There are four primary types of seismic waves: compressional or primary waves, known as P waves; shear or secondary waves, known as S waves; Love waves; and Rayleigh waves. P waves and S waves propagate outward in all directions, including deep into the planet's interior, much like a rock dropped in a pond. Hence, they are called body waves, since they travel through the body of the Earth. Love waves and Rayleigh waves are considered surface waves because they primarily travel along the planet's surface. When it comes to earthquakes, body waves tend to exert a bigger impact on human societies than the surface waves. S waves typically cause most of the violent shaking and subsequent damage near the epicenter of a quake. However, P waves change the volume of the material through which they travel, alternating between compression and expansion. They are technically a type of sound wave.
The four kinds of waves travel at different speeds. A wave's travel time equals the distance it traveled divided by its average speed. The further a location from an earthquake's epicenter, the greater the difference in the arrival times of the various types of waves. It follows that the fastest waves show up on a seismograph first, alerting people to the fact than an earthquake has occurred. Additionally, the further an earthquake monitoring system from a quake, the greater the depth at which it can detect the quake.
Velocities of seismic waves depend on a number of factors. One major factor is density. Waves travel faster through denser materials. Since the Earth's composition becomes denser with depth, waves travel faster as they penetrate deeper. Another factor is the composition of the Earth's material itself. For instance, seismic waves move more quickly through solid material than water. A third contributor to wave velocity is temperature -- hot temperatures, such as those found in molten areas, slow the movement of seismic waves. Other factors affecting wave velocity include the frequency of waves emanating from the epicenter as well as the quantity and shape of cracks and fissures in the ground.
All seismic waves travel very quickly, so much so that scientists measure their velocities in kilometers per second. Overall, P waves travel faster than the other wave types. In fact, their formal name, primary waves, derives from a synonym for the word for first. P waves travel at velocities of about 5 to 7 kilometers per second in the Earth's crust. S waves are the second-fastest wave type, moving at velocities of about 3 to 4 kilometers per in the Earth's crust. Love waves usually arrive next, with velocities of 2 to 4.4 kilometers per second, and Rayleigh waves typically arrive last, traveling just fractions of a second slower than Love waves.