Why California’s ‘Earthquake Gate’ Keeps Scientists Up at Night
Cajon Pass is more than a busy mountain gateway. Its complex geology could help reshape the next major earthquake’s reach and intensity.
Drive north from Los Angeles toward Las Vegas on I-15 and about one hour in you’ll find yourself snaking through one of the most geologically precarious parts of North America.
Cajon Pass, also known as the gateway to Southern California, has served as a natural navigable route through the towering San Bernardino and San Gabriel mountain ranges for centuries. It’s an industrial corridor, boasting major highways, train lines, aqueducts, underground gas pipes, and fiber optic cables. Each day, an average of 160,000 vehicles travel on the highway, while power lines above provide electricity to thousands, and natural gas flows in high-pressure pipes underground.
The pass also sits above two of the most dangerous fault lines in the world, poised for rupture: the San Jacinto and the San Andreas, where the Pacific tectonic plate and the North American plate meet.
Cajon Pass is a critical site for the next major earthquake that rips through this region, according to a new study that found tectonic stress is, in some parts, at its highest level in a thousand years. Scientists said that the pass could act as a potential “earthquake gate,” either blocking large ruptures from passing through both faults or allowing them to occur across the two systems.
The findings, released this June in the Journal of Geophysical Research: Solid Earth, have reignited longstanding concerns among seismologists and emergency managers about this Californian chokepoint, the lifelines that run through it, and what it can withstand when the next rupture occurs.
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“Cajon Pass keeps me up at night,” says Ken Hudnut, who helped the U.S. Geological Survey develop seismic resiliency initiatives for the state of California. “We all know the San Andreas fault is loaded. It's ready to go. For years the scientific community has been saying so."
A High-Magnitude Scenario
Far beneath us, the Pacific and North American tectonic plates are rubbing against each other. The Pacific plate is moving faster, snagging on the North American plate as it passes it. When the stress of those snags suddenly releases, we experience an earthquake.
The June study found that tectonic stress has steadily built along the two fault zones that meet at Cajon Pass, growing in some places to its highest level in 1,000 years. In the past, when stresses rose in concert, like they are right now, the pass has acted as a gateway, allowing the rupture to go through both fault systems, and producing a larger earthquake. The findings can’t help us predict the next catastrophic quake, but they do imply that when it comes, it could be major—around 7.5 on the Richter Scale.
“We are primed to have another one in our generation,” says Liliane Burkhard, lead author of the June paper and a geophysicist at the University of Bern. She added that her study could further refine investigations into how these fault junctions behave in other parts of the world. Turkey’s North Anatolian fault, for example, has certain branches that could be possible earthquake gates, says Burkhard, as do faults in China, Nepal, and New Zealand.
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If a major earthquake was to rip through the southern part of the San Andreas fault, the results would be catastrophic. Seismology models have predicted more than 1,800 deaths, 50,000 injuries, and $200 billion in damage in the event of a 7.8 magnitude quake.
At Cajon in particular, highways would be affected. Rail lines would be destroyed. Fiber optic cables could be ripped apart, disconnecting Southern California from much of its Internet traffic. And gas lines could combust, according to the 2008 ShakeOut Scenario report led by Lucy Jones, a former USGS seismologist who developed the methodology used for earthquake advisories in the state of California. The shaking could trigger landslides that would topple transmission towers and bury petroleum and fiber lines. Some 75,000 cubic meters of debris would cut off all roads going through the pass.
"This earthquake's inevitable. We just don't know exactly when,” says Jones, adding that “any sort of lifeline crossing the fault will be cut off.”
How This Danger Zone Became So Critical
It’s no accident that humans built a gateway along these faults; in fact, the faults actually built the gateway.
The San Andreas fault was created some 25 million years ago when the North American tectonic plate and the Pacific plate began to chafe against each other in what seismologists call a strike-slip motion. At that point, the fault formed the boundary between the continent and the ocean.
But about 8 million years ago the plate boundary stepped inland, creating a big bend around a large area that includes the Cajon Pass. About 6-7 million years later, newer faults formed, including the San Jacinto.
Scientists believe those new faults are a direct response to the formation of the big bend, indicating the plate boundary is simplifying itself.
“Why should the strain go around this big stepover here if it could just cut straight through?” says Rebecca Dorsey, a professor of earth sciences at the University of Oregon.
Those shifts set into motion the forces that are coming to a head at Cajon Pass today.
“The faults created the mountains, but they also created valleys within the mountains,” says Jones. “That naturally became the place where—well, where else would you go through these mountains?”
The first roads through the pass were built in the mid-1800s, as California became U.S. territory after the Mexican-American War. From there the pass became a well-trodden, albeit perilous, route for gold-seekers and westward pioneers, oblivious to what lay beneath them.
They were tipped off in 1857 when the last major earthquake—and one of the most powerful ever recorded in Southern California—rattled some 217 miles down the San Andreas and decimated the region. Though roughly 1,500 people were living in what is now the Los Angeles metropolitan area, the approximately 7.9 magnitude event miraculously killed just two people. Now nearly 13 million people reside there.
The Fort Tejon Earthquake, as it became known, didn’t stop people from continuing to settle.
Seismology is a relatively young science, only emerging in the 60s or 70s, says Kate Scharer, a seismologist at the USGS who co-authored the June paper and has been studying the frequency of ground rupturing earthquakes on major faults in Southern California since the early 2000s. So, infrastructure was already placed in locations that the field would advise against today.
"This region is unique because the mountains are so tall and the faults are so active,” notes Sharer.
Can We Protect Ourselves From the Next Big One?
Now that we’ve begun to understand what is underneath us, says Jones, there are ways to build differently.
The state of California said it is aware of the risk and acknowledges the critical infrastructure that runs through the pass could be destroyed in the event of a catastrophic earthquake.
“Damage to or closure of that route could have a significant impact on supply chains, power systems, communication infrastructure, and transportation networks,” says a spokesperson from the California Governor’s Office of Emergency Services. A task force was formed to coordinate between the private sector and the government to ensure those lifelines are safely restored, the spokesperson said.
Other entities, like the region’s largest utilities company, said they are working to assess risk. Southern California Edison acknowledged in a 2026 planning report that important transmission corridors run over the San Andreas at Cajon Pass, crossing steep landscape susceptible to landslides. The company has invested some $300 million in seismic hardening from 2016-2025.
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SCE also said it would spend roughly $13 million over the coming years retrofitting transmission corridors, some of which run near Cajon over the San Andreas fault.
SoCal Gas, the nation's largest natural gas distribution utility, said it has since 2012 shored up its seismic safety measures. In the Cajon Pass, “the company has equipped two transmission pipelines with automated valves at each end of the pass and installed a fault isolation valve on the south end to help enable rapid isolation of pipeline segments during emergencies or other significant events,” says Raul Gordillo, a spokesperson for the company. “These efforts are part of SoCalGas' ongoing investment in system safety, reliability, and operational preparedness,” he adds.
There are other ways to build more safely—but they come with tradeoffs. When oil was discovered in the Beaufort Sea, scientists engineered an earthquake-safe pipeline above ground across the Denali fault on slats that allowed it to move back and forth across the Alaskan tundra. When an earthquake struck the region in the early 2000s, the pipeline didn’t break, staving off an oil spill and ecological disaster.
But it’s much harder to retrofit infrastructure rather than building it from scratch. And pipelines above ground closer to populated areas are a risk in themselves; they’re much easier to tamper with or even blow up.
When building or retrofitting isn’t an option, says Jones, the other alternative is to plan. She believes more could be done.
“We know exactly where this will happen,” says Jones. “That's the thing that's frustrating.”
