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As shown in Figure 8--3, earthquakes tend to be concentrated in particular zones on the earth's surface that coincide with the boundaries of the tectonic plates into which the earth's crust is divided (Figure 8--4). As the plates move relative to each other along the plate boundaries, they tend not to slide smoothly but to become interlocked. This interlocking causes deformations to occur in the rocks on either side of the plate boundaries, with the result that stresses build up. As the rocks deform on either side of the plate boundary, they store energy--and the amounts of such energy that can be stored in the large volumes of rock involved can be truly massive. When the fault ruptures, the energy stored in the rocks is released in a few seconds--partly as heat and partly as shock waves. These waves constitute the earthquake (21). The resultant vibrational energy is then transmitted through the earth's surface, and when it reaches the surface, it may cause damage and collapse of structures, which in turn may kill and injure the occupants of these structures. These large and inexorable forces are responsible for the band of seismic activity that extends along the Pacific rim to South America and to Japan (Figure 8--3).