California isn’t where it used to be — literally.
Over the past 1,000 years, the state has slowly shifted about half a football field. That’s because earthquakes move the state an average movement rate of about 45 to 50 millimeters per year. If it continues at this rate, “Los Angeles and San Francisco will one day be adjacent to one another” someday, according to the U.S. Geological Survey,
Some sections of California’s crust creep gradually along faults. Others remain locked for long periods, accumulating strain until it is released in earthquakes that can rapidly reshape the landscape. That combination of steady motion and episodic rupture keeps California in constant geologic flux, even though most of the change is measured in inches per year.
Work looking at California’s fault systems, including the San Andreas Fault, shows just how uneven that strain can be, with stress building across connected faults over long periods before being released in larger, multi-segment earthquakes.
Why the San Andreas Fault is so risky
A new study from the University of Hawaiʻi at Mānoa finds that major fault zones across Southern California are among the most highly strained in at least 1,000 years, raising the potential for a larger, multi-fault rupture rather than a more isolated earthquake.
Scientists emphasize the region is not showing signs of an imminent quake, but say the San Andreas and San Jacinto faults—stretching through the Inland Empire and near Los Angeles, San Bernardino and Riverside counties—are “critically loaded” after more than 160 years since the last major rupture.
Published in the Journal of Geophysical Research: Solid Earth, the research highlights how stress is building across interconnected fault segments, including a key junction at Cajon Pass that could help determine whether future ruptures stop or spread across multiple faults.
That long-term buildup fits into what seismologists say is the fault’s typical behavior. Earthquakes along the San Andreas system have historically occurred on roughly multi-century cycles, but key sections of the southern fault have gone significantly longer without a full “large rupture,” UCLA earthquake engineering professor Jonathan Stewart told USA TODAY.
He noted that more than 300 years have passed since the last major rupture in the southern segment, well beyond what is seen in past patterns. That does not mean an earthquake is imminent, Stewart said, but it does indicate a growing buildup of strain.
When a major rupture does occur, researchers say the impacts could be widespread. One of the biggest concerns is California’s water infrastructure, which relies on long-distance aqueducts that cross fault zones.
“An earthquake like this would rupture most, if not all of the major aqueducts bringing water into Southern California,” Stewart told USA TODAY. “Most people will not be in a collapsed structure after this earthquake, but everybody’s going to be affected by water problems.”
San Andreas Fault map
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.
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.
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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: California has moved half a football field, thanks to earthquakes
Reporting by Brandi D. Addison, USA TODAY NETWORK / Palm Springs Desert Sun
USA TODAY Network via Reuters Connect
By Brandi D. Addison, USA TODAY NETWORK | USA TODAY Network
