Seismic Waves and Structure of the Earth s Interior Seismic waves provide a unique means of investigating the nature of the earth s interior below the depths from which samples can be obtained. They provide information the density and physical state of earth materials at depth. Body-wave velocities are proportional to the density of the material through which they are traveling. Velocity increases with density .
Seismic waves travel fastest through dense , cold , solid materials.
The deflection or change in direction of seismic body waves as they move across a boundary between two materials of different properties. Refraction of seismic waves indicates that the earth is layered, not homogeneous. The boundary between crust and mantle , defined using seismic velocities. Refraction and higher seismic wave velocities in the upper mantle make it possible to determine the thickness of the crust. The lithosphere-asthenospere boundary represents a change in physical state .