Editing San Andreas Fault (section)

== Plate boundaries ==
[[File:Mormon Rocks at the San Andreas Fault, California, USA as viewed from a FPV drone.webm|thumb|Mormon Rocks is an example geological formation along the San Andreas Fault.]]

The [[Pacific plate]], to the west of the fault, is moving in a northwest direction while the [[North American plate]] to the east is moving toward the southwest, but relatively southeast under the influence of [[plate tectonics]]. The rate of slippage averages about {{convert|33|to|37|mm|in|sp=us}} a year across California.<ref>{{cite web |url=http://pubs.usgs.gov/pp/1990/1515/ |title=Present-Day Crustal Movements and the Mechanics of Cyclic Deformation |work=The San Andreas Fault System, California |last=Wallace |first=Robert E. |access-date=2007-10-26}}</ref>

The southwestward motion of the North American plate towards the Pacific is creating compressional forces along the eastern side of the fault. The effect is expressed as the Coast Ranges. The northwest movement of the Pacific plate is also creating significant compressional forces which are especially pronounced where the North American plate has forced the San Andreas to jog westward. This has led to the formation of the Transverse Ranges in Southern California, and to a lesser but still significant extent, the [[Santa Cruz Mountains]] (the location of the [[Loma Prieta earthquake]] in 1989).

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, California earthquake|Landers earthquake]] in 1992 – suggests the plate boundary may be shifting eastward away from the San Andreas towards Walker Lane.<ref>{{cite journal |last1=Faulds |first1=James E. |last2=Henry |first2=Christopher D. |last3=Hinz |first3=Nicholas H. |title=Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary |journal=Geology |date=2005 |volume=33 |issue=6 |pages=505 |doi=10.1130/G21274.1|bibcode=2005Geo....33..505F }}</ref><ref>{{cite journal |last1=Busby |first1=C. J. |title=Birth of a plate boundary at ca. 12 Ma in the Ancestral Cascades arc, Walker Lane belt of California and Nevada |journal=Geosphere |date=1 October 2013 |volume=9 |issue=5 |pages=1147–1160 |doi=10.1130/GES00928.1|bibcode=2013Geosp...9.1147B |doi-access=free }}</ref>

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.<ref>[http://www.geologypage.com/2012/08/san-andreas-fault.html San Andreas Fault]. Geologypage. Retrieved from July 21st, 2020.</ref>