Published In The Journal Of Geophysical Research: Solid Earth And Built From A Physics-Based Computer Model Fed A Millennium Of Earthquake History Reconstructed From Radiocarbon-Dated Sediments And Tree-Ring Records, The Study Identifies Cajon Pass As An “Earthquake Gate” That Could Either Block Or Allow A Joint Rupture Of Both Fault Systems Simultaneously. An Event That Would Strike Los Angeles, San Bernardino, Riverside, And The Coachella Valley At Once
According to the University of Hawaiʻi System News, Newsweek and ABC7, tectonic stress along the San Andreas and San Jacinto fault systems in Southern California has now reached, and in some places exceeded, the highest levels seen in the past 1,000 years, according to research led by Earth scientists at the University of Hawaiʻi at Mānoa.
The study, published in the peer-reviewed Journal of Geophysical Research: Solid Earth, carries direct implications for seismic hazard assessments across one of the most densely populated and infrastructure-critical corridors in the United States.
How the researchers built a thousand-year stress map. The team, led by Liliane Burkhard, constructed a physics-based computer model designed to simulate how stress accumulates and releases along the southern San Andreas and San Jacinto fault systems, including at Cajon Pass; the geological junction connecting the two systems.
To calibrate the model, researchers fed it a millennium-long earthquake history of the region, reconstructed from geological evidence including radiocarbon dating of displaced sediments and tree-ring records.
Running this simulation forward to the present day allowed the team to estimate how much stress has actually built up across the fault systems over that thousand-year window, and to compare today’s loading against every prior point in that record.
The result: stress at a millennial peak, and in places already past it. The study found that several fault segments are now at, or in some cases above, the highest stress values recorded across the entire thousand-year reconstruction.
Burkhard summarised the headline finding directly, noting that several segments are now at or above the highest values seen in the past millennium, and that the broader system may now be capable of producing a large rupture that runs through both fault systems together rather than just one.
Stress that would ordinarily be released through periodic large earthquakes has instead continued accumulating, the study found, climbing to what researchers describe as unprecedented levels for the region. globalsecurity
Why more than 160 years matters. Part of what makes this finding significant is timing. More than 160 years have elapsed since the last major rupture along this section of the San Andreas system. In fault mechanics, stress along a locked fault segment behaves somewhat like tension building in a coiled spring: it accumulates steadily between major slip events, and the longer the interval since the last release, the more stored energy has had time to build.
Combining a historically long quiet period with stress levels already at or above the highest points in the millennial record is what led Burkhard to describe the current state of the system as critically loaded; language chosen specifically because it describes a physical condition, not a forecast.
Cajon Pass: the “earthquake gate” that decides how bad it could get. Perhaps the most consequential structural finding in the study concerns Cajon Pass, the narrow geological junction where the San Andreas and San Jacinto fault systems meet.
The researchers describe Cajon Pass as functioning like a gate: sometimes it blocks a rupture on one fault system from spreading into the other, and sometimes it allows the rupture to pass through, turning what would have been a single-fault earthquake into a joint rupture involving both systems simultaneously.
Whether that gate opens or stays shut during any given rupture appears to depend on how closely aligned the stress levels are between the two fault systems at the precise moment of rupture; a condition that becomes statistically more likely to occur the longer both systems remain under sustained, elevated loading, exactly the situation the new modelling now describes.
Why a joint rupture would be categorically worse than a single-fault quake. A rupture confined to one fault system, however large, has a roughly predictable footprint based on decades of seismic hazard modelling for that fault alone. A joint rupture spanning both the San Andreas and San Jacinto systems through Cajon Pass would be a fundamentally different and more destructive event, releasing a longer rupture length and correspondingly greater total energy across a wider geographic area.
The populations directly in the path of such an event include Los Angeles, San Bernardino, Riverside, and the Coachella Valley; collectively home to many millions of people and containing some of the most economically and logistically critical infrastructure corridors on the West Coast, including major transportation arteries that pass directly through or near Cajon Pass itself.
What the study explicitly does not claim. The researchers are emphatic on one point: this is not a prediction of when an earthquake will happen. Earthquake timing remains fundamentally unpredictable with current science, and the study makes no attempt to assign a date, month, or even a probability window to a future rupture.
What the research does provide is a rigorously quantified picture of present-day accumulated stress relative to the historical record; a foundation that can meaningfully inform engineering, planning, and policy decisions even though it cannot answer the question of when.
What this is actually useful for. Burkhard and her co-authors frame the study’s primary value as practical rather than predictive: a contribution to national and global earthquake hazard research built on quantitative modelling rather than speculation.
The findings are intended to feed directly into improved seismic hazard analyses for Southern California, with downstream implications for infrastructure investment planning, updates to regional building codes, and the strengthening of emergency preparedness systems across the corridor most exposed to a Cajon Pass joint rupture scenario.
The modelling framework itself is also designed to generalise: the same physics-based approach used here for Cajon Pass can, the researchers note, be applied to other complex multi-fault junctions around the world, positioning this study as a reusable tool for global seismic hazard work rather than a one-off regional analysis.
The bottom line. A thousand years of earthquake history, reconstructed from tree rings and radiocarbon-dated sediment, now shows Southern California’s two most consequential fault systems carrying more stored stress than at any point in that entire record, with a geological gate between them that history suggests can, under the right conditions, let that stress through both systems at once.
The science cannot tell anyone when. It can, for the first time with this level of rigor, tell millions of people living along that corridor exactly how loaded the spring currently is.
To check out our previous coverage on natural disasters, scientific research, and global hazard preparedness, read our articles here.
