Earth's Average Seismic Structure

The main elements in Earth's interior, the core, the mantle, and the crust were outlined in the Earth's Composition page. By studying the propagation characteristics (travel times, reflection amplitudes, dispersion characteristics, etc.) of seismic waves for the last 90 years we have learned much about the detailed nature of Earth's interior. Great progress was made quickly because for the most part Earth's interior is relatively simple, divided into a sphere (the inner core) surrounded by roughly uniform shells of iron and rock. Models that assume the Earth is perfectly symmetric can be used to predict travel times of P-waves that are accurate to a few seconds for a trip all the way across the planet.

The diagram below is a plot of the P- and S-wave velocities and the density as a function of depth into Earth. The top of the Earth is located at 0 km depth, the center of the planet is at 6371 km.

Velocity and density variations within Earth based on seismic observations. The main regions of Earth and important boundaries are labeled. This model was developed in the early 1980's and is called PREM for Preliminary Earth Reference Model.

Several important characteristics of Earth's structure are illustrated in the chart. First note that in several large regions such as in the lower mantle, the outer core, and inner core, the velocity smoothly increases with depth. The increase is a result of the effects of pressure on the seismic wave speed. Although temperature also increases with depth, the pressure increase resulting from the weight of the rocks above has a greater impact and the speed increases smoothly in these regions of uniform composition.

The shallow part of the mantle is different; it contains several important well-established and relatively abrupt velocity changes. In fact, we often divide the mantle into two regions, upper and lower, based on the level of velocity heterogeneity. The region from near 400 to 1000 km depth is called the transition zone and strongly affects body waves that "turn" at this depth and arrive about 20°-30° distant from a shallow earthquake. In this depth range the minerals that make up the mantle silicate rocks are transformed by the increasing pressure. The atoms in these rocks rearrange themselves into compact structures that are stable at the high pressures and the result of the rearrangement is an increase in density and elastic moduli, producing an overall increase in wave speed.

Graphite in "lead" pencils and diamonds are a more common example of atoms rearranging themselves under different conditions - they are both composed of carbon. The different arrangement and bonding of the carbon atoms in the two materials produces dramatically different properties. Diamonds are formed under enormous pressures; all natural diamonds formed at depths of about 150-200 km, and were brought to the surface by volcanic activity. At the high pressures the carbon atoms are squeezed into a tight arrangement that makes them one of the hardest materials. In contrast, the low-pressure arrangement of carbon in graphite creates the slippery, soft character of "lead" that we use for pencils.

The two largest contrasts in material properties in the Earth system are located near the surface and the core-mantle boundary. Both are compositional boundaries and the core-mantle boundary is the larger contrast. Other sharp contrasts are observable, the inner-core outer-core boundary is relatively sharp, and velocities increase from the liquid to the solid.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.