Southern California Faults at Highest Stress in 1,000 Years

Tectonic stress along Southern California's San Andreas and San Jacinto fault systems has reached levels not seen in the past millennium.

A new study published in the Journal of Geophysical Research: Solid Earth warns that the region is in a "critically loaded state" — though researchers emphasize there are no signs of an imminent rupture.

Key Findings from the Research

• Stress has exceeded previous millennium-high levels in some areas of the fault system.
• The system is operating under unusually high stress within its long-term seismic cycle.
• The Cajon Pass, a critical junction between the two fault systems, may act as an "earthquake gate" — capable of either blocking or linking ruptures.
• More than 160 years have passed since the last major rupture in the region.

"A rupture involving both fault systems could be more damaging than a single-fault event," said lead author Liliane Burkhard of the University of Hawaiʻi at Mānoa.

The potential threat is amplified by the proximity to major population centers, including Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

Understanding the San Andreas Fault

The San Andreas Fault is a strike-slip plate boundary, according to the U.S. Geological Survey, where the Pacific and North American plates slide past each other horizontally. Large earthquakes involve sideways movement, not the splitting of land.

Earthquake Preparedness

California's Earthquake Early Warning system uses a network of seismic sensors to provide alerts before strong shaking arrives. Warnings are broadcast via:

• Cell phone alerts
• The MyShake app
• Wireless Emergency Alerts

Recommended Safety During an Earthquake

Authorities advise:

• Drop to hands and knees
• Cover your head and neck
• Hold on until shaking stops
• Stay indoors unless you are near a tsunami zone