In 1906, the seismologist Henry Reid developed the “elastic rebound theory” to explain earthquakes. When rocks begin to press against each other, they initially bend, like a spring, to accommodate the opposing forces. Eventually, when the rocks reach a point where they cannot bend further, they break. The bent rocks snap back, or rebound, to their original shape. The break is the fault itself, and the shock waves emanating from the rebound are the earthquake. The shock waves vibrate through the Earth, making it “ring” like a bell.
A fault is a rock fracture along which movement occurs. Normal faults develop where the crust stretches apart, as in the East African Rift Valley. In thrust faults, which are found at subduction zones, the rocks on one side of the fault are pushed up and over those on the other side. A third type of fault is the strike-slip fault, where the rocks on either side of the fault slip by each other horizontally. The San Andreas Fault is a strike-slip fault.
Learn more about earthquakes in the Gottesman Hall of Planet Earth.