Summary (tl;dr)

A new study led by Liliane Burkhard reveals that stress levels on Southern California's San Andreas and San Jacinto fault systems have reached their highest point in 1,000 years, significantly increasing the potential for a major earthquake.

Essential Background

Earthquakes occur along fracture zones in the Earth's crust where large tectonic plates slide past each other, leading to a build-up of stress that is suddenly released. The San Andreas and San Jacinto faults are two of the most significant of these zones in Southern California, accommodating the majority of the region's plate motion. The last major earthquake to heavily impact the wider Los Angeles area was the 7.9 magnitude Fort Tejon earthquake in 1857, and tectonic stress has continuously accumulated along these fault segments since then.

The Full Story

Liliane Burkhard, a scientist at the University of Bern and a research affiliate at the University of Hawaiʻi at Mānoa, led an international research team that developed a physics-based computer model simulating 1,000 years of earthquake history along the southern San Andreas and San Jacinto fault systems. Their study, published on June 3, 2026, in the Journal of Geophysical Research: Solid Earth, indicates that current stress levels on multiple fault segments are at or have exceeded the highest values observed in the past millennium. The researchers also identified the Cajon Pass, located northeast of Los Angeles, as a critical "earthquake gate" that can either block or allow earthquake ruptures to cross between the San Andreas and San Jacinto faults. The study highlights that the present-day stress conditions between the two fault systems are closely aligned, a configuration historically associated with joint ruptures that could lead to more severe consequences.

Why It Matters

These unprecedented stress levels mean the fault system is in a "critically loaded state," raising concerns about the potential for a large, through-going earthquake involving both fault systems. Such an event could have devastating implications for heavily populated areas of Southern California, including Los Angeles, San Bernardino, Riverside, and the Coachella Valley. While the study does not predict when an earthquake will occur, its findings are vital for refining seismic hazard assessments and informing critical decisions related to infrastructure planning, emergency preparedness, and building codes in the region.

Geographic Location

• Southern California, United States (location of the San Andreas and San Jacinto fault systems and focus of the study)
• Cajon Pass, San Bernardino County, California, United States (critical junction between the San Andreas and San Jacinto fault systems)
• Los Angeles, Los Angeles County, California, United States (major population center at risk)
• San Bernardino, San Bernardino County, California, United States (major population center at risk)
• Riverside, Riverside County, California, United States (major population center at risk)
• Coachella Valley, Riverside County, California, United States (major population center at risk)