Most earthquakes are believed to occur according to the elastic rebound theory, which was developed after the San Francisco earthquake of 1906. In this theory an earthquake represents a sudden release of strain energy that has built up over a period of time.
In Fig. 2.3a a block of rock traversed by a pre-existing fracture (or fault) is being strained in such a way as eventually to cause relative motion along the plane of the fault. The line AB is a marker indicating the state of strain of the system, and the broken line the location of the fault. Relatively small amounts of strain can be accommodated by the rock (Fig. 2.3b). Eventually, however, the strain reaches the level at which it exceeds the frictional and cementing forces opposing movement along the fault plane (Fig. 2.3 c). At this point fault movement occurs instantaneously (Fig. 2.3d). The 1906 San Francisco earthquake resulted from a displacement of 6.8 m along the San Andreas Fault. In this model, faulting reduces the strain in the system virtually to zero, but if the shearing forces persist, strain would again build up to the point at which fault movement occurs. The elastic rebound theory consequently implies that earthquake activity represents a stepwise response to persistent strain.
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