San Andreas Fault stress hits 1,000-year high, raising quake risk

Tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has reached — and in some areas exceeded — the highest levels seen in the past 1,000 years, according to new research led by Earth scientists at the University of Hawai?i at Ma?noa.

Researchers say the system is not showing signs of an imminent rupture, but is operating under unusually high stress in a long-term seismic cycle that could support large earthquakes, including multi-fault events.

The study, published in the Journal of Geophysical Research: Solid Earth, suggests the region is in a “critically loaded state,” with stress building across multiple fault segments. One key area of focus is Cajon Pass, a junction between the two fault systems that may act as an “earthquake gate,” either blocking ruptures from crossing between faults or allowing them to link into a single larger event.

Lead author Liliane Burkhard, a research affiliate at the University of Hawai?i’s Institute of Geophysics and Planetology and scientist at the University of Bern, said the system is highly stressed after more than 160 years since the last major rupture.

“Right now, with stress at historically high levels across the region and more than 160 years elapsed since the last major rupture, the system is in a critically loaded state,” Burkhard said.

Researchers say a rupture involving both fault systems could be significantly more damaging than a single-fault earthquake due to its size and proximity to major population centers, including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

Can the San Andreas Fault fully break?

The San Andreas Fault is not a crack that can “split open” and drop a chunk of California away from the continent. It’s a strike-slip plate boundary, meaning the Pacific Plate and North American Plate are sliding past each other horizontally, not pulling apart, according to the U.S. Geological Survey.

So even in a very large earthquake, what happens is sideways movement along the fault, not a breakup of the landmass. Parts of California can shift suddenly by feet or even tens of feet during a major rupture, but both sides remain part of the same crustal system.

However, over millions of years, continued movement along the fault could gradually bring Los Angeles and San Francisco into closer proximity, geologic models suggest.

In a major rupture, strong shaking could last tens of seconds to more than a minute. The most severe damage would occur near the fault and in areas built on soft or water-saturated soils, which can amplify shaking and increase the risk of liquefaction.

Scientists note that infrastructure built across active faults is especially vulnerable, as surface rupture can directly offset roads, buildings, and other structures that span the fault trace. (Source: USA Today)