San Andreas Fault at highest stress in 1,000 years

A new study says the San Andreas and San Jacinto fault systems are in a “critically loaded” state, with stress levels reaching the highest seen in about 1,000 years. Researchers warn the region is not showing signs of an imminent rupture, but the unusually hig

For the third week in a row. people in earthquake country have heard the same uneasy phrase in different forms: “critically loaded.” In this case. the words come from a new scientific study describing the San Andreas and San Jacinto fault systems as having stress levels that reach—and in some areas exceed—the highest seen in the past 1. 000 years.

The research. led by Earth scientists at the University of Hawaiʻi at Mānoa and published in the Journal of Geophysical Research: Solid Earth. paints a careful picture. It does not suggest the region is about to break open tomorrow. But it does say the system is operating under unusually high stress inside a long-term seismic cycle—stress building across multiple fault segments and capable. researchers say. of supporting large earthquakes. including multi-fault events.

One focus is Cajon Pass, a junction between the San Andreas and San Jacinto fault systems. Researchers describe it as a potential “earthquake gate,” a place where ruptures could either be blocked from crossing between faults or link together into a single larger event.

Liliane Burkhard. the study’s lead author and a research affiliate at the University of Hawaiʻi’s Institute of Geophysics and Planetology and a scientist at the University of Bern. tied the concern to timing 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.

The study’s warning is not just about the size of a hypothetical quake—it’s about what that size could mean for where people live. A rupture involving both fault systems could be significantly more damaging than a single-fault earthquake because of its size and proximity to major population centers. including Los Angeles. San Bernardino. Riverside. and the Coachella Valley.

The science also emphasizes what a “San Andreas rupture” actually looks like. The San Andreas Fault is not a crack that can “split open” and drop a chunk of California away from the continent. It is a strike-slip plate boundary. In other words. the Pacific Plate and North American Plate slide past each other horizontally. not pulling apart. according to the U.S. Geological Survey.

Even in a very large earthquake, the movement is sideways 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.

The earthquake descriptions matter because they shape how damage spreads. Surface rupture is one of the clearest physical expressions of an earthquake. showing permanent deformation where two sides of a fault slip past one another. according to the Pacific Northwest Seismic Network. But most earthquakes do not produce surface rupture, the U.S. Geological Survey says. 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 does occur, it can produce either horizontal or vertical offsets depending on fault type. Strike-slip faults, such as the San Andreas Fault, typically cause horizontal displacement, while dip-slip faults can produce vertical displacement. Some earthquakes involve a combination of both.

A major rupture also brings a brutal kind of certainty for the people who experience it: shaking can last tens of seconds to more than a minute. Scientists say the most severe damage would occur near the fault and in areas built on soft or water-saturated soils. where shaking can be amplified and where liquefaction risk rises. They also note that infrastructure built across active faults is especially vulnerable. because surface rupture can directly offset roads. buildings. and other structures that span the fault trace.

Earthquakes can feel mysterious until you map them onto how the planet actually moves. The Earth has four layers: the inner core, outer core, mantle, and crust. The crust and top of the mantle make up the lithosphere. described by the USGS as a kind of skin around the surface. That skin is not one solid piece; it’s divided into puzzle-like fragments called tectonic plates, which move slowly.

As those plates move past one another, they bump and collide at their edges. Stress accumulates along those edges until it becomes great enough to create cracks called “faults.” The line where faults move against each other is called the “fault line.” Energy is released when friction between fault lines is overcome. triggering seismic waves that cause an earthquake.

That is why California is so earthquake-prone. The state sits along the boundary between the Pacific Plate and the North American Plate. It also has more than 500 active faults, making it one of the most earthquake-prone regions in the United States. The San Andreas Fault is roughly an 800-mile system running through much of the state and forming 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.

Earthquakes are not limited to land either. 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.

What people want most—prediction—has a hard boundary. Earthquakes are not predicted. Neither the U.S. Geological Survey nor scientists have predicted a major earthquake, and there is no expectation that this will change in the near future.

But California does have a system designed to buy seconds. 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. especially for those closer to the epicenter.

The warning 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.

If the timing of a quake is unknowable, preparedness still isn’t. During an earthquake. officials recommend people “Drop to your hands and knees. ” “Cover your head and neck under sturdy furniture if possible. ” and “Hold on until shaking stops.” They also advise staying indoors unless someone is near a known coastal tsunami risk zone. and staying informed by getting weather alerts via text.

Back in the lab, Burkhard and co-authors are not calling for panic over an imminent rupture. They are describing a different kind of risk: a fault system with stress pushed unusually high—across multiple segments. concentrated enough that Cajon Pass could matter. and framed by more than 160 years since the last major rupture.

San Andreas Fault San Jacinto Fault earthquake stress critically loaded Cajon Pass multi-fault earthquake Journal of Geophysical Research: Solid Earth Earthquake Early Warning MyShake UC Berkeley U.S. Geological Survey