The idea behind seismology is that the Earth (and other Terrestrial planets) are like bells; when struck they ring. As a consequence, if we disturb the Earth, for example, by dropping an asteroid on it or by say, an earthquake, waves will be generated which propagate through and around the Earth.

There are two kinds of waves which we will consider for this course:

The waves are defined by the direction in which the material is disturbed as compared to the direction in which the wave propagates. For example,

• for the top picture, the S(econdary) waves, the waves moves from left-to-right, while the disturbance moves up and down (perpendicular to the direction of wave propagation). This wave is like the disturbance you would make if you snapped a whip or if you were to shake a rope. These waves are also known as shear waves or transverse waves.

• in the bottom picture, the P(rimary) waves, the wave again moves from left-to-right, while now the disturbance is in the same direction. This wave is like a wave you make with a slinky (a spring). The spring alternately compresses and expands with the coils of the spring moving in the same direction in which the wave propagates. These waves are also known as pressure waves or longitudinal waves.

P waves travel more quickly through the Earth than do S waves. The speed at which they move (and, in fact, whether they can propagate or not at all) is determined by the properties of the material. The P waves can travel through both liquid and solid material moving 5-6 km per second through the Earth while S waves can only propagate through solid material moving at 3-4 km per second through the Earth. S waves will not propagate through liquid material. So, people wait for an earthquake and then monitor the arrival of the seismic waves, the P and S waves. Depending upon when the waves arrive and the type of wave which arrives, one can deduce the interior structure of the Earth.