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Figure 2. Diagrams of near-surface ground motions produced by seismic waves. The P and S waves, (a) and (b) respectively, travel through the earth in all directions from the focus of the earthquake; the first wave to reach an observer during an earthquake is the P wave. Two types of surface waves shown in (c) and (d), travel along the ground surface, somewhat like water waves, and arrive after the S waves. The direction the wave travels is indicated by the arrow below each diagram; the direction of ground movement caused by each wave is indicated by the solid arrows on the diagrams. P and S waves cause the ground to vibrate in mutually perpendicular directions. (Modified from "Earthquakes" by Bruce A. Bolt. Copyright @1978, 1988 W.H. Freeman Company. Reprinted with permission) .

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