Three fault segments running beneath northern California and its 15 million people are overdue for a major earthquake, including one section that lies near the dams and canals that supply much of the state's water.
The three fault segments and one other in the region are loaded with enough tension to produce quakes of magnitude 6.8 or greater, according to a geological study published on Monday.
They include the little-known Green Valley fault, which lies near key dams and aqueducts northeast of San Francisco. Underestimated by geologists until now, the fault running between the cities of Napa and Fairfield is primed for a magnitude-7.1 quake, according to researchers from the US Geological Survey and San Francisco State University.
The water supplies of the San Francisco Bay Area, Southern California and the farm-rich Central Valley depend on the man-made water system that links to the Sacramento and San Joaquin rivers, noted James Lienkaemper, the US Geological Survey geologist who was lead author of the study. The Green Valley fault is last believed to have ruptured sometime in the 1600s.
The study shows the state "needs to consider more seriously" the earthquake risk in that area, Lienkaemper said by phone.
All four vulnerable fault segments belong to the San Andreas fault system, the geological dividing line that marks where the western half of California shifts northwest and away from the rest of North America at about 6cm a year.
Northern California recorded its biggest earthquake of a quarter-century August 24, when a magnitude-6.0 quake hit Napa, north of San Francisco.
Seismologists estimate seven quakes of 7.3 magnitude or more have hit California just since the 1800s, most of them when the state's population was a fraction of what it is now.
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