Physics Photo of the Week

March 27, 2013

Shifted Fence
This wooden fence used to be connected in a straight line.  It stretches across the San Andreas Fault at Point Reyes National Seashore just north of San Francisco, California.  The massive 1906 earthquake that destroyed much of San Francisco was centered at Point Reyes.  The western side of the fault (where the camera is looking) shifted northward by about 5 meters represented by the displaced fence - all in a matter of about 20 seconds!  A wooden fence doesn't last very long (about 30 years at most), but this fence has been preserved and reconstructed by the National Park Service so visitors can appreciate the offset induced by the 1906 earthquake.  Pt Reyes National Seashore was created in 1962 to preserve the grasslands, open country, and prevent development very close to San Francisco.

The map of Point Reyes at right (photographed from a relief map at the Point Reyes Visitor Center clearly shows the San Andreas Fault as the narrow Tomales Bay to the north and the continuing narrow valley to the south.  The offset fence is on a short trail near the visitor center just south of the end of Tomales Bay.  The Point Reyes peninsula west of the fault has been "rapidly" transported from the Montery Bay area south of San Francisco through many earthquakes during the past several million years.   Click on the map for a larger image.

The outer parts of the peninsula consist of open grasslands, scattered beef and dairy farms (allowed to remain after creation of the national seashore), and magnificent views, wildlife, sea mammals, and extensive deserted beaches.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to

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