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

New research finds the San Andreas and San Jacinto fault systems are in a “critically loaded” state, with stress levels that could support large, multi-fault earthquakes near major California cities.

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 Hawaiʻi at Mānoa.

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 Hawaiʻi’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.

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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.

There are other tectonic settings on Earth where continents rift apart and eventually form new oceans — such as the East African Rift, which is in the process of splitting into two tectonic plates — but the San Andreas system is not one of them. It is a strike-slip boundary, where plates slide past each other rather than spreading apart.

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

What would happen if the San Andreas Fault ruptured?

Surface rupture occurs when movement along a fault deep within the Earth breaks through to the surface. According to the Pacific Northwest Seismic Network, it is one of the clearest physical expressions of an earthquake, showing permanent deformation where two sides of a fault slip past one another.

Most earthquakes do not produce surface rupture, according to the U.S. Geological Survey. Some faults do not reach the surface, and even when they do, rupture does not always propagate all the way upward during a given event.

When surface rupture occurs, it can produce either horizontal or vertical offsets, depending on the type of fault. Strike-slip faults, such as the San Andreas Fault, typically cause horizontal displacement, whereas dip-slip faults can produce vertical displacement. Some earthquakes involve a combination of both.