After the devastating 1906 San Francisco, California earthquake, a fault trace was discovered that could be followed along the ground in a more or less straight line for 270 miles. It was found that the earth on one side of the fault had slipped compared to the earth on the other side of the fault by up to 21 feet (7 m). This fault trace drew the curiosity of a number of scientists, especially since nobody had yet been able to explain what was happening within the earth to cause earthquakes. Up until this earthquake, it had generally been assumed that the forces leading to the occurrence of earthquakes must be close to the locations of the earthquakes themselves.
Harry Fielding Reid, after studying the fault trace of the 1906 earthquake, postulated that the forces causing earthquakes were not close to the earthquake source but very distant. Reid's idea was that these distant forces cause a gradual build up of stress in the earth over tens or hundreds or thousands of years, slowly distorting the earth underneath our feet. Eventually, a pre-existing weakness in the earth--called a fault or a fault zone--can not resist the strain any longer and fails catastrophically. This is something like pulling a rubber band gradually until the band snaps. This theory is known as the "elastic rebound theory."
The following animation shows a bird's eye view of a country road that cuts through an orchard. Passing right down the middle of the orchard, and across the road, is a fault zone. The animation shows how the earth is gradually distorted about the fault, in response to distant forces, eventually leading to sudden slip or displacement along the fault--what we call an earthquake.
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