A geological study conducted by researchers from the U.S. Geological Survey and San Francisco State University is warning that four fault segments running beneath Northern California or nearby are primed for an earthquake of 6.8 magnitude or more.
The faults are the Green Valley fault between Napa and Fairfield, which underlies areas near key dams and aqueducts, and could manifest a magnitude 7.1 earthquake; the northern Calaveras and Hayward faults in the east San Francisco Bay Area; and the Rodgers Creek fault that lies north of San Francisco.
The study was published in the Bulletin of the Seismological Society of America, according to the AP. James Lienkaemper, the lead author noted that water supplies throughout the state rely on the man-made water route connecting the Sacramento and San Joaquin rivers, and an earthquake on the Green Valley fault would threaten that. The last occurrence of a temblor on that fault came in the 1600s.
The San Andreas fault system underlies all four fault segments; the San Andreas fault divides California between its western half, which moves northwest 2 inches a year, from the eastern half of the state.
Of the 1,250 miles making up the five major branches of the San Andreas fault, about two-thirds feature fault creep, tiny movements that release strain on the faults. (If there is no fault creep, the fault is estimated to be locked, and a candidate for an earthquake from the resultant stress.)
The largest magnitude earthquake in Northern California in 25 years struck Aug. 24 in Napa, a 6.0 magnitude temblor. According to seismologists, seven temblors of 7.3 magnitude or more have struck in the state since the 1800s.