The “earthquake gate” that stopped the San Andreas disaster is under the most pressure in 1,000 years

It’s like a scene from a Hollywood movie: a major earthquake along the San Andreas Fault hits Los Angeles, destroying infrastructure, starting fires and causing landslides. For decades, scientists have been investigating when and how this catastrophe might unfold.

Now, researchers report that such an earthquake could be more widespread and damaging than previously thought. Their study found that the southern portions of the San Andreas Fault and parts of the adjacent San Jacinto fault line are locked and loaded with the highest stress levels in 1,000 years — increasing the likelihood of a major earthquake. If a strong earthquake hits any of the fault zones, researchers say seismic aftershocks could flow into its neighbor through an “earthquake gateway” and spread damage from north of Los Angeles across San Bernardino, Riverside and the Coachella Valley simultaneously.

“We’re talking loosely about faulting that is delayed, but it’s important to see a physical estimate that suggests the system is at its highest level in 1,000 years,” said Matthew Weingarten, a geologist at San Diego State University who was not involved in the study.

Earthquakes occur when sudden slip along a fault — a break in the Earth’s crust where masses of rock move against each other — releases energy that has built up over time. Stress builds up when tectonic forces move the crust, but parts of the fault are locked and unable to slide freely.

Less than 60 miles northeast of downtown Los Angeles, pressure has been building for more than a century along the southern San Andreas and San Jacinto fault systems. Both fractures form the boundaries of the Pacific and North American tectonic plates, which have been sliding past each other a few centimeters each year while other areas are closed. As a result, some parts of the faults build up tension like a coiled spring with nowhere to move.

In fact, previous analyzes have shown that there is a greater than 50% chance of an earthquake of magnitude 6.7 or higher occurring along the southern extension of the San Andreas Fault in the coming decades.

But the San Andreas and San Jacinto fault systems also meet at a junction called the Cajon Corridor, which researchers say acts as an “earthquake gateway.” The gate can stop or transmit large ruptures between the two cracks. In 1812, the 7.5-magnitude Wrightwood earthquake occurred, which researchers suspect crossed the Cajon Pass, extending along the San Andreas and San Jacinto fault systems and causing 40 deaths.

Today, if an earthquake traveled across the Cajon Corridor and along both faults, scientists say the consequences would be severe and widespread, affecting critical infrastructure such as major highways, railways and energy corridors across several cities at once.

“In terms of severity, a joint tear crossing the Cajon Pass can reach about 7.4 to 7.8 degrees and affect a much larger area than a single fault event,” said Liliane Burkhard, lead author of the study and a geophysicist at the University of Bern in Switzerland.

To determine what caused the earthquake to travel through the Cajon Corridor, the team reconstructed the last 1,000 years of seismic activity along the two faults, tracking how stress builds and releases. They found that the earthquakes passed through the intersection when both sides of the corridor had similar levels of high pressure — and “that’s the configuration we’re approaching today,” Burkhard said.

According to simulations, the San Jacinto Bernardino section emerged with the highest fatigue load anywhere during the 1,000-year reconstruction, recording 3.6 MPa. This reading exceeds its previous peak nearly 50 years ago. The southern portion of the Mojave in San Andreas recorded 2.8 MPa and exceeded its record compressive load from a decade ago.

Previous simulations showed that ruptures traveling through a cajon passage occurred when the pressure difference between the two parts was only 0.3 MPa. The gap is currently 0.8 MPa, said Burkhard, who conducted much of this research at the University of Hawaii at Manoa.

“The idea is not that stress builds up over time, which we’ve known for a long time, but that the balance of stress across the junction may decide whether the next earthquake remains under control or develops into a much larger rupture,” said Weingarten, whose research group has modeled earthquake stress and its triggering on the San Andreas Fault.

Burkhard said the main message of the new research is not to panic but to act urgently. City managers and emergency responders should plan for joint ruptures along the San Andreas and San Jacinto Fault as a realistic possibility under current stress conditions, not a remote worst-case scenario.

“Southern California faces a significant and growing seismic risk, and the time to prepare is now, not after the next earthquake,” she said.

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