Seismic energy is energy in the form of elastic waves that are propagated through the earth's interior or along its surface, or have the form of standing waves (free oscillations) in the planet as a whole. The source of seismic waves may be either natural—an earthquake—or artificial, for example, a chemical or nuclear explosion, or a weight dropped on the earth's surface. The energy source, whatever it is, imposes a strain—a change in shape and volume—on the surrounding earth material, which resists the strain because of its elastic properties. As the strained material tries to recover its original shape and volume, oscillations are produced, which are transferred to material slightly farther away from the source. In this way, seismic waves are produced that spread out from the source in all directions. In detail, the mechanics of seismic sources...