New earthquake study finds San Andreas fault is primed for a big quake

An earthquake is overdue along Southern California’s “critically stressed” San Andreas and San Jacinto faults, according to a new study.

As stress builds on a fault over centuries, it builds pressure that has to be released in an earthquake. In the study, scientists found that the San Andreas and San Jacinto faults are under more stress than at any point in the last 1,000 years, meaning that a massive earthquake could be on the way.

“Because it’s been quite a long time since the Southern San Andreas or the San Jacinto have had a large earthquake, we’ve accumulated a lot of stress,” said Kate Scharer, a co-author of the study and a geologist with the U.S. Geological Survey.

Using geological evidence, including tree-ring records and sediment samples, a team of scientists created a computer model that shows how pressure accumulates along faults over time. Then they ran the model up to the present day to estimate how much stress is now building beneath our region. They found that pressure has been gradually building since the last Big One in 1857, one of California’s largest seismic events on record.

“The idea that all of those segments of the fault could have enough stress for an imminent future earthquake was already there,” said Harold Tobin, the director of the Pacific Northwest Seismic Network and a professor at the University of Washington who was not involved in the study. “This [study] puts it on more of a quantitative, rigorous scientific basis.”

One area of interest is the Cajon Pass, the narrow corridor between the San Gabriel and San Bernardino mountains.

“Cajon Pass could act as an ‘earthquake gate,’ like a junction that either stops or transmits large ruptures between the San Andreas and San Jacinto faults depending on stress conditions,” said Liliane Burkhard, the lead author of the study and a research affiliate in the Hawaiʻi Institute of Geophysics and Planetology.

The pass is a place where a major earthquake could jump from one fault system to another, Burkhard said. It could allow the rupture to spread farther across Southern California and affect millions more people across the Coachella Valley and San Bernardino County.

Going forward, Burkhard hopes to study other earthquake-prone regions where several fault systems interact and create risks that remain difficult to predict.

Scientists agree that Southern California will experience another major earthquake. The challenge is that no one knows exactly when.

“It could happen today, tomorrow, or in 10 years, or in 30 years,” said Ahmed Elbanna, director of the Statewide California Earthquake Center and a professor at USC who was not involved in the study. “On geological time scales, these are all very short.”

So it’s a question of when, not if.

“We should certainly expect to experience large earthquakes in our lifetimes,” Scharer said.