<P> Studies of the relative motions of the Pacific and North American plates have shown that only about 75 percent of the motion can be accounted for in the movements of the San Andreas and its various branch faults . The rest of the motion has been found in an area east of the Sierra Nevada mountains called the Walker Lane or Eastern California Shear Zone . The reason for this is not clear . Several hypotheses have been offered and research is ongoing . One hypothesis--which gained interest following the Landers earthquake in 1992--suggests the plate boundary may be shifting eastward away from the San Andreas towards Walker Lane . </P> <P> Assuming the plate boundary does not change as hypothesized, projected motion indicates that the landmass west of the San Andreas Fault, including Los Angeles, will eventually slide past San Francisco, then continue northwestward toward the Aleutian Trench, over a period of perhaps twenty million years . </P> <P> The San Andreas began to form in the mid Cenozoic about 30 Mya (million years ago). At this time, a spreading center between the Pacific Plate and the Farallon Plate (which is now mostly subducted, with remnants including the Juan de Fuca Plate, Rivera Plate, Cocos Plate, and the Nazca Plate) was beginning to reach the subduction zone off the western coast of North America . As the relative motion between the Pacific and North American Plates was different from the relative motion between the Farallon and North American Plates, the spreading ridge began to be "subducted", creating a new relative motion and a new style of deformation along the plate boundaries . These geological features are what are chiefly seen along San Andreas Fault . It also includes a possible driver for the deformation of the Basin and Range, separation of the Baja California Peninsula, and rotation of the Transverse Range . </P> <P> The main southern section of the San Andreas Fault proper has only existed for about 5 million years . The first known incarnation of the southern part of the fault was Clemens Well - Fenner - San Francisquito fault zone around 22--13 Ma . This system added the San Gabriel Fault as a primary focus of movement between 10--5 Ma . Currently, it is believed that the modern San Andreas will eventually transfer its motion toward a fault within the Eastern California Shear Zone . This complicated evolution, especially along the southern segment, is mostly caused by either the "Big Bend" and / or a difference in the motion vector between the plates and the trend of the fault and its surrounding branches . </P>

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