Earthquake Los Angeles: San Andreas at critical stress

Scientists say key sections of the San Andreas and San Jacinto faults near Cajon Pass are at their highest tectonic strain in 1,000 years, raising earthquake Los Angeles risk. A new model does not predict when the next Big One will strike, but it finds stress conditions approaching those that preceded major joint ruptures historically.

Key Takeaways

• Researchers model peak tectonic stress in 1,000 years on key San Andreas and San Jacinto segments near Cajon Pass.
• Modeled stress reaches about 2.8 megapascals on the San Andreas northwest of the pass and 3.6 on the San Jacinto southeast.
• USGS estimates cite a roughly 60% chance of a magnitude 6.7 or greater quake in the Los Angeles region by 2045.
• Scientists stress the finding is not a prediction of when a quake will hit—preparedness is the core message.

What did scientists find about San Andreas stress?

According to reporting on a study published in the Journal of Geophysical Research: Solid Earth, a computer model estimated seismic strain on three key segments of the San Andreas and San Jacinto faults where they meet at Cajon Pass in San Bernardino County—about 50 miles northeast of downtown Los Angeles.

Lead author Liliane Burkhard, of the University of Bern and the University of Hawaii, said the model found tectonic stress higher than at any point in a 1,000-year record, with conditions approaching those that historically preceded large joint ruptures across both fault systems. USGS geologist Kate Scharer, a co-author, noted that earthquake energy keeps accumulating and is released mainly through large quakes, not small ones.

The model drew on estimated quake timing over the last millennium, satellite observations of plate motion, and estimates of crust rigidity. Coverage of the research also notes the San Andreas accounts for about 90% of tectonic slipping between the North American and Pacific plates in Southern California.

Why does this matter for earthquake Los Angeles risk?

Key sections have gone more than a century without significant quakes, and that quiet has allowed strain to build. The study calculated current stress of 2.8 megapascals on the San Andreas just northwest of Cajon Pass—above prior highs of 2.7 before large quakes in 1469 and 1691—and 3.6 megapascals on the San Jacinto southeast of the pass, above a previous high of 2.9 before a quake in 1249.

USGS figures cited in coverage put a 60% chance of a magnitude 6.7 or greater earthquake striking the Los Angeles region by 2045, and about a 1-in-5 chance of such a quake on the San Andreas within L.A. County and the Inland Empire. A hypothetical magnitude 7.8 southern San Andreas quake in a 2008 USGS estimate could bring violent shaking across L.A., Orange, Riverside, San Bernardino, Ventura, and Imperial counties and an estimated 1,800 deaths—far beyond the localized impact of the 1994 Northridge magnitude 6.7 quake.

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When is the next Big One expected to hit?

No one can name a date. Researchers and reporters covering the study emphasize the model is not a forecast of timing. Intervals on the San Andreas northwest of Cajon Pass are often roughly 100 to 150 years between big quakes, Scharer said, though gaps near 200 years are known; southeast of the pass, big events often come every 200 to 250 years.

The last southern California “Big One” on the San Andreas was the 1857 Fort Tejon quake, estimated at magnitude 7.9. Scientists acknowledge model limits, including assumptions about how completely big ruptures reset stress. Burkhard’s message, echoed in local coverage, is simple: take the elevated loading seriously and get prepared. Full reporting is available from the Los Angeles Times.