It’s like a scene from a Hollywood movie. A massive earthquake along the San Andreas Fault ripples through Los Angeles, destroying infrastructure, starting fires, and triggering landslides. For decades, scientists have studied when and how this disaster unfolds.
Researchers now report that such earthquakes can be more widespread and damaging than previously thought. Their study found that the southern portion of the San Andreas Fault and the adjacent San Jacinto Fault Line are locked in place and loaded to the highest stress levels in 1,000 years, increasing the likelihood of a significant earthquake. In the event of a strong earthquake in either fault zone, the rumble could travel through “seismic gates” to neighboring fault zones, spreading damage simultaneously from north Los Angeles through San Bernardino, Riverside and the Coachella Valley, researchers said.
“While we talk loosely about the fault being ‘overdue,’ it’s important to look at the physics-based estimate that the fault is at its highest point in 1,000 years,” said Matthew Weingarten, a geologist at San Diego State University who was not involved in the study.
Earthquakes occur when a sudden slip along a fault (a crack in the Earth’s crust where chunks of rock collide with each other) releases energy that has built up over time. Stress builds up due to tectonic movements, but parts of the fault become locked and cannot slide freely.
Stress has been building for more than a century along the Southern San Andreas and San Jacinto fault systems, located less than 90 miles northeast of downtown Los Angeles. Both rifts form the boundary between the Pacific and North American tectonic plates, where the plates slide past each other by a few centimeters each year while other zones remain fixed. As a result, some segments of the fault have increased tension, like a coiled spring with nowhere to move.
In fact, past analyzes indicate that there is a greater than 50 percent chance of a magnitude 6.7 or greater earthquake occurring south of the San Andreas Fault within the next few decades.
But the San Andreas and San Jacinto fault systems also meet at a confluence point called Cajon Pass, which researchers say acts as an “earthquake gate.” Gates can stop or transmit large ruptures between two faults. In 1812, the magnitude 7.5 Wrightwood earthquake that researchers suspect crossed Cajon Pass rippled along both the San Andreas and San Jacinto fault systems, killing 40 people.
Scientists now say that if an earthquake were to occur along the Cajon Pass and both faults, the effects would be severe and far-reaching, affecting critical infrastructure such as major highways, railroads and energy corridors in multiple cities simultaneously.
“In terms of severity, a co-rupture over the Cajon Pass could reach magnitudes around 7.4 to 7.8 and have a much wider impact than a single fault event,” said study lead author Lilian Burckhardt, a geophysicist at the University of Bern in Switzerland.
To determine what caused earthquakes to pass through the Cajon Pass, the researchers reconstructed the past 1,000 years of seismic activity along the two faults, tracing how stress built up and was released. They found that the earthquake passed through the confluence when both sides of the pass had similar levels of high stress, and “that’s the configuration we’re approaching today,” Burkhardt said.
According to simulations, the San Jacinto Bernardino segment exhibited the highest stress loading of 1,000 years of restoration, registering 3.6 megapascals. This measure is above its peak about 50 years ago. The Mojave South section of the San Andreas recorded 2.8 megapascals, surpassing the record stress load from 10 years ago.
Previous simulations showed that failure would pass through Cajon Pass if the stress difference between the two segments was only 0.3 megapascals. Now, the difference is 0.8 megapascals, said Burkhard, who conducted much of the research at the University of Hawaii at Manoa.
“This insight is not that stress increases over time, which is something we’ve known for a long time,” said Weingarten, whose research group modeled seismic stress and triggering on the San Andreas Fault. “The balance of stress across the joint may determine whether the next earthquake remains suppressed or grows into a larger fault.”
Burkhardt said the underlying message of the new study is to act quickly rather than panic. City managers and emergency responders should plan for joint rupture along the San Andreas and San Jacinto faults as a realistic possibility under current stress conditions, rather than a distant worst-case scenario.
“Southern California faces serious and increasing earthquake risk, and now is the time to prepare, not wait until the next earthquake hits,” she said.
