New research finds tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has reached— and in some places exceeded—the highest levels seen in the past 1,000 years. Scientists say there’s no sign of an imminent rupture, but the re
On a calendar marked in geologic time, the San Andreas and San Jacinto fault systems are showing something few seismologists like to see: stress levels at the highest point in at least a thousand years.
The research. led by Earth scientists at the University of Hawaiʻi at Mānoa. concludes that tectonic stress across Southern California’s fault systems has reached—and in some areas surpassed—the highest levels recorded in the past 1. 000 years. The study is not describing an imminent break. Instead. it depicts a system under unusually high strain. moving through a long-term seismic cycle in a way that could still allow a major earthquake to happen.
In their paper. published in the Journal of Geophysical Research: Solid Earth. the researchers describe the region as “critically loaded. ” with stress building across multiple fault segments rather than staying localized. One focal point is Cajon Pass, a junction between the San Andreas and San Jacinto fault systems. There. the faults may function like an “earthquake gate”—either preventing ruptures from crossing from one system to the other. or helping them link into a single larger event.
Lead author Liliane Burkhard—described in the study as 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 and more than 160 years elapsed since the last major rupture. the system is in a critically loaded state. ” Burkhard said.
The stakes extend beyond the geology. The study says a rupture that involves both fault systems could be significantly more damaging than an earthquake limited to one fault. largely because of the size of such an event and its proximity to major population centers. Those include Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
At the same time. the earthquake risk message here is complicated: the research does not predict when the shaking could arrive. and it does not claim the ground is about to fail on a specific day. What it does say is that the system is carrying an unusually heavy load—enough to support the possibility of large events when the right rupture conditions eventually align.
The research also ties directly to a question people often ask when they hear the words “San Andreas”: is this fault a crack that could split California away from the continent?. It isn’t. The San Andreas Fault is a strike-slip plate boundary. meaning the Pacific Plate and North American Plate slide past each other horizontally. not apart. according to the U.S. Geological Survey.
Even during a very large earthquake, the fault motion 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.
That distinction matters when thinking about what residents would feel. Surface rupture happens when movement along a fault deep within Earth breaks through to the surface. The Pacific Northwest Seismic Network describes surface rupture as 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 never 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 horizontal or vertical offsets depending on the type of fault. Strike-slip faults, including the San Andreas Fault, typically cause horizontal displacement, while 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 be near the fault and in areas built on soft or water-saturated soils. where shaking can amplify and increase the risk of liquefaction. Scientists 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.
The article’s underlying story connects the same set of facts in a way that’s hard to ignore: stress is unusually high across multiple fault segments. Cajon Pass could influence whether ruptures stay contained or link up. and a multi-fault event would place the region’s population centers at greater risk.
That tension sits inside a broader reality about California. The state lies along the boundary between two massive tectonic plates—the Pacific Plate and the North American Plate—which are constantly moving and sliding past each other at different speeds. California also has more than 500 active faults, making it one of the most earthquake-prone regions in the United States. Among them are the San Andreas Fault. a roughly 800-mile system running through much of the state and forming the primary boundary between the 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. The study describes that junction as one of the most seismically active offshore regions along the West Coast.
For residents, one of the most practical questions is whether earthquakes can be predicted. They can’t. Neither the U.S. Geological Survey nor scientists have predicted a major earthquake, and there is no expectation this will change in the near future.
What does exist is warning. 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 help people 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 even after an earthquake has already passed. especially for those closer to the epicenter.
The system is delivered through multiple channels, including Android Earthquake Alerts, Wireless Emergency Alerts, and the MyShake app. MyShake—developed by the UC Berkeley Seismological Laboratory—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.
When the shaking does come. the basic instructions remain clear: drop to your hands and knees; cover your head and neck under sturdy furniture if possible; hold on until shaking stops. Stay indoors unless you are near a known coastal tsunami risk zone. and stay informed by getting weather alerts via text.
As the new findings land, the message is not that the fault is about to rupture tomorrow. It’s that the region is carrying an unusually heavy burden—one that could, under the right conditions, turn into something far bigger than a single fault event.
San Andreas Fault San Jacinto fault tectonic stress 1 000-year high Cajon Pass earthquake gate critically loaded state earthquake early warning MyShake