Two of California's most dangerous fault systems have reached earthquake stress levels not seen in at least 1,000 years, and researchers say only a magnitude 7 or larger event can release the pressure now building beneath the feet of nearly 24 million people.
A peer-reviewed study published June 3 in the Journal of Geophysical Research: Solid Earth examined three segments of the San Andreas and San Jacinto faults near the San Bernardino Mountains. Using computer simulations and a painstaking reconstruction of a millennium of seismic history, drawn from radiocarbon-dated sediments, tree-ring records, and historical accounts, researchers found that two of those three segments have reached or exceeded the highest stress levels observed in the entire 1,000-year record.
The findings land at a moment when Californians are already on edge. A magnitude 5.6 earthquake recently struck Mendocino County, injuring several people, knocking out power to roughly 8,000 PG&E customers, and triggering nearly 657,000 early-warning alerts through the MyShake app. A magnitude 7 quake, the minimum size scientists say is needed to release the stress described in the new study, would be more than 125 times stronger.
The study zeroes in on Cajon Pass in San Bernardino County, the narrow corridor where the San Andreas and San Jacinto faults converge. Lead author Liliane Burkhard, a planetary geologist at the University of Bern in Switzerland, described the pass as an "earthquake gate", a place that can determine whether a rupture continues along the fault system or stops cold.
Burkhard told The Sacramento Bee that the stakes extend far beyond shaking:
"It is a chokepoint for major highways, rail lines and energy infrastructure, so disruption there would have consequences well beyond the immediate shaking."
She added that a major event at Cajon Pass "could affect transportation and energy corridors that the broader region depends on." Anyone who has driven Interstate 15 through that pass, the main artery linking Los Angeles to Las Vegas and the Inland Empire, understands what that means in practical terms.
Newsmax reported that the last major rupture in the area was a magnitude 7.9 earthquake in 1857, which stopped at Cajon Pass. That left southern fault sections locked and accumulating stress for nearly 170 years. The new study suggests those sections are now primed for a through-going rupture that could jump between both fault systems, a scenario that would threaten communities from Los Angeles to the Coachella Valley.
Burkhard's own summary of the findings was blunt. As she told reporters:
"Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems."
Ahmed Elbanna, director of the Statewide California Earthquake Center and a professor of earth sciences and engineering at the University of Southern California, offered context that should give no one comfort. He told The Sacramento Bee that the scientific community has long understood the risk.
"The authors themselves, and the community, do not find the findings of the study surprising."
What the study does is put hard numbers on a threat that seismologists have discussed for years. Elbanna noted that California has not experienced a major urban earthquake since the 1994 Northridge quake, more than three decades of relative quiet in a region sitting on some of the most active fault lines on the planet.
He described Southern California's crust as "critically stressed and ready to rupture in a large-sized event," adding the caveat that haunts every earthquake forecast: "Whether that event would happen tomorrow, or in 10 years, that's the part that we don't know."
The broader pattern of elevated stress across California's fault systems makes the uncertainty harder to dismiss. This is not a single data point. It is a trend confirmed by multiple lines of evidence stretching back a millennium.
The study estimates that a magnitude 7 or larger earthquake along these fault segments could threaten nearly 24 million people across Southern California, including greater Los Angeles and the Inland Empire. That figure encompasses one of the most densely populated and economically productive regions in the country.
Angie Lux, a seismologist at the Berkeley Seismology Laboratory, offered some reassurance about California's building codes, which are among the strictest in the nation for seismic resilience. She told The Sacramento Bee:
"You might see cracks or have things falling down, but the structures themselves should not fail."
That is cold comfort for anyone who remembers the Northridge earthquake's $20 billion-plus damage toll, or who has watched California's infrastructure age while the state's political leadership has poured billions into high-speed rail and other projects that have little to do with hardening the built environment against the next major temblor.
Elbanna stressed that continued investment in earthquake preparedness and warning technology remains critical. The New York Post noted that stress levels on both the San Andreas and San Jacinto fault systems have grown more similar over time, creating conditions that may make it easier for an earthquake to jump between the two systems, the very scenario the study flags as a worst case.
The new study arrives amid a stretch of heightened seismic activity up and down the West Coast. The Mendocino County quake caused thousands of dollars in damage from merchandise falling off store shelves and left communities rattled. But it was far from an isolated event.
Recent months have brought multiple earthquakes along the California and Oregon coastline, and an earthquake swarm near Brawley in Southern California topped 350 individual quakes before slowing down. Farther east, a swarm of quakes rattled western Nevada as the region's crust continues to pull apart.
None of these events individually signals that "the big one" is imminent. But together, they paint a picture of a seismically active region where the question is not whether a catastrophic earthquake will strike, but when, and whether the people and infrastructure in its path are ready.
California's political class has spent the past decade focused on climate mandates, housing policy fights, and social spending. Earthquake preparedness, by contrast, rarely makes the front page until the ground starts shaking. The state's building codes are strong on paper, but enforcement varies, and millions of older structures, particularly unreinforced masonry buildings and soft-story apartment complexes, remain vulnerable.
The MyShake early-warning system performed well during the Mendocino County event, pushing out nearly 657,000 alerts. That is a genuine success. But an alert that arrives seconds before a magnitude 7+ earthquake is no substitute for retrofitted bridges, hardened water systems, and emergency supply chains that can function when Cajon Pass, the chokepoint for highways, rail, and energy infrastructure, is impassable.
Elbanna's observation that the scientific community is not surprised by these findings should itself be a scandal. If the experts have known for years that Southern California's faults are critically stressed, the relevant question is what elected officials and emergency planners have done with that knowledge. The answer, measured against the scale of the threat, is not enough.
The earth does not care about budget cycles or political priorities. It moves on its own schedule. And right now, the science says the clock has been ticking for a very long time.
