Because seismic waves from earthquakes (or surface explosions) can pass through the entire earth, the behavior of these waves permits deductions about the rocks through which they have passed.
Seismic refraction. The direction of travel of a seismic wave, like that of a beam of light, can be bent, or refracted, when it passes into or out of different mediums. This seismic refraction occurs only if the mediums have different densities or strengths, which change the velocity of the seismic wave (Figure 1).
A distinct rock boundary is not necessary to bend a seismic wave. Seismic waves begin to develop a slightly curved path as they move away from the source, a result of many small refractions as the waves pass through the different rock types of the crust.
Seismic reflection. Seismic reflection is the return of some of the energy from seismic waves that have penetrated downward from the surface or near‐surface, hit a rock boundary, and bounced back to the surface (Figure 2). Since the time of departure and return are known from the seismogram, the depth to the rock boundary can be calculated.