Could earthquake gate open? San Andreas fault at highest stress level in 1,000 years

A study found parts of the San Andreas and San Jacinto faults are critically loaded after long stress build-up. The findings sharpen planning for a possible linked rupture through Cajon Pass without predicting when a quake will strike.

In Short

• Researchers modelled fault behaviour using geological records spanning the past millennium
• Cajon Pass may determine whether ruptures spread between two major faults
• An opened gate could produce a larger, more destructive connected earthquake

A new scientific study has revealed that parts of Southern California's infamous San Andreas Fault system are now under the highest levels of tectonic stress seen in at least 1,000 years, raising fresh concerns about the region's earthquake risk.

Researchers from the University of Hawai‘i at Manoa and collaborators found that stress along the San Andreas and neighbouring San Jacinto fault systems has reached, and in some places exceeded, levels recorded over the past millennium.

The findings were published in the Journal of Geophysical Research: Solid Earth.

The study does not predict when an earthquake will occur. However, scientists say the results suggest the fault network is in a "critically loaded" state after more than 160 years without a major rupture along key sections of the southern San Andreas Fault.

To investigate the fault systems, researchers built a sophisticated computer model that simulated how stress accumulates and is released during earthquakes. The model incorporated a 1,000-year earthquake history reconstructed from geological clues, including displaced sediments, radiocarbon dating and tree-ring records.

Their findings point to a particularly important location known as Cajon Pass, a junction where the San Andreas and San Jacinto faults come close together. Scientists describe it as a potential "earthquake gate" that can either block a rupture from spreading between the two fault systems or allow a single earthquake to jump from one fault to the other.

If the gate "opens," a rupture could potentially involve both fault systems simultaneously, producing a much larger and more destructive earthquake than a typical single-fault event.

Such a scenario would have significant implications for millions of residents living in and around Los Angeles, San Bernardino, Riverside and the Coachella Valley, as well as critical transportation, energy and water infrastructure across Southern California.

The researchers found that the stress levels on the two fault systems are currently unusually aligned, creating conditions that could favour a larger connected rupture. Stress that would normally be released through major earthquakes has continued to build over time, leaving the region under unprecedented strain.

Despite the alarming findings, scientists emphasise that the study is not a forecast.

"This is not a prediction of when an earthquake will happen," lead author Liliane Burkhard said. Instead, the research provides a clearer understanding of the range of earthquake scenarios that emergency planners, engineers and policymakers should prepare for.

The study highlights the importance of continued investment in earthquake-resistant infrastructure, updated hazard assessments and public preparedness in one of the world's most seismically active urban regions.

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