Stress at historically high levels, the potential for a larger, multi-fault earthquake has increased.
Beneath the surface of Southern California, a millennium's worth of geological tension has quietly accumulated along two of the region's most consequential fault systems. Scientists at the University of Hawaiʻi at Mānoa have determined that the San Andreas and San Jacinto faults are now 'critically loaded'—carrying stress levels unseen in a thousand years—raising the prospect that a future rupture could cascade across both systems simultaneously, shaking millions of lives from Los Angeles to the Coachella Valley. No earthquake can be foretold, but the earth itself is keeping a long and patient account, and the balance has rarely been this high.
- The San Andreas and San Jacinto faults have been silently accumulating tectonic stress for over 160 years since their last major rupture, and researchers now describe them as critically loaded at levels not seen in a millennium.
- A key junction called Cajon Pass could act as an 'earthquake gate'—the point where a rupture on one fault might leap to the other, transforming two separate events into a single catastrophic chain reaction.
- If both faults ruptured together, shaking would be stronger, longer-lasting, and far more widespread, with the densest damage concentrated near fault lines and in areas built on soft or water-saturated soils prone to liquefaction.
- California's Earthquake Early Warning system—delivering alerts through cell phones and the MyShake app—offers residents precious seconds to drop, cover, and hold on before the strongest waves arrive.
- Scientists are careful to stress that this is not a prediction of an imminent event, but a call to recognize that the longer the fault system stays locked, the greater the stored energy waiting to be released.
Beneath Southern California, two massive fault systems—the San Andreas and the San Jacinto—are carrying more stress than at any point in the past thousand years. Researchers at the University of Hawaiʻi at Mānoa, publishing in the Journal of Geophysical Research: Solid Earth, describe the system as 'critically loaded,' meaning the accumulated tectonic pressure has reached levels capable of supporting a powerful earthquake. More troubling still, the conditions may now favor a rupture large enough to link both faults into a single event.
At the heart of the concern is Cajon Pass, where the two faults converge. Lead researcher Liliane Burkhard describes this junction as an 'earthquake gate'—a point that could either contain a rupture or allow it to cascade across both systems. With stress building for more than 160 years since the last major break, the potential for a larger, multi-fault earthquake has meaningfully grown.
The consequences of such an event would be far-reaching. A combined rupture would produce stronger, longer shaking across a wide corridor stretching from Los Angeles through San Bernardino, Riverside, and the Coachella Valley. The worst damage would fall near the fault trace itself and in areas built on soft soils, where shaking is amplified and liquefaction becomes a serious risk. One persistent myth can be set aside: California will not fall into the ocean. The San Andreas is a strike-slip fault—the plates slide horizontally past each other, not apart.
While no scientist can predict when the fault will move, California has invested in early warning infrastructure. The state's Earthquake Early Warning system, along with the UC Berkeley-developed MyShake app, can push alerts to cell phones before the strongest shaking arrives—giving residents critical seconds to drop, cover, and hold on. The research carries no prediction of an imminent event, but its message is unmistakable: for millions living above one of the most stressed fault systems in recorded history, preparation is no longer a matter of choice.
Beneath Southern California, two massive fault systems are under more stress than they have been in a thousand years. Scientists at the University of Hawaiʻi at Mānoa have published research showing that the San Andreas and San Jacinto faults—which run through the Inland Empire, San Bernardino, and Riverside counties near millions of homes—are what researchers call "critically loaded." This means the tectonic stress building in the rock has reached levels that could support a powerful earthquake, possibly one large enough to link both fault systems into a single, catastrophic rupture.
The study, published in the Journal of Geophysical Research: Solid Earth, focuses on a crucial junction called Cajon Pass, where the two faults meet. This point may act as an "earthquake gate"—either preventing ruptures from crossing between the faults or allowing them to connect into one massive event. Lead researcher Liliane Burkhard, a scientist at the University of Bern and research affiliate at the University of Hawaiʻi's Institute of Geophysics and Planetology, notes that the system has been accumulating stress for more than 160 years since the last major rupture. "Right now, with stress at historically high levels across the region," she said, the potential for a larger, multi-fault earthquake has increased.
What makes this different from a single-fault rupture is the amplification of damage. If both the San Andreas and San Jacinto faults ruptured together, the shaking would be stronger and last longer across a much wider area—from Los Angeles to San Bernardino, Riverside, and the Coachella Valley. The strongest damage would concentrate near the fault itself and in areas built on soft or water-saturated soils, which amplify shaking and increase the risk of liquefaction. Buildings and infrastructure that cross the fault trace would face direct offset from surface rupture, potentially rendering them unusable.
A common misconception needs clarifying: California will not fall into the ocean. The San Andreas Fault is a strike-slip boundary, meaning the Pacific Plate and North American Plate slide past each other horizontally at roughly 45 millimeters per year—about the speed fingernails grow. Even in a very large earthquake, the motion is sideways, not vertical separation. Over the past thousand years, California has shifted roughly 140 to 160 feet northwest along the fault, roughly half a football field of movement. But this is gradual, distributed motion across a network of crustal pieces, not a catastrophic breaking apart.
When a major rupture does occur, surface rupture can produce horizontal offsets of feet or even tens of feet. Strong shaking could last tens of seconds to more than a minute. The damage would be most severe near the fault and in poorly constructed areas, but the state's infrastructure would face significant strain across a broad region. California sits atop more than 500 active faults, making it one of the most earthquake-prone regions in the United States. The San Andreas alone stretches roughly 800 miles and forms the primary boundary between the two plates.
While earthquakes cannot be predicted—neither the U.S. Geological Survey nor any scientist has successfully predicted a major earthquake—California has built defenses. The state's Earthquake Early Warning system uses seismic sensors and ground-motion monitoring to send alerts to cell phones before the strongest shaking arrives. Even a few seconds of warning can allow people to Drop, Cover, and Hold On, or put critical systems into safe mode. The system operates through Android Earthquake Alerts, Wireless Emergency Alerts, and the free MyShake app, developed by UC Berkeley's Seismological Laboratory, which issues warnings for earthquakes typically magnitude 4.5 or greater.
The research does not suggest an imminent earthquake. But it does suggest that the longer the system remains locked, the greater the potential energy accumulates. For millions of residents across Southern California, the message is clear: preparation is not optional. The fault system is under stress unseen in a millennium, and when it finally moves, the consequences will be felt across the entire region.
Citações Notáveis
Right now, with stress at historically high levels across the region— Liliane Burkhard, lead researcher at University of Hawaiʻi at Mānoa
A Conversa do Hearth Outra perspectiva sobre a história
What does "critically loaded" actually mean in practical terms?
It means the rock is under so much stress that it's primed to break. Think of it like a rubber band stretched to its limit—it hasn't snapped yet, but it's reached the point where any small additional tension could trigger the release.
Why does Cajon Pass matter so much?
It's the junction where the two fault systems meet. If a rupture starts on one fault and reaches that point, it could either stop there or jump to the other fault. If it jumps, you get one massive earthquake instead of two separate ones, and the shaking amplifies across a much larger area.
The article mentions 160 years since the last major rupture. Is that a long time geologically?
In human terms, it's generations. Geologically, it's relatively recent. But stress has been accumulating the entire time, and the longer the wait, the more energy builds up. That's what makes the current state so significant.
If California doesn't fall into the ocean, what actually happens during a major rupture?
The ground shifts sideways. Los Angeles could move several feet northwest relative to the rest of the continent. Buildings crack, roads offset, infrastructure breaks. But the state stays attached to the continent. The damage comes from the shaking and the ground deformation, not from separation.
How much warning would people actually get from the Early Warning system?
It depends on how close you are to the epicenter. For people far from the fault, you might get several seconds. For people right on top of it, the warning might arrive as the shaking begins or even after. But even a few seconds is enough time to get under a table or away from falling objects.
What should someone in Los Angeles actually do with this information?
Know where the sturdy furniture is in your home and workplace. Have a plan. Download the MyShake app. Keep emergency supplies accessible. This isn't about panic—it's about being ready for something that will eventually happen, even if we don't know when.