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Roger Weller, geology instructor

San Andreas Fault
by Victoria  Hermosilla
Physical Geology
Spring 2007



The San Andeas Fault-Divisions Among Us


            Hardly any border or law affecting the U.S. can have as much of an impact as the San Andreas Fault not only within the U.S., but also around the world.  At approximately 800 miles in length, the San Andreas Fault is literally tearing North America apart bit by bit (Wikipedia).  However, to fully understand the magnitude of this natural line in the sand, more research and knowledge needs to be given.  Details of plate tectonics, fault structures, and the devastation from the earthquakes produced from the fault need to be understood first in order to somehow appreciate the San Andreas Fault. 

            First of all, how would such a line in the earth’s crust even begin to be formed?  It all began about 180 million years ago when the super continent Pangaea began to break up (Wikipedia).  Largely referred to as continental drift, large plates of the Earth’s crust began to shift around and bump into each other thereby forming the earth’s continents as civilization knows it now (Plate). 

Map of Pangaea


Approximately 180 million years ago.


Present day Earth altimetry and bathymetry.  Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.

Present day Earth.  Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.

            So as can be seen from the illustrations, as the earth’s plates and continents have been shifting around for millions of years.  Such large masses of land carry, well, a lot of mass.  The force of such masses meeting, crashing into each other, or being torn apart has severe affects on the earth’s topography: particularly when it comes to mountains and fault lines. 

            The meeting of two plates or land masses leads to an inquiry of faults.  Basically, faults are the result of two plates moving next to each other.  Normal faults are the result of two plates being pulled away from each other which results in tension (Faults).  Reverse faults are the result of two plates meeting, and then squeezing together which results in compression (Faults).  Last but not least, a strike-slip fault is the result of two plates meeting and then sliding in opposite directions.  That sliding is referred to as lateral movement (Faults).


           The San Andreas Fault is a strike-slip fault (Webster’s). 


San Andreas fault gif

            Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo. (Photograph by Robert E. Wallace, USGS.)

              The Pacific Plate and the North American Plate are moving in opposite directions laterally.  This is obvious from earthquakes such as the Great Quake of 1906 where the two sides of the fault shifted 3 feet laterally (Webster’s).  The fault cuts through from the southeastern desert of California, bends in the Los Angeles area, and then turns north again to San Francisco where it dives beneath Pacific Ocean in the Tomales Bay (Wikipedia).  From the San Andreas Fault, many more faults branch out into the California landscape.  Most notably of the offshoots are the San Jacinto Fault, the Owens Valley Fault, the Banning Fault, and the Garlock Fault.

Map of large faults in California

            Since the Pacific Plate and the North American Plate which meet at the San Andreas Fault are continuously sliding against each other, there are many earthquakes which occur each year in California.  Geologists estimate the plates slide at an average rate of approximately an inch each year (Wikipedia).  The friction and movement would therefore cause many earthquakes all along the fault as the crust releases the tension and ‘jumps’ into its new position.

Map of earthquake occurance

The map shows the location of all earthquakes of magnitude 1.5 and larger in the California-Nevada region during 1980. Literally thousands of small earthquakes occur in California each year, providing scientists with clear indications of places where faults cut the Earth's crust.


            People living in California must be in a constant state of awareness.  The ground upon which their infrastructure is built is shifty ground.  So, safety precautions such as drills, emergency kits, and knowledge of earthquakes must be taken.  Additionally, special construction for buildings and houses must be used to prevent great loss of life if a large earthquake were to strike.

            Thrust into place and kept alive by the forces behind continental drift, the San Andreas Fault is the earth in action.  More interesting still is the strike-slip structure of the fault which allows it to shift a few centimeters each year.  That continual shifting is the reason behind so many earthquakes in California where the fault is located.  All in all, the San Andreas Fault is a truly fascinating bit of geology which is alive and well. 



Works Cited

Faults and Stresses. Home page. 2 May 2007. 

Plate Tectonics. Home page. 2 May 2007.

“San Andreas Fault.” Webster’s Family Encyclopedia. 1st ed. 1988.

USGS. San Andreas Fault. 24 Jun. 1997.

Wikipedia. Home page. 1 May. 2007. 2 May 2007.