6.3: Earthquake Waves

Earthquake Waves

As an earthquake occurs, two different types of waves are produced: 1) body waves surface waves Links to an external site., so termed because they travel through the body of the Earth, and 2) that travel along the Earth’s surface. There are two types of body waves: P-waves and S-waves. P-waves , or primary waves Links to an external site., are compressional waves that move back and forth, like the action of an accordion. As the wave passes, the atoms in the material it is travelling through are compressed and stretched (Figure 6.6). Movement is compressional parallel to the direction of wave propagation, which makes P-waves the fastest of the seismic waves. These waves can travel through solids, liquids, and gases, because all materials can be compressed to some degree.

S-waves , or secondary waves Links to an external site., are shear waves that move material in a direction perpendicular to the direction of travel (Figure 6.7). S-waves can only travel through solids and are slightly slower than P-waves. Think of a wave movement like the wave created by fans in a stadium that stand up and sit down. Body waves are often responsible for the jerking and shaking motions felt during an earthquake.

Surface waves, which are only propagated along the surface of the Earth, are slower than body waves. Additionally, their movement is often not handled by buildings well, so these waves are responsible for considerable damage to structures. These waves move slowly, which often results in an increase in amplitude. Love waves are the faster surface waves, and they move material back and forth in a horizontal plane that is perpendicular to the direction of wave travel (Figure 6.8). Rayleigh waves make the Earth’s surface move in an elliptical, rolling motion, like the movement in an ocean wave (Figure 6.8). Combined, this results in ground movement that is both up and down and side-to-side.

Table 6.1: Summary table of the different seismic waves and their arrival order to a distance seismic station (CC-BY 4.0, Chloe Branciforte, own work).

Category	Type	Velocity (miles per second)	Arrival placement at distant seismogram
Body	Primary (P)	5 mi/s	First
Body	Secondary (S)	3 mi/s	Second
Surface	Love (L)	2.8 mi/s	Third
Surface	Rayleigh (R)	2.2 mi/s	Fourth

 

Want to view seismic waves in action? Visit the Incorporated Research Institutions for Seismology (IRIS) for videos showing wave motion Links to an external site..