How does the study of seismic waves give us clues about the internal structure of the Earth?

Answer 1

There are many types of seismic waves. The two most easily understood are compressional (P-waves) and shear (S-waves).

Compressional waves travel through the earth in a more direct route than do S-waves. This is because the P-waves travel through both solids and liquids (gases, too, but that's not important in this discussion). S-waves do not travel through liquids, and so must take a longer route when liquids are encountered.

The "liquid" difference is very important, for an Earthquake on the other side of the globe produces both P-waves and S-waves. There is, then a shadow zone in which NO S-waves can be detected, and the shadow zone corresponds to the outer, liquid core. The P-waves just go right on through (both are refracted as they get to zones of different densities).

Artificial waves produced to help detect oil and gas reservoirs do so (mainly) by reflection. The wave travels from the surface downward, and each layer of different density reflects some of it back, to detectors (at the surface). The time it took to travel to a particular rock bed is measured, and a bunch of these two-way travel times are plotted as a seismic line, with detector points scattered either along a line or within a grid. Computer programs put all of them together to create a Seismic Line, in which the orientation of rock layers can be determined. This is of the utmost importance to Oil and Gas exploration companies.

Answer 2

Because seismic waves act like any other wave. When they encounter a region where the density of the medium through which they're tracelling changes, part of their energy is reflected and part of it is refracted. By measuring the amount of reflection and refraction from a large number of points scattered over a large enough area, you can then begin to build up a 'picture' of how the subsurface strata are laying.

Doug