The San Andreas Fault and San Jacinto Fault Zone have reached their highest stress levels in 1,000 years, according to a study by researchers at the University of Hawaii at Manoa, leading scientists to wonder when California’s next “Big One” will occur.

The fault lines have reached “unprecedented levels,” according to the study, which was recently published in Journal of Geophysical Research: Solid Earth. The study suggests that stress normally released in large earthquakes has continued to build as 160 years have passed since the last “major rupture.”

The study’s lead author, Liliane Burkhard, said the fault system is in a “critically loaded state.”

“Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems,” Burkhard said. 

The study also found that the Cajon Pass in Southern California may act as an “earthquake gate,” which can sometimes block large earthquakes from striking along the San Andreas Fault and San Jacinto Fault Zone at the same time. However, the Cajon Pass could also “facilitate a joint rupture,” the study states. An earthquake simultaneously striking both fault lines would be “significantly more damaging” and affect highly populated areas of Los Angeles, San Bernardino, Riverside and the Coachella Valley, according to the study.

“The conditions that determine whether the ‘earthquake gate’ at Cajon Pass opens or stays closed appear to be related to how closely the stress levels on the two fault systems are aligned with each other at the time of rupture,” Burkhard said. “Right now, with stress at historically high levels across the region.”

Scientists conducted the research by feeding 1,000 years of earthquake history from the region into a computer model. The extensive history was gathered by reconstructing geological evidence through radiocarbon dating of displaced sediments and tree-ring records. 

This research can help determine earthquake threat levels and help experts plan for disasters, the University of Hawaii at Manoa said.

“This is not a prediction of when an earthquake will happen,” Burkhard said. “However, studies like this are important contributions to national and global earthquake hazard research in that we are using rigorous, quantitative science to better understand the risk facing millions of people. What we can say is that the system is critically stressed, and that physics-based models like this one give us a clearer picture of the range of scenarios we should be prepared for. That information matters for hazard assessments, infrastructure planning, and emergency preparedness.”

California residents are strongly encouraged to prepare for the next major earthquake. Earthquake preparedness tools and information are available at earthquake.ca.gov.

“If you live in or travel to areas that could be impacted by an earthquake, preparation is vital, as these events can strike suddenly, at any time, with little warning,” the state website reads.