The ‘earthquake gate’ stopping a San Andreas disaster is under its highest stress in 1,000 years

WASHINGTON — It’s like a scene from a Hollywood movie: A major earthquake along the San Andreas fault ripples through Los Angeles, decimating infrastructure, sparking fires and triggering landslides. For decades, scientists have been investigating when and how this disaster could unfold.

Now, researchers report that such a quake could be more widespread and damaging than previously thought. Their study found that southern portions of the San Andreas fault and parts of the adjacent San Jacinto fault line are locked and loaded to their highest stress levels in 1,000 years, increasing the probability of a significant earthquake. If a strong earthquake hits either fault zone, the researchers said the seismic rumbles could cascade into its neighbor through an “earthquake gate” and spread damage from north of Los Angeles through San Bernardino, Riverside and the Coachella Valley simultaneously.

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“We talk loosely about faults being ‘overdue,’ but it’s important to see a physics-based estimate that the system is sitting at a 1,000-year high,” said Matthew Weingarten, a geologist at San Diego State University who was not involved in the study.

Earthquakes happen when a sudden slip along a fault, a fracture in Earth’s crust where rock masses move against one another, releases energy built up over time. Stress accumulates as tectonic forces move the crust, but parts of the fault are locked and unable to slip freely.

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

In fact, the chances are more than 50% that an earthquake of 6.7-magnitude or higher occurs along the southern stretch of the San Andreas fault in the upcoming decades, previous analysis showed.

But the San Andreas and San Jacinto fault systems also meet at a junction called the Cajon Pass, which the researchers said acts as an “earthquake gate.” The gate can either stop or transmit large ruptures between the two faults. In 1812, the 7.5-magnitude Wrightwood earthquake, which the researchers suspect crossed the Cajon Pass, rippled along both the San Andreas and San Jacinto fault systems and caused 40 deaths.

Today, if an earthquake were to travel through the Cajon Pass and along both faults, scientists said the consequences would be severe and widespread, affecting critical infrastructure such as major highways, railways and energy corridors over several cities simultaneously.

“In terms of severity, a joint rupture crossing Cajon Pass could approach around a magnitude 7.4 to 7.8 and affect a far larger area than a single-fault event,” said Liliane Burkhard, the study’s lead author and geophysicist at the University of Bern, Switzerland.

To determine what causes an earthquake to travel through the Cajon Pass, the team reconstructed the last 1,000 years of seismic activity along the two faults, tracking how stress accumulated and released. They found that earthquakes passed through the junction when both sides of the pass had similar levels of high stress – and “that is the configuration we are approaching today,” Burkhard said.

According to the simulation, the San Jacinto Bernardino segment stood out with the highest stress load anywhere in the 1,000-year reconstruction – registering 3.6 megapascals. That reading exceeds its previous peak from nearly 50 years ago. The Mojave South segment of the San Andreas recorded 2.8 megapascals and surpassed its own record stress load from a decade ago.

Past simulations showed ruptures traveled through Cajon Pass when the stress difference between the two segments was only 0.3 megapascals. Currently, the gap is measuring 0.8 megapascals, said Burkhard, who conducted much of this research at the University of Hawaiʻi at Mānoa.

“The insight isn’t that stress builds over time, which we’ve long known,” said Weingarten, whose research group models earthquake stress and triggering on the San Andreas Fault, “but that the balance of stress across the junction may decide whether the next earthquake stays contained or grows into a much bigger rupture.”

The underlying message of the new research is not to panic but act with urgency, Burkhard said. City managers and emergency responders should plan for joint ruptures along the San Andreas and San Jacinto faults as a realistic possibility with the 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 and not after the next earthquake,” she said.