Southern California’s San Andreas and San Jacinto fault systems are at their highest levels of tectonic stress in 1,000 years in what scientists describe as a “critically loaded state”, according to a study published earlier this month.

“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,” Liliane Burkhard, the lead author of the study published in the Journal of Geophysical Research: Solid Earth, said in a statement.

Cajon Pass, which sits at the junction of the faults, could play a key role, acting as an “earthquake gate” that can block large ruptures from traveling between the faults or involve both systems in one event, according to the research. It’s been more than a century since the last major event, and stress “has continued to accumulate and is now at unprecedented levels”, which has increased the chances of a large quake in the future.

“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,” said Burkhard.

“Right now, with stress at historically high levels across the region and more than 160 years elapsed since the last major rupture, the system is in a critically loaded state.”

The team of scientists at the University of Hawaii at Mānoa used a computer model to see how stress accumulates and releases in the fault systems and input 1,000 years of earthquake history to simulate how much stress is currently present. Their research found that Cajon Pass could “facilitate a joint rupture of both the San Andreas and San Jacinto faults simultaneously”, and have impacts across southern California.

Their research does not predict when such a quake might happen, but is necessary to better understand the risks, Burkhard said.

“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.”

Californians have long been preparing themselves for the “big one”. The last major devastating and destructive quake to hit the state was 1994’s Northridge earthquake in Los Angeles. The disaster, which unfolded just before dawn, destroyed 87,000 homes and businesses and killed more than 60 people.