Burkhard Says San Andreas Fault Stress Level Hit 1,000-Year High

New research says the san andreas fault stress level across Southern California’s San Andreas and San Jacinto fault systems has reached or exceeded the highest levels seen in the past 1,000 years. Lead author Liliane Burkhard said the system is in a critically loaded state after more than 160 years since the last major rupture.

The study, led by Earth scientists at the University of Hawaiʻi at Mānoa and published in the Journal of Geophysical Research: Solid Earth, says the system is not showing signs of an imminent rupture. Researchers say the stress is building across multiple fault segments in a long-term seismic cycle that could support large earthquakes, including multi-fault events.

Burkhard and the 160-year span

Burkhard is a research affiliate at the University of Hawaiʻi’s Institute of Geophysics and Planetology and a scientist at the University of Bern. She said, "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 new study places that warning in a specific framework: the last major rupture was more than 160 years ago, and the present stress level now matches or exceeds the highest levels seen in a 1,000-year record. That does not point to a rupture today, but it does show how much load has accumulated across the fault network.

Cajon Pass and linked faults

Researchers said Cajon Pass, a junction between the San Andreas and San Jacinto fault systems, may act as an earthquake gate. It could block ruptures from crossing between faults or allow them to link into a single larger event.

That possible linkage is the practical concern for nearby population centers. Researchers said a rupture involving both fault systems could be significantly more damaging than a single-fault earthquake because of its size and proximity to Los Angeles, San Bernardino, Riverside, and the Coachella Valley.

The San Andreas Fault is a strike-slip plate boundary where the Pacific Plate and North American Plate slide past each other horizontally. Researchers said it is not a crack that would split California away from the continent, and most earthquakes do not produce surface rupture. For residents and planners, the message from the study is narrower and sharper: the region is under unusually high stress, but the data do not point to an imminent break.