The stretch of the San Andreas Fault running through Southern California has not broken in a major earthquake since 1857, at least 300 years by some geologists’ accounting of the fault’s slip history, while stress has continued to build underneath Los Angeles, Orange and San Diego counties. A scenario built by the U.S. Geological Survey to plan for the next rupture puts the toll at roughly 1,800 deaths and $213 billion in damage if a magnitude-7.8 earthquake tears through the same ground today.
Two hundred miles of fault that skipped its turn
The last major rupture on the southern San Andreas was the Fort Tejon earthquake of January 9, 1857, a magnitude-7.9 event that tore roughly 350 kilometers of fault line and was felt as far away as Sacramento. Only two deaths were recorded, largely because the region was sparsely settled at the time; a repeat today would strike a corridor now home to more than 20 million people across Los Angeles, Orange, Riverside, San Bernardino and San Diego counties. The fault has produced comparably large ruptures elsewhere since then, including the magnitude-7.8 earthquake that devastated San Francisco in 1906, but that break occurred on the fault’s northern segment, roughly 350 miles from the section that has stayed locked. Geologic surveys of offset stream channels and buried fault trenches put the interval since the southern segment’s last big slip at a minimum of 300 years, and some estimates run longer, since the record of prehistoric ruptures thins the further back researchers try to read it.
Geologist Andrew Lawson of the University of California, Berkeley first mapped the fault as a continuous feature in 1895, more than a decade before the 1906 earthquake demonstrated how far a single rupture could travel along it. The southern extension he traced runs from the Salton Sea, near the U.S.-Mexico border, north through the Coachella Valley, along the base of the San Bernardino Mountains and into the Transverse Ranges above Los Angeles, a geometry that concentrates the fault’s bend directly under some of the region’s fastest-growing suburbs.
Geophysicist Yuri Fialko of the Scripps Institution of Oceanography published stress measurements in the journal Nature in 2006 suggesting the southern segment had accumulated enough strain to produce an earthquake as large as any in the fault’s recorded history, a finding that reframed how urgently seismologists talk about the gap.
“Locked, loaded and ready to roll”
Thomas Jordan, director of the Southern California Earthquake Center, told the National Earthquake Conference in Long Beach that the southern San Andreas is “locked, loaded and ready to roll.” Jordan described the fault’s accumulated strain in blunter terms than most agency reports allow, comparing the system to a spring “wound very, very tight” after more than a century and a half without the release that plate motion alone should have already forced.
Plate motion along the fault runs at roughly an inch and a half a year, enough to have shifted the two sides past each other by about 16 feet since 1857 had the segment been slipping steadily. It has not. That accumulated, unreleased offset is the physical basis for describing the southern San Andreas as overdue rather than simply active.
The ShakeOut Scenario’s 1,800 deaths and $213 billion
The 1,800-death, $213 billion estimate comes from the ShakeOut Scenario, a 2008 U.S. Geological Survey study led by seismologist Lucile Jones that modeled a hypothetical magnitude-7.8 earthquake rupturing the southern San Andreas from the Salton Sea to the mountains north of Los Angeles. The report, co-written with more than a dozen engineers and social scientists, broke the projected damage into building collapses, fire losses and the failure of aqueducts and pipelines that carry most of Southern California’s water supply across the fault line itself.
Jones and her co-authors built the scenario not as a prediction of when the earthquake will happen but as a planning tool, intended to push cities, utilities and hospitals to fix the specific vulnerabilities the model exposed rather than treat the hazard as an abstraction. Retrofits to older water pipelines and hospital buildings that followed the report’s publication were credited, in later state assessments, with lowering some of the original loss estimates for particular systems, though the fault’s overall hazard did not change.
Where the 7 percent comes from
The 7 percent figure describing the odds of a magnitude-8.0 or larger earthquake somewhere on the San Andreas system within 30 years comes from UCERF3, the Uniform California Earthquake Rupture Forecast issued in November 2013 by a working group that includes the USGS, the California Geological Survey and academic seismologists. The model does not point to a single date or location; it treats the southern San Andreas as one of several faults statewide capable of producing that scale of event, though its long quiet period and measured strain make it the segment specialists return to most often when the subject comes up.
The Earthquake Country Alliance, a public-private partnership that includes the USGS and state emergency officials, notes that no instrument can predict the day a fault will finally slip. The alliance’s own guidance stops short of a date and instead points residents back to the same recommendation every version of the forecast has carried since Fialko’s 2006 paper: treat the overdue segment as a when, not an if, and plan accordingly.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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