SEISMOLOGY

The idea behind seismology is that the Earth (and the other Terrestrial planets) are like bells; when struck they will 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.

We consider two kinds of waves for this course, shear and pressure waves

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 is you snapped a whip or if you were to shake a rope. These waves are also known as Shear 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.

• Directly opposite an earthquake it is clear that S-waves will not be observed because they cannot propagate through the Earth's liquid core. This forbidden region is actually quite large and occurs for propagation angles = 105^o to 180^o, the so-called Shadow Zone.

• P waves also show a Shadow Zone, but it is more complicated. P waves bend (refract) as they move through the Earth. The bending produces a Shadow Zone which runs from angles 104^o to 140^o. P waves can be observed directly opposite to the earthquake, but they disappear over the Shadow Zone.

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.