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. 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 interseismic period, the Earth's plates (see plate tectonics) move relative to 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 deformation builds at the rate 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.
Read more about this topic: Elastic-rebound Theory
Famous quotes containing the words theory and/or explained:
“A theory of the middle class: that it is not to be determined by its financial situation but rather by its relation to government. That is, one could shade down from an actual ruling or governing class to a class hopelessly out of relation to government, thinking of gov’t as beyond its control, of itself as wholly controlled by gov’t. Somewhere in between and in gradations is the group that has the sense that gov’t exists for it, and shapes its consciousness accordingly.”
—Lionel Trilling (1905–1975)
“Nothing fortuitous happens in a child’s world. There are no accidents. Everything is connected with everything else and everything can be explained by everything else.... For a young child everything that happens is a necessity.”
—John Berger (b. 1926)