Tectonic stress building beneath Southern California could have far-reaching consequences for millions of residents from the Coachella Valley to Los Angeles. New research from the University of Hawaiʻi at Mānoa warns the region’s major fault systems are under some of the highest strain seen in at least 1,000 years.
Scientists say the San Andreas and San Jacinto faults — which run through the Inland Empire, San Bernardino and Riverside counties and sit near densely populated areas — are not signaling an imminent earthquake. But the system is “critically loaded,” raising the potential for a powerful rupture that could affect communities across Southern California.
The study suggests stress is building across multiple connected fault segments, increasing the likelihood of a larger, multi-fault earthquake rather than a more isolated event — a scenario that could amplify damage.
Published in the Journal of Geophysical Research: Solid Earth, the study points to Cajon Pass as a key area of concern. The junction between the two fault systems 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 a 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,” Burkhard said.
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.
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.
Could California fall into the ocean?
California is not going to fall into the ocean. It sits on rigid blocks of Earth’s crust that are part of two moving tectonic plates: the Pacific Plate and the North American Plate. These plates meet along the San Andreas Fault, where they slide horizontally past one another at roughly 45 millimeters per year, about the rate your fingernails grow.
This type of boundary is called a transform fault, meaning the primary motion is sideways rather than vertical. Because the motion is lateral, there is no geologic mechanism for California to drop or “fall” into the ocean, according to the U.S. Geological Survey. Instead, over millions of years, parts of coastal California gradually shift northwest relative to North America, slowly reshaping the region but not separating it from the continent.
How many feet has California moved over the last 1,000 years?
Over the past 1,000 years, California has shifted roughly 140 to 160 feet northwest along the San Andreas Fault, based on an average movement rate of about 45 to 50 millimeters per year. That adds up to roughly half a football field of motion over a millennium.
This movement is not smooth or evenly distributed. Some sections of the fault slowly “creep,” while others remain locked for long periods and release energy in earthquakes. And it’s not the entire state moving as a single rigid block; instead, it’s a network of crustal pieces shifting along a broad fault system.
San Andreas Fault map
What causes earthquakes?
The Earth has four layers: the inner core, outer core, mantle, and crust. The crust and top of the mantle make up another area called the “lithosphere,” which acts like a skin surrounding the Earth’s surface.
The lithosphere, however, is not a single piece but is divided into puzzle-like fragments called tectonic plates, according to the USGS. These parts of the lithosphere are not stationary and move slowly.
As tectonic plates move past one another, they occasionally bump or collide. This places stress on the plates’ edges. When the stress becomes too great, it creates cracks called “faults.” The point where these faults move against each other is called the “fault line.”
When there is too much friction between the fault lines, energy is suddenly released, triggering seismic waves that cause an earthquake.
Why are earthquakes so common in California?
California sits along the boundary between two massive tectonic plates — the Pacific Plate and the North American Plate. These plates are constantly moving, sliding past one another at different speeds.
The state is also home to more than 500 active faults, according to geologists, making it one of the most earthquake-prone regions in the United States. The most famous is the San Andreas Fault, a roughly 800-mile system that runs through much of the state and forms the primary boundary between the two plates.
Other major fault systems include the Hayward Fault in the Bay Area, the Calaveras Fault in Central California, and the San Jacinto and Elsinore faults in Southern California.
But earthquakes are not limited to land. Many occur offshore, including near the Mendocino Triple Junction, where the Pacific, North American, and Gorda plates all meet. That junction creates one of the most seismically active offshore regions along the West Coast.
Can earthquakes be predicted?
Earthquakes are not predicted. Neither the U.S. Geological Survey nor scientists have predicted a major earthquake, and there’s no expectation that this will change in the near future.
What is California’s Earthquake Early Warning system?
California’s Earthquake Early Warning system uses seismic sensors, ground-motion monitoring, and alerting technology to deliver warnings to people via cell phones before the strongest shaking arrives, according to the official website.
Officials say even a brief warning can allow people to take protective actions such as “Drop, Cover and Hold On,” or put devices and systems into a safe mode. In some cases, alerts may arrive just as shaking begins or after an earthquake has already passed, particularly for those closer to the epicenter.
The system is delivered through multiple channels, including Android Earthquake Alerts, Wireless Emergency Alerts, and the MyShake app.
Developed by the UC Berkeley Seismological Laboratory, MyShake is a free smartphone application that issues audio and visual warnings for earthquakes typically magnitude 4.5 or greater, or when weak shaking is expected. It is available on iPhone and Android devices, as well as select computer platforms.
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Brandi D. Addison covers weather across the United States as the Weather Connect Reporter for the USA TODAY Network. She can be reached at baddison@gannett.com.
This article originally appeared on Palm Springs Desert Sun: San Andreas Fault stress hits 1,000-year high in Southern California
Reporting by Brandi D. Addison, USA TODAY NETWORK / Palm Springs Desert Sun
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By Brandi D. Addison, USA TODAY NETWORK | USA TODAY Network
