The San Andreas fault zone has been a very significant source of major California earthquakes. From 1812 to 1906 it generated four major earthquakes of M ~7 or larger in two pairs on two major portions of the fault. A pair of major earthquakes occurred on the central to southern region, where the 1857 faulting overlapped the 1812 earthquake faulting. A pair of major earthquakes occurred on the northern region, where the 1906 faulting overlapped the 1838 earthquake faulting.
These maps show an estimate of the likelihood of earthquake ground motions, based on a probabilistic seismic hazard analysis. Such analysis incorporates seismic and geologic information to consider the probability of all possible damaging earthquakes, calculates the potential range of ground motions for each potential earthquake, and arrives at a level of ground shaking that has a given probability, using the formulation first developed by Cornell (1968).