Tectonic stress along the southern San Andreas and neighboring San Jacinto fault systems has reached its highest level in 1,000 years, significantly elevating the risk of a major earthquake in Southern California, according to a study published this month.

The peer-reviewed research, published in the Journal of Geophysical Research: Solid Earth, warns that the two fault lines are so critically strained that a rupture on one could spill over into the other, potentially triggering a massive, multi-fault catastrophe, reports LiveScience on Wednesday.

The 'Earthquake Gate' scientists identified Cajon Pass, a mountain pass between the San Bernardino Mountains and the San Gabriel Mountains, as a critical "earthquake gate."

Located where the San Jacinto fault branches off from the San Andreas fault, this junction can either halt a rupture or allow it to pass through, depending on the alignment of stress levels between the two systems.

"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," said Liliane Burkhard, the study's first author and a planetary geologist at the University of Bern and the University of Hawaii at Manoa.

The two fault systems have historically been highly active, causing 36 earthquakes of magnitude 6.4 or greater over the last millennium.

A major earthquake in 1812 successfully breached the Cajon Pass gate, proving a joint rupture is physically possible.

Southern California's last major event was a magnitude 7.9 earthquake in 1857, which ruptured a 205-mile segment of the San Andreas fault.

That rupture stopped at Cajon Pass, leaving the southern sections locked and accumulating stress ever since.

Geologists noted that nearly 170 years of seismic silence along this southern stretch means the faults are primed for an Earth-shattering release.

Unlike the 19th century, a modern rupture would tear through heavily populated areas, threatening millions of residents in Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

To calculate the modern risk, researchers reconstructed 1,000 years of tectonic activity using physical evidence of past earth movements, including displaced sediment data and historical tree ring records.

The data was integrated into a computer model that simulates how stress accumulates and transfers through the Earth's crust over centuries.

The model revealed that because both faults currently share similarly extreme stress levels, the Cajon Pass gate is highly susceptible to letting a rupture pass through.

This could result in a "joint rupture" across two branches of the San Andreas, or a worst-case "tripartite rupture" involving both branches of the San Andreas and the San Jacinto fault simultaneously.

While the study cannot predict the exact timing or probability of an impending earthquake, researchers emphasized that the data is crucial for state and local infrastructure planning, emergency preparedness, and updated hazard assessments.