Seismic waves are waves of energy caused by a sudden disturbance beneath the earth, such as an earthquake. A seismograph measures seismic waves to determine the level of intensity of these disturbances. There are several different types of seismic waves, such as the P, or primary wave, and the S, or secondary wave, and they are important differences between them.
P waves travel faster than S waves, and are the first waves recorded by a seismograph in the event of a disturbance. P waves travel at speeds between 1 and 14 km per second, while S waves travel significantly slower, between 1 and 8 km per second. The S waves are the second wave to reach a seismic station measuring a disturbance.
Type of Wave
Primary waves are made up of compression waves, also known as push-pull waves. The individual waves, therefore, push against one another, causing a constant parallel, straight motion. S waves are transverse waves, which means they vibrate up and down, perpendicular to the motion of the wave as they travel. In an S wave, particles travel up and down and the wave moves forward, like the image of a sine wave.
P waves can travel through any kind of material, whether it is a solid, liquid or gas because of their wave movement. S waves, on the other hand, can only move through solids and are stopped by liquids and gases. For this reason, they are sometimes referred to as shear waves because they are unable to alter the volume of the material that they pass through. This also accounts why less S waves are recorded than P waves.
S waves are generally larger than P waves and are often responsible for the damage that an earthquake may cause. Since the particles move up and down in a S wave, they move the earth around them with greater force, and are able to actually shake the surface of the earth. P waves, though easier to record, are significantly smaller and do not cause as much damage because they compress particles in only one direction.