Friday, 17 July 2026, 6:51
A new study from the University of Hawaiʻi reveals that tectonic stress along the Southern San Andreas and San Jacinto faults has climbed to its highest point in a millennium. Geologist Matthew Herman, the lead researcher, explains that stress has been accumulating for more than 160 years—since the last major rupture on the Southern San Andreas Fault, the 1857 Fort Tejon earthquake, which measured nearly magnitude 7.9.
“The stresses have built up, and built up, and built up, as there haven’t been any big earthquakes, and now we’re at a point in time where we’ve built up, where in historical times there have been earthquakes at these stress levels,” Herman told KERO.
The study, published in the Journal of Geophysical Research: Solid Earth, suggests the region is in a “critically loaded state,” with stress building across multiple fault segments. Lead author Liliane Burkhard, a research affiliate at the University of Hawaiʻi’s Institute of Geophysics and Planetology and 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 model is based on a number of factors: the estimated timing of earthquakes on the faults over a record of the last 1,000 years; satellite observations of how fast tectonic plates are moving; and estimates of how rigid Earth’s crust is, which determines how much stress can be accommodated and then released, said U.S. Geological Survey geologist Kate Scharer, one of the study’s co-authors. The computer model “found that tectonic stress has now reached higher levels than at any point in that entire record. From the model, we see that the conditions that historically preceded large joint ruptures crossing both fault systems are now approaching,” Burkhard said.
The 1906 San Francisco earthquake, another massive San Andreas event, devastated the city. But the San Andreas isn’t the only fault Californians face. Herman noted that Kern County residents experienced the Ridgecrest earthquakes not long ago, which caused significant damage in eastern Kern County. In the early 1950s, the White Wolf Fault, just south of Bakersfield, inflicted heavy damage on downtown Bakersfield and Tehachapi.
One key area of focus is Cajon Pass, a junction between the two fault systems that may act as an “earthquake gate,” either blocking ruptures from crossing between faults or allowing them to link into a single larger event. Researchers say a rupture involving both fault systems could be significantly more damaging than a single-fault earthquake due to its size and proximity to major population centers, including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.
While California has seen sizable temblors since the great San Francisco quake of 1906, the state’s faults are capable of producing intense shaking over a much larger area than seen during more modern events such as the Sylmar, Loma Prieta or Northridge quakes. “We keep accumulating that earthquake energy, and it has to be released. And the only way it gets released is through large earthquakes,” said Scharer. “The small ones don’t really do it.”
The last major temblor to strike urban Southern California, the magnitude 6.7 Northridge earthquake of 1994, resulted in severe damage and the deaths of about 60 people. But it was also mostly limited to a relatively small area of Los Angeles County. By comparison, a hypothetical magnitude 7.8 earthquake on the southern San Andreas would simultaneously bring violent shaking to L.A., Orange, Riverside, San Bernardino, Ventura and Imperial counties and could result in 1,800 deaths, according to a U.S. Geological Survey estimate published in 2008.
California experiences thousands of earthquakes each year, though most are too small to feel. To mitigate potential damage, the state enacted some of its most stringent building codes in the 1970s. However, Herman cautioned, “Buildings that were built before those building codes were in effect need to be retrofitted. Not all buildings have been retrofitted.”
Despite the alarming findings, Herman urged calm: “This changes nothing. You should have been prepared yesterday; you should be prepared today. And if you’re not, you should be prepared tomorrow.”
Experts recommend having an emergency kit, knowing to drop, cover, and hold on, and making a family communication plan before the next earthquake strikes. “No earthquake scientist can tell you when the earthquake is going to happen. All we can do is prepare ourselves in California,” Herman said.
For residents of Calaveras County and the broader region, including San Andreas, this study is a stark reminder of the seismic forces beneath their feet. The Southern San Andreas Fault runs near many communities, and the San Jacinto Fault adds another layer of risk. While the study does not predict an imminent quake, it highlights the importance of local emergency planning.
The Southern San Andreas Fault has a history of major ruptures, including the 1857 Fort Tejon earthquake and the 1906 San Francisco earthquake. These events reshaped California’s approach to seismic safety, leading to modern building codes. The new research builds on decades of fault monitoring, using advanced modeling to track stress accumulation over centuries.
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. However, over millions of years, continued movement along the fault could gradually bring Los Angeles and San Francisco into closer proximity, geologic models suggest. 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.
The study offers no specific prediction, but it reinforces the need for ongoing preparedness. Californians should maintain emergency kits, practice drop-cover-hold-on drills, and ensure their homes and workplaces are retrofitted where necessary. As Herman put it, “All we can do is prepare ourselves in California.”
