Elastic-rebound theory
Elastic rebound
The elastic rebound theory is an explanation for how
energy is spread during earthquakes. As rocks on oppo-
site sides of a fault are subjected to force and shift, they
accumulate energy and slowly deform until their inter-
nal strength is exceeded. At that time, a sudden move-
ment occurs along the fault, releasing the accumulated
energy, and the rocks snap back to their original unde-
formed shape.
In geology, the elastic rebound theory was the first theory
to satisfactorily explain earthquakes. Previously it was
thought that ruptures of the surface were the result of
strong ground shaking rather than the converse suggested
by this theory.
Ancient cultural explanations of earthquakes were often
along the lines of the mythical Japanese Namazu:A giant
catfish with the islands of Japan on his back. A demigod,
or daimyojin, holds a heavy stone over his head to keep
him from moving. Once in a while the daimyojin is dis-
tracted so Namazu moves and the Earth trembles.
1 The theory explained
Following the great 1906 San Francisco earthquake,
Harry Fielding Reid examined the displacement of the
ground surface around the San Andreas Fault.[1] From
his observations he concluded that the earthquake must
have been the result of the elastic rebound of previously
stored elastic strain energy in the rocks on either side of
the fault. In an seismic period, the Earth’s plates (see
plate tectonics) move relativeto each other except at most
plate boundaries where they are locked. Thus, if a road
is built across the fault as in the figure panel Time 1, it is
perpendicular to the fault trace at the point E, where the
fault is locked. The far field plate motions (large arrows)
cause the rocks in the region of the locked fault to accrue
elastic deformation, figure panel Time 2. The deforma-
tion builds at therate of a few centimeters per year, over a
time period of many years. When the accumulated strain
is great enough to overcome the strength of the rocks, an
earthquake occurs. During the earthquake, the portions
of the rock around the fault that were locked and had not
moved 'spring' back, relieving the displacement in a few
seconds that the plates moved over the entire interseismic
period (D1 and D2 in Time 3). The time period between
Time 1 and Time 2 could be months to hundreds of years,
while the change from Time 2 to Time 3 is seconds. Like
an elastic band, the more the rocks are strained the more
elastic energy is stored and the greater potential for an
event. The stored energy is released during the rupture
partly as heat, partly in damaging the rock, and partly as
elastic waves. Modern measurements using GPS largely
support Reid’s theory as the basis of seismic movement,
though actual events are often more complicated.
2 References
[1] Reid, H.F., The Mechanics of the Earthquake, The Cali-
fornia Earthquake of April 18, 1906, Report of the State
Investigation Commission, Vol.2, Carnegie Institution of
Washington, Washington, D.C. 1910
3 External links
•http://earthquake.usgs.gov/regional/nca/1906/
18april/reid.php
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