Study Finds 1,000-Year Stress Highs at San Andreas Fault Line

New research says the san andreas fault line at Cajon Pass has reached stress levels matching 1,000-year highs. The junction sits about 50 miles from downtown Los Angeles, where two major Southern California fault systems meet and could shape how a future rupture spreads.

The study, published this month in the Journal of Geophysical Research, used computer simulations of the last 1,000 years of rupture history. Researchers estimated stress levels of 2.8 on the Mojave South segment, 1.8 on the North San Bernardino segment and 3.6 on the San Jacinto Bernardino segment, making the San Jacinto segment the most heavily loaded part of the junction.

Cajon Pass Junction

Cajon Pass links the Mojave South and North San Bernardino segments of the San Andreas Fault with the San Jacinto Bernardino segment of the San Jacinto Fault. The study said the junction could act as an earthquake gate, sometimes halting a rupture there and other times allowing it to jump between the two fault systems.

That interaction matters for a densely populated region because the study suggested the two fault systems may interact when their stress levels become equal. The report said stress has reached high levels across that area, not just on one segment.

Journal of Geophysical Research

Over the past 1,000 years, the San Andreas and San Jacinto fault systems have produced at least 36 earthquakes of magnitude 6.4 or greater, and it has been over 100 years since they produced a major earthquake. The new simulation work places those events in a longer rupture history and points to the Cajon Pass junction as the place where future ruptures may be redirected.

For residents and planners near Los Angeles, the practical takeaway is that the junction is not just a point on a map. It is the part of the system the study says can control whether shaking stays on one fault or spreads across both connected systems.