California Fault Stress at 1,000-Year Peak

TL;DR

• A landmark study from the University of Bern, published in the Journal of Geophysical Research: Solid Earth, has modelled 1,000 years of seismic history on the San Andreas and San Jacinto faults and found that current tectonic stress levels exceed anything recorded in the last millennium.
• The San Jacinto Fault's Bernardino section is experiencing 3.6 MPa of Coulomb stress — the highest value in the entire 1,000-year simulation — while the San Andreas's Mojave section sits at 2.9 MPa, both far above historical rupture thresholds.
• The study identifies the Cajon Pass "earthquake gate" as the critical junction: when stress alignment between the two faults occurs, a joint rupture could produce a magnitude 7.5–7.8 event affecting Los Angeles, San Bernardino, Riverside, and the Inland Empire simultaneously.
• Researchers emphasise this is not a prediction — the model cannot forecast timing — but the stress accumulation, combined with a 169-year seismic drought in Southern California, places the region in a state of unprecedented readiness for a major rupture.

The Quiet Before the Storm

Southern California has not experienced a major earthquake in over 169 years. The last great rupture — the Fort Tejon earthquake of 1857, estimated at magnitude 7.9 — tore through approximately 350 kilometres of the San Andreas Fault, from Parkfield to the Cajon Pass. Since then, the southern San Andreas and its sister fault, the San Jacinto, have remained in a prolonged seismic quiet period that has increasingly troubled geoscientists.

Now, a landmark study led by Dr Liliane Burkhard at the University of Bern, Switzerland, has quantified exactly how much stress has accumulated during that silence — and the results are sobering. Using a physics-based, four-dimensional earthquake cycle model that reconstructs 1,000 years of seismic history, the international research team has found that tectonic stress along the southern San Andreas and San Jacinto fault systems has reached — and in some locations exceeded — the highest levels recorded in the last millennium.

Published on 3 June 2026 in the Journal of Geophysical Research: Solid Earth, the study introduces a critical new concept: the Cajon Pass "earthquake gate" — a fault junction that may determine whether the next major rupture remains confined to a single fault or cascades across both systems in a catastrophic joint event.

The Research Team and Methodology

Who Conducted the Study

The research was led by Dr Liliane Burkhard of the Division of Space Research and Planetary Sciences at the Physics Institute, University of Bern, with collaborators from:

How the Model Works

The team constructed a physics-based earthquake cycle model — a computational simulation that tracks how stress accumulates, transfers, and releases across an entire fault network over centuries. The model, known as Maxwell, is a semi-analytic Fourier-transform simulator that represents an elastic crust resting atop a viscoelastic mantle.

Key features of the model:

• Spatial domain: approximately 450 × 900 kilometres, encompassing 38 major fault segments from the Carrizo Plain to the Borrego Mountain region.
• Temporal resolution: stress fields calculated at 10-year intervals, with annual-resolution simulations around major earthquake events.
• Input data: the model was fed a 1,000-year earthquake record reconstructed from geological evidence including radiocarbon dating, tree-ring anomalies, historical documentation of ground ruptures, geodetic observations, fault locking depths, and geological slip rates.

"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."
— Dr Liliane Burkhard, University of Bern

The result is the most sophisticated stress-state assessment of the southern San Andreas system ever produced — a dynamic, physics-grounded picture of how the fault network has evolved from the medieval period to the present day.

Key Finding: Stress at Historic Highs

The headline finding is unambiguous: tectonic stresses in the region are currently at their highest level in the last 1,000 years.

The Numbers

The model quantifies stress in megapascals (MPa) of Coulomb stress — a measure of how close a fault segment is to failure. The higher the value, the more primed the fault is to rupture.

The San Jacinto–Bernardino segment's 3.6 MPa reading is particularly alarming — it exceeds the stress level present before any major rupture included in the simulation record. The Mojave South segment, meanwhile, carries the highest stress accumulation rate in the system, meaning it is loading faster than any other segment.

"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. The results show that stresses in the region are currently at their highest level in the last 1,000 years."
— Dr Liliane Burkhard

The "Earthquake Gate": Cajon Pass

What Is an Earthquake Gate?

The study's most conceptually important contribution is the identification of Cajon Pass as an "earthquake gate" — a fault junction that does not simply block or permit ruptures, but responds dynamically to the stress conditions on both sides.

Cajon Pass is a narrow corridor northeast of Los Angeles where the San Andreas and San Jacinto faults converge. It is one of the most tectonically complex junctions in North America. Major highways (including Interstate 15), railroads, and energy infrastructure run directly through it.

Historical Precedent: Gate Open vs. Gate Closed

The model identifies two contrasting historical behaviours at Cajon Pass:

The difference between these two outcomes, the researchers found, lies not in the absolute stress on any single fault, but in the relative alignment of stress between the two systems.

The Alignment Principle

"The decisive factor is not only how much stress has built up on a single fault but how aligned the stresses on the two fault systems are. When the stress on both faults rises in concert over time, toward similarly high levels, conditions favor a large joint rupture crossing both systems."
— Dr Liliane Burkhard

When stress levels on the San Andreas and San Jacinto systems are closely aligned — both high, both primed — the gate tends to open, and a rupture can cascade across both faults. When they are out of step, ruptures are more likely to terminate at the junction.

The current situation is concerning on both counts. Not only are absolute stress levels at historic highs, but the two fault systems are approaching the relative stress configuration that historically preceded joint ruptures. The San Jacinto–Bernardino at 3.6 MPa and Mojave South at 2.8 MPa represent a system where both major faults are highly and similarly stressed.

"So not only is it concerning that the stresses are reaching historic highs, 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."
— Dr Liliane Burkhard

What a Joint Rupture Would Mean

A joint rupture of the San Andreas and San Jacinto faults crossing Cajon Pass would be a fundamentally different — and far more severe — event than one confined to a single fault.

Affected Regions

The impact zone would include some of the most densely populated and infrastructure-critical corridors in the United States:

• Greater Los Angeles metropolitan area (population ~18.5 million)
• San Bernardino and Riverside (Inland Empire, population ~4.6 million)
• Coachella Valley (including Palm Springs)
• Cajon Pass itself — through which run Interstate 15, major rail lines, gas and oil pipelines, and high-voltage transmission lines

Comparison: The 1994 Northridge Earthquake

For context, the 1994 Northridge earthquake — a magnitude 6.7 event on a blind thrust fault, not even on the San Andreas — caused an estimated $49 billion in damage (in 2025 dollars) and killed 57 people. A joint San Andreas–San Jacinto rupture would likely exceed magnitude 7.8 and could approach or surpass the 1857 Fort Tejon event (M 7.9), with shaking affecting a far larger and more densely populated region than in 1857.

Important Caveats: What the Study Does NOT Say

Dr Burkhard and her team have been careful to emphasise the limits of their findings:

"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."

Key caveats include:

1. No temporal prediction — the model cannot forecast when an earthquake will occur. Earthquake occurrence remains fundamentally unpredictable in the short term.

2. Model assumptions — the results depend on model parameters, fault geometries, and rheological assumptions. Different parameterisations could yield different stress estimates.

3. Stress is not fate — high stress indicates elevated probability, not certainty. Faults can remain critically stressed for decades before rupturing.

4. Not all stress leads to large earthquakes — some stress may be released through aseismic creep, small earthquakes, or other mechanisms.

The Supercomputing Dimension

The study represents a significant advance in computational seismology. The Maxwell model's calculations involve repeated evaluation of three-dimensional fault dislocations, viscoelastic relaxation processes, and Coulomb stress interactions across an entire regional fault network — computations that would have been impossible only a few decades ago.

The model assimilates:

• Paleoseismic records spanning 1,000 years
• Geological slip rates for 38 fault segments
• Geodetic measurements of present-day crustal motion
• Rheological models of crust and mantle behaviour

The team even simulated hypothetical future rupture sequences, including scenarios where multiple fault segments fail together. These experiments revealed that a complete rupture involving all major Cajon Pass fault strands would produce the largest stress release observed in the entire modelled history.

The LA-GRID Tool

Alongside the study, Dr Burkhard developed LA-GRID (Los Angeles Geospatial Risk and Infrastructure Dashboard) — a free, web-based, interactive tool that visualises seismicity and fault data for the Los Angeles region, with live updates on earthquakes and wildfires.

Global Applicability

While the study focuses on California, its framework is explicitly designed to be portable. The earthquake gate concept and the physics-based cycle modelling approach can be applied to other complex fault junctions worldwide, including:

• The North Anatolian Fault (Turkey), where a similar stress transfer cascade produced a sequence of magnitude 7+ earthquakes from 1939 to 1999
• The Alpine Fault (New Zealand), which has a well-documented ~300-year recurrence interval and is considered late in its seismic cycle
• The Dead Sea Transform (Middle East)
• The Enriquillo–Plantain Garden fault zone (Haiti)

What This Means for California

For California's 39 million residents, the study's message is clear: the seismic hazard is real, it is quantifiable, and it is at levels not seen in a millennium. The findings have direct implications for:

• Hazard assessment — providing physics-based inputs for probabilistic seismic hazard maps used in building codes
• Infrastructure planning — informing retrofit priorities for bridges, hospitals, schools, and utilities in the Cajon Pass corridor and surrounding regions
• Emergency preparedness — giving emergency managers a clearer picture of worst-case rupture scenarios for planning purposes
• Insurance and finance — enabling more accurate catastrophe modelling for the reinsurance industry

Publication Details

Full citation: Burkhard, L. M. L., Smith-Konter, B. R., Scharer, K. M., & Sandwell, D. T. (2026). "Cajon Pass and the Southern San Andreas Fault System: Earthquake Cycle Stress Accumulation and Present-Day Loading." Journal of Geophysical Research: Solid Earth. DOI: 10.1029/2025JB033213

Published: 3 June 2026

Institutions: University of Bern, University of Hawaiʻi at Mānoa, U. S. Geological Survey (Pasadena), Scripps Institution of Oceanography (UC San Diego)

Sources

• Burkhard, L. M. L., et al. (2026). Journal of Geophysical Research: Solid Earth. 
• University of Bern media release, 8 June 2026: "Earthquake model reveals: Stress in California at record level"
• EarthSky, 9 June 2026: "California faults under record stress, study finds"
• Phys.org, 8 June 2026: "California's tectonic stress has reached record level, earthquake model reveals"
• Supercomputing Online, 8 June 2026: "Supercomputers reveal dangerous stress buildup beneath Southern California"
• LA-GRID Interactive Dashboard