Seismology

Using earthquakes to probe the Earth

Using seismographs placed around the Earth, we can measure the vibrations due to earthquakes. Earthquakes send waves of various types through the Earth:

P waves (primary or pressure) - travel through Earth
S waves (secondary, or shear) - also travel through Earth
Surface waves - travel along the surface (provide no information on interior)

A seismogram:

How can we use these to probe the Earth?

P waves pass through liquid and solid, but their speed and direction depend on density changes (refraction).
S waves have a different dependance on density, and cannot travel through liquid.

Use these properties to "map out" the interior of the Earth:

For example:

No S waves on the opposite side of the Earth --> Liquid Core!
Size of the shadow zones give size of the core
Travel time differences map out density structure inside Earth
Shadow zones map out layer interfaces