shaking we call an earthquake.

The San Andreas and San Jacinto faults are among the most significant fault systems in Southern California. Between them, they absorb much of the tectonic movement in the region. To the northeast of Los Angeles, the two systems converge at Cajon Pass — a complex geological meeting point where a rupture moving along one fault can, given the right circumstances, keep going onto the other.

The significance stems from the fact that the wider Los Angeles area has gone without a comparable major earthquake since the magnitude 7.9 Fort Tejon quake in 1857. Throughout that lengthy span, tectonic strain kept accumulating along portions of the fault network, leaving open questions about how a subsequent major rupture could play out.

What This Means

The report describes circumstances that would permit a rupture to move across both fault systems. No timing for an earthquake is forecast. The scientists characterize Cajon Pass as a “earthquake gate”, a point where faults meet and might decide whether a major earthquake stays confined to one fault or spreads across both systems. The research appears in Journal of Geophysical Research: Solid Earth.

A look at how the two quakes compare proves telling. The 1857 Fort Tejon rupture ended at Cajon Pass. The 1812 Wrightwood quake pushed right through it. The model points to stress conditions rather than fixed barriers as what decides whether a rupture stops or keeps going.

The study’s findings are troubling. They describe a rare moment when pressure built up on both fault systems simultaneously. Such an alignment has preceded ruptures crossing both systems in the past.

Source material: “Scientists find an “earthquake gate” as California fault stress hits a 1,000-year high,” ScienceDaily.

California · earthquake · fault stress · San Andreas · San Jacinto