Environmental & Earth Science

Seismic Wave

March 22, 2011, 11:04 am

A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the Earth's surface (Rayleigh and Love waves) or through the Earth's interior (P and S waves).

Elastic Waves

A wave is a disturbance that moves with time. Mechanical waves like sound or water waves require a physical medium to move through (e.g., air or water), while other waves like light can travel through a vacuum.

Mechanical waves distort the medium that they travel through - for example, ripples in water. Many media are not permanently deformed by waves traveling through them and return to their prior form after the wave has passed. This property is called elasticity.  Waves that move through media with the property of elasticity are called elastic waves.

Seismic waves are elastic waves because the ground deforms in response to their passing. The ground returns to its former shape once the waves have passed.

There are two primary ways in which an elastic wave can move through the body of a medium (both generically called body waves).

  • Compressional (or longitudinal) waves cause a deformation in the medium in the same direction as the wave is moving. The medium appears to be temporarily compressed and then expands as the wave moves through that part of it.
     
  • Shear (or transverse) waves cause a deformation in the medium in a perpendicular direction (at right angles) to the direction that the wave is moving. Imagine a rope lying on a table which you shake side to side along the surface of the table. The wave moves along the rope directly away from you with the rope displacing perpendicular to the direction of the wave.

Seismic body waves can occur in both of these forms:

  • Compressional body waves are known as P waves.
     
  • Shear body waves are known as S waves.

In addition, elastic waves can move along the surface of a medium. In seismology there are two significant surface waves:

  • Love waves are purely transverse surface waves.
     
  • Rayleigh waves cause an elliptical or "rolling" motion with no net transverse, or perpendicular motion.

From a practical point of view, surface waves are significant because they are the primary cause of the physical destruction found at the Earth's surface when an earthquake occurs.

P Waves

  • The fastest wave, and therefore the first to arrive at a given location.
  • Also known as compressional waves, the P wave alternately compresses and expands material in the same direction it is traveling.
  • Can travel through all layers of the Earth.
  • Generally felt by humans as a bang or thump.

caption Graphic image showing P Wave.

 

S Waves

  • The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling.
  • Also know as shear waves.

caption Graphic image showing S Wave.

 

Surface Waves

  • Surface waves follow the P and S waves.
  • Also known as Rayleigh and Love waves.
  • These waves travel along the surface of the Earth.

caption Graphic image showing Surface Wave.

 

Rayleigh Waves

A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular motion.

caption Graphic image showing Rayleigh Wave.

 

Love Waves

A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

caption Graphic image showing Love Wave.

 

Putting It All Together

When an earthquake occurs, it releases energy in the form of waves that radiate from the earthquake source in all directions. The different types of energy waves shake the ground in different ways and also travel through the Earth at different velocities.

caption Graphic image of a seismogram and the arrivals of P, S and surface waves.

caption Graphic image showing the relative velocities of the various types of seismic waves.

 

Further Reading

The following links provide some more information about seismic waves and earthquakes.

Glossary

Citation

Survey, U. (2011). Seismic Wave. Retrieved from http://www.eoearth.org/view/article/164604

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