The Waves Below Your Feet

Using Waves to Understand Earth's Interior

Do you ever wonder what's below your feet? Not just a few yards or meters below, but way down deep - many miles or kilometers below the surface?

Have you wondered how scientists know the basic structure of Earth's interior or how the interior structure relates to plate tectonic boundaries?

One way that scientists study Earth's interior is to study the seismic waves produced by earthquakes. By measuring the speed of seismic waves as they travel through different kinds of materials, scientists infer or draw conclusions about the basic structure and material of Earth's interior. An earthquake produces both primary (P) waves and secondary or shear (S) waves. One important difference between P and S waves is that shear waves cannot travel through liquids (for example, Earth's outer core) like primary waves can. By taking and analyzing thousands of measurements of the time it takes for S waves to travel from an earthquake to a receiver, scientists have produced three-dimensional models of Earth's crust and mantle.

Once they have measured the speed at which S waves move through a material, scientists can infer the density and temperature of the material. For example, in the crust, shear waves travel faster when moving through basaltic (oceanic) material than granitic (continental) material because basalt is denser than granite. In the mantle, shear waves travel faster through regions that are colder and therefore denser, and slower in warmer and therefore less dense material. Comparing the speed (velocity) of S waves at two different depths below the surface will allow you to infer information about how deep Earth's continental crust is, as well as the depth of the transition to the mantle below ocean basins and beneath continents.

In addition to providing clues about the depth and temperature of the crust and mantle, the speed of shear waves can help us examine the structure of plate boundaries. Analyses of S wave velocities on either side of a plate boundary can help identify places where tectonic plates are moving towards (converging) or away from (diverging) each other.

Consider the following questions as you go through this chapter:

• Do changes in shear wave velocities near plate boundaries support or refute the theory of plate tectonics?
• How fast do shear waves move at progressively deeper levels of Earth?
• How can shear wave velocities be used to estimate the depth of Earth's crust?