A new study finds southern sections of the San Andreas and parts of the adjacent San Jacinto faults are locked at their highest stress levels in 1,000 years, raising the odds of a major quake. Researchers say an “earthquake gate” at Cajon Pass could let a rupt
The fear with earthquakes isn’t just the shaking. It’s the possibility that one rupture doesn’t stay in its lane.
For the third week in a row, the science coming out of southern California has carried that exact warning—one rooted in a grim measure: stress.
Researchers reported that major quake conditions along the San Andreas fault and the nearby San Jacinto fault line are sitting at their highest levels in 1. 000 years. The finding points to a system that isn’t just “due. ” but physically primed. with southern portions of the San Andreas and parts of the adjacent San Jacinto locked and loaded. If a strong earthquake strikes either fault zone. scientists say the seismic energy could cascade into its neighbor through an “earthquake gate. ” spreading damage from north of Los Angeles through San Bernardino. Riverside and the Coachella Valley simultaneously.
Matthew Weingarten. a geologist at San Diego State University who was not involved in the study. said the key point is seeing the stress through physics rather than loose forecasts. “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. ” he said.
Earthquakes happen when sudden slip releases energy that has built up over time along a fracture in Earth’s crust. Stress accumulates as tectonic forces move the crust, but some segments remain locked—unable to slip freely—like tension coiled in rock.
Less than 60 miles northeast of downtown Los Angeles. stress has been building for more than a century along the southern San Andreas and San Jacinto fault systems. Both fractures form part of the boundary between the Pacific and North American tectonic plates. Those plates have been sliding past each other a few centimeters each year. while other zones are locked. tightening the system segment by segment.
The stakes are not theoretical. Previous analysis cited in the study found that the chances are more than 50 percent that an earthquake of 6.7-magnitude or higher occurs along the southern stretch of the San Andreas fault in upcoming decades.
But the new research focuses on a junction where one disaster could become two.
The San Andreas and San Jacinto fault systems meet at Cajon Pass. which the researchers describe as an “earthquake gate.” The gate can either stop or transmit large ruptures between the two faults. In 1812, the Wrightwood earthquake—magnitude 7.5—rippled along both systems. Researchers suspect it crossed the Cajon Pass, and it caused 40 deaths.
Today, the concern is that a similar crossing could be far more consequential.
If an earthquake were to travel through Cajon Pass and along both faults. scientists say the effects would likely be severe and widespread. The consequences would reach beyond a single corridor. threatening critical infrastructure such as major highways. railways. and energy corridors across several cities at once.
Liliane Burkhard. the study’s lead author and a geophysicist at the University of Bern. Switzerland. put a number on what that joint rupture might look like. “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. ” she said.
To understand when the gate opens. the team reconstructed the last 1. 000 years of seismic activity along both faults. tracking how stress accumulated and released. They found that earthquakes passed through the junction when both sides of the pass showed similar levels of high stress. and Burkhard said. “that is the configuration we are approaching today.”.
In 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—exceeding 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.
The researchers also compared what’s happening now to earlier conditions in which ruptures did—or did not—cross the pass. Past simulations showed ruptures traveled through Cajon Pass when the stress difference between the two segments was only 0.3 megapascals. Currently, Burkhard said, the gap is measuring 0.8 megapascals.
Weingarten framed the central lesson this way: the point isn’t simply that stress builds over time—scientists have known that for a long time. “but that the balance of stress across the junction may decide whether the next earthquake stays contained or grows into a much bigger rupture,” he said.
The message, Burkhard said, is not to panic. It’s to prepare with urgency.
She told city managers and emergency responders to plan for joint ruptures along the San Andreas and San Jacinto faults as a realistic possibility under current stress conditions. not as 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.
For people in the region—those who commute on major highways. rely on rail connections. and depend on energy corridors—this is the kind of warning that doesn’t end when the news cycle does. It asks a simple question of preparation: if the gate is under its highest stress in 1. 000 years. what does “ready” look like before the ground decides?.
San Andreas fault San Jacinto fault Cajon Pass earthquake gate southern California seismic risk earthquake stress Liliane Burkhard Matthew Weingarten Wrightwood earthquake Los Angeles San Bernardino Riverside Coachella Valley