<P> The propagation velocity of the waves depends on density and elasticity of the medium . Velocity tends to increase with depth and ranges from approximately 2 to 8 km / s in the Earth's crust, up to 13 km / s in the deep mantle . </P> <P> Earthquakes create distinct types of waves with different velocities; when reaching seismic observatories, their different travel times help scientists to locate the source of the hypocenter . In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth's interior, and man - made vibrations are often generated to investigate shallow, subsurface structures . </P> <P> Among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, and surface waves, which travel at the Earth's surface . </P> <P> Other modes of wave propagation exist than those described in this article; though of comparatively minor importance for earth - borne waves, they are important in the case of asteroseismology . </P>

A seismic wave that travels along the earth surface