Seismologists, who study earthquakes and the propagation of waves through the Earth, often refer to two primary types of seismic waves: P waves and S waves. P waves, also known as primary waves, are the fastest moving seismic waves. They are called "P" because they are typically the first waves to reach seismic instruments after an earthquake. P waves are compressional waves, meaning they cause particles in the Earth's material to move back and forth in the same direction as the wave is traveling. S waves, on the other hand, are known as secondary waves or shear waves. They travel more slowly than P waves and are the second type of wave to be detected. S waves are characterized by their side-to-side, or up-and-down, motion, which is why they are also referred to as shear waves. This motion is due to the fact that S waves can only travel through solids, unlike P waves which can travel through both solids and liquids. In summary, P waves are called primary waves because they are the first to arrive, and S waves are called secondary waves because they follow after P waves. Both are crucial for seismologists to determine the location and magnitude of an earthquake. read more >>

Compressional waves are also called P-Waves, (P stands for "primary") because they are always the first to arrive. They gave us the first jolt last Friday. Shear waves propagate more slowly through the Earth than compressional waves and arrive second, hence their name S- or secondary waves.read more >>