San Andreas and San Jacinto faults reach extreme stress as California’s ‘earthquake gate’ hits a 1,000-year high point

However, a very important intersection between two major fault zones in Southern California may now show unusual patterns of stress according to a novel model of an earthquake. Scientists analysing the San Andreas Fault and the San Jacinto Fault report that stress modelled in a location known as Cajon Pass, north of Los Angeles, has now reached extremely high values in comparison to the last 1,000 years of simulations performed by these scientists.

However, this study does not predict an upcoming earthquake. This research gives insight into the buildup of stress since the last major earthquakes and how a future rupture will behave.

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This research, conducted by Liliane Burkhard of the University of Bern, along with colleagues from the University of Hawai’i at Mānoa, US Geological Survey and Scripps Institution of Oceanography, appeared in the Journal of Geophysical Research: Solid Earth.

Citing the example of Cajon Pass, which is considered an “earthquake gate,” since the rupture may stop there, while under certain stress conditions it may also proceed from one fault system to another.

Such a difference may be significant for Southern California, where the size and extent of an earthquake not only depend on its starting point, but also on how far it extends.

Cajon Pass could determine how far an earthquake travels

The San Andreas and San Jacinto fault systems are among the most important sources of earthquake hazard in Southern California. Cajon Pass sits close to where sections of the two systems interact, making it an unusual point in the regional fault network.

The new research suggests the pass does not behave as a permanent barrier.

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Instead, whether a rupture crosses the junction may depend partly on the relative stress on neighbouring fault segments. When stress levels on the two systems become more similar and sufficiently high, the model shows that ruptures are more likely to continue across the junction. When the stress levels are more out of balance, a rupture is more likely to stop.

That is the basis for the researchers' “earthquake gate” description. The geological history provides examples of both behaviours.

While the 1857 magnitude 7.9 Fort Tejon earthquake broke a substantial portion of the San Andreas Fault, it failed to propagate further down into the Cajon Pass area in the model. This contrasts with the 1812 Wrightwood earthquake, which was a break that propagated across the junction of both fault systems.

This allows for research on what allows some major earthquakes to stay within the fault and other earthquakes to connect nearby faults.

A 1,000-year earthquake history reveals unusually high stress

Rather than looking at only modern-day observations, the scientists have created an analysis of the way earthquakes occurred over the course of approximately 1,000 years.

They have used a four-dimensional model to study change through both time and space based on the paleoseismic dataset compiled from radiocarbon dating, tree ring data, and reports of surface ruptures.

The model then calculates the effects each earthquake had on the stress of adjacent segments and how geologic processes increased the stress between earthquakes.

The result is impressive.

The Coulomb stress of the Mojave South fault segment of the San Andreas Fault in 2025 will be 2.8 MPa. The study says this is the highest modelled mean stress reached on that segment during the years.

On the San Jacinto-Bernardino segment, the modelled stress was even higher, at about 3.6 MPa. That exceeds the highest value recorded for that segment in the model's previous 1,000 years.

The North San Bernardino segment, meanwhile, was modelled at about 1.8 MPa.

The researchers say the combination matters because the San Andreas and San Jacinto segments are not simply experiencing high stress independently. Their relative stress levels have also moved into a range associated in the model with joint ruptures.

Why two highly stressed faults matter

An earthquake that remains on one fault can be very destructive. A rupture that connects neighbouring fault systems can potentially affect a much larger area.

That is why the researchers are interested in the current relationship between the San Andreas and San Jacinto systems rather than focusing on a single stress measurement.

The model found that the San Jacinto-Bernardino section has accumulated stress beyond the highest level seen during the modelled historical record. The neighbouring Mojave South section is also near its upper historical range.

The study identifies characteristic pre-earthquake stress ranges for different segments. For Mojave South, the modelled range is about 1.2–2.7 MPa; for North San Bernardino, 0.4–1.6 MPa; and for San Jacinto-Bernardino, 1.2–2.9 MPa.

Those figures should not be interpreted as countdown numbers.

They are a part of a model that relates to stress levels prior to previous earthquakes in relation to the current simulation. The purpose of using these is to understand fault interactions, and not to pinpoint a particular threshold for when an earthquake will happen.

The key observation is that similar stress conditions on the two major fault systems have appeared alongside modelled through-going ruptures in the past.

That makes the junction an important area for earthquake-hazard research.

High stress is not an earthquake forecast

However, the first criterion that should be noted is that it is the simplest one to overlook amid all those sensational headlines – the fact that the researchers do not predict the time when another earthquake will happen in Southern California.

Scientists do not have enough data now to be able to predict that kind of event based on fault stress measurements.

Instead, the new work provides a physics-based way to examine possible rupture behaviour. The authors say the findings can help improve seismic hazard assessments by showing how stress develops and moves between interconnected fault segments.

The study is particularly relevant to the wider Los Angeles and Southern California region because a rupture involving multiple fault sections could have consequences across a broad area.

Cajon Pass itself is also an important transportation corridor, adding another reason for scientists and emergency planners to understand how a large earthquake could propagate through the junction.

The research therefore offers something more useful than a prediction.

It identifies a specific geological connection where the behaviour of one fault may influence what happens on another.

For Southern California, that means the next major earthquake will not necessarily be a simple story of one fault breaking on its own. The interaction between faults — and the stress conditions at Cajon Pass — could help determine how far a rupture ultimately travels.

FAQ

1. What is the “earthquake gate” in California?
The term refers to Cajon Pass, a junction near where sections of the San Andreas and San Jacinto fault systems interact. Researchers found that its stress conditions may influence whether an earthquake rupture stops at the junction or continues across it.

2. Has California’s fault stress really reached a 1,000-year high?
The study's model indicates that stress on some fault segments is at or above the highest levels reached during its reconstructed 1,000-year earthquake history. The San Jacinto-Bernardino segment reached about 3.6 MPa, above its previous modelled maximum, while Mojave South reached about 2.8 MPa, its highest modelled mean value in the period.

3. Does the study predict a major earthquake soon?
No. The researchers explicitly frame the work as an assessment of the fault system's current stress state and possible rupture behaviour, not a prediction of when an earthquake will occur.

4. Why is Cajon Pass important to Los Angeles and Southern California?
Because it lies at an important junction between major fault systems. The model suggests that when stress conditions on neighbouring segments become sufficiently similar, a rupture may be more capable of crossing the junction and involving more than one fault system.