When you look at a seismogram the wiggles you see are an indication that the ground is being, or was, vibrated by seismic waves. Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions. You can picture this concept by recalling the circular waves that spread over the surface of a pond when a stone is thrown into the water. An earthquake is a more complicated process than a stone splashing into water, and the seismic waves that are set up during an earthquake are more varied than those on the pond.
The are many different seismic waves, but all of basically of four types:
Compressional or P (for primary)
Transverse or S (for secondary)
Love
Rayleigh
An earthquake radiates P and S waves in all directions and the interaction of the P and S waves with Earth's surface and shallow structure produces surface waves.Near an earthquake the shaking is large and dominated by shear-waves and short-period surface waves. These are the waves that do the most damage to our buildings, highways, etc. Even in large earthquakes the intense shaking generally lasts only a few tens of seconds, but it can last for minutes in the greatest earthquakes. At farther distances the amplitude of the seismic waves decreases as the energy released by the earthquake spreads throughout a larger volume of Earth. Also with increasing distance from the earthquake, the waves are separated apart in time and dispersed because P, S, and surface waves travel at different speeds. Seismic waves can be distinguished by a number of properties including the speed the waves travel, the direction that the waves move particles as they pass by, where and where they don't propagate. We'll go through each wave type individually to expound upon the differences. The first two wave types, P and S , are called body waves because they travel or propagate through the body of Earth. The latter two are called surface waves they the travel along Earth's surface and their amplitude decreases with depth into Earth.
