SAN FRANCISCO (KABC) -- Three fault segments running beneath Northern California and one other in the region are overdue for a major earthquake, according to a geological study published Monday.



The U.S. Geological Survey determined that about 40 miles of fault lines in Northern California appear to be locked. That means pressure is building along the fault lines, which could trigger a very powerful earthquake.



Large stretches of the San Andreas fault are primed for earthquakes of magnitude 6.8 or greater. The fault running between the cities of Napa and Fairfield is primed for a magnitude-7.1 quake.



The study also found the Green Valley fault in Napa and Solano counties appears to be ready for a large earthquake as well. That fault is near key dams and aqueducts northeast of San Francisco that supply the San Francisco Bay Area, Southern California and the farm-rich Central Valley's water supply.



The surveys measure fault creep, movements of fractions of inches that slowly release strain on some faults. When no fault creep is recorded, a fault is considered locked, and stress builds until an earthquake unlocks it.



Roughly two-thirds of the 1,250 miles that comprise the five major branches of the San Andreas fault feature fault creep, the study concludes.



The Associated Press contributed to this report.





