Vibrations from an earthquake are categorised as P or S waves. They travel through the Earth in different waves and at different speeds. They can be detected and analysed.
P and S Waves
A wave is a vibration that transfers energy from one place to another without transferring matter (solid, liquid or gas). Light and sound both travel in this way. Energy released during an earthquake travels in the form of waves around the Earth. Two types of seismic wave exist, P- and S-waves. They are different in the way that they travel through the Earth. P waves (P stands for primary) arrive at the detector first. They are longitudinal waves which mean the vibrations are along the same direction as the direction of travel. Other examples of longitudinal waves include sound waves and waves in a stretched spring. S waves (S stands for secondary) arrive at the detector of a seismometer second. They are transverse waves which mean the vibrations are at right angles to the direction of travel. Other examples of transverse waves include light waves and water waves.
Both types of seismic wave can be detected near the earthquake centre but only P-waves can be detected on the other side of the Earth. This is because P-waves can travel through solids and liquids whereas S-waves can only travel through solids. This means the liquid part of…