When a seismic wave comes into being a boundary that isolates rocks of different elastic properties, it experiences reflection and refraction. There is a unique kind of complexity because conversion between the wave types generally also takes place at such a boundary: an incident P or S wave can generate reflected P and S waves and refracted P and S waves. Boundaries between structural layers also trigger scattered and diffracted waves. These additional waves are in part held accountable for the complexities noticed in ground motion during earthquake episodes.The range of frequency of seismic waves is quite large, from as high as the audible range (higher than 20 hertz) to as low as the frequencies of the free fluctuations of the whole Earth, with the gravest period being 54 minutes. Weakness or depletion of the waves in rock thrust upon high-frequency restricts, and in small to moderate earthquakes the dominant frequencies stretch in surface waves from about 1 to 0.1 hertz.