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In April 1906 the great San Francisco earthquake struck and most of the city either toppled or was burned. In late 1906 the state of California formed the California State Earthquake Investigation Commission to study the San Francisco earthquake and to determine
the risk to the state of possible future earthquakes. Reid was asked to serve as a member of
this nine-member commission.
This commission study turned Reid’s interest toward earthquakes and fault lines. He
mapped and studied the San Andreas fault line and roamed the central California coastal region mapping other fault lines. Always he searched for an answer to the question: What
caused earthquakes?
Reid carefully studied the rocks along California fault lines and concluded that they
suffered from long-term physical stress, not just from the jolt of a sudden earthquake. Reid
saw that great stresses must have existed in the rocks along the San Andreas fault line for
centuries—even for millennia—before the earthquake happened.
That meant that the fault lines had to have existed first and stress along them caused the
earthquake. Stress built up and built up in the rocks until they snapped. That “snap” was an
earthquake.
Reid developed the image of the rock layers along fault lines acting like rubber bands.
Stresses deep in the earth along these fault lines pulled the rocks in different directions,
causing these rocks to stretch—like elastic. Once the stress reached the breaking point, the
rocks elastically snapped back—causing an earthquake.
Fault lines caused earthquakes, not the other way around. That meant that studying
fault lines was a way to predict earthquakes, not merely study their aftermath. Reid had discovered the significance of the earth’s spider web maze of fault lines.
Fun Facts: The destructive San Francisco earthquake of 1906 horizontally shifted land surfaces on either side of the San Andreas fault up to 21
ft (6.4 m).
More to Explore
Branley, Franklyn. Earthquakes. New York: HarperTrophy, 1998.
Harrison, James. Discover Amazing Earth. New York: Sterling Publishing Co., 2005.
Sherrow, Victoria. Great Scientists. New York: Facts on File, 1998.
Thomas, Gordon. The San Francisco Earthquake. New York: Day Books, 1996.
Ulin, David. The Myth of Solid Ground. New York: Penguin, 2005.