Low-frequency acoustic energy released within the Earth's crust and mantle mostly propagates - depending on the density and elasticity of the medium - through several types of seismic waves categorized as body waves (longitudinal P and transverse S) or surface waves (long L and ground roll R). The importance of seismic wave research lies not only in our ability to understand and predict earthquakes and tsunamis, it also reveals information on the Earth's composition and features in much the same way as it led to the discovery of Mohorovicic's discontinuity
