Waves

A wave is a disturbance that transmits energy. Waves are very common in nature: light is a wave, sound is a wave, ocean surf is generated by waves, and even matter has wave-like properties. The "disturbance" can be an alternating electromagnetic field strength (light), a variation in water height (ocean waves), a variation in material density (sound waves), or a distortion of the shape of the ground (seismic waves).

If you've felt Earth shake during an earthquake or explosion then you've felt seismic waves. These vibrations travel outward in all directions from their source. Waves generated by large earthquakes can be detected throughout the world and are routinely recorded and analyzed by seismologists.

  • Seismic waves are generated by many different processes:
  • Earthquakes
  • Volcanoes
  • Explosions (especially nuclear bombs)
  • Wind
  • Planes (supersonic)
  • People
  • Vehicles

The range of ground motion amplitudes that are of interest in earthquake studies is very large and seismometers we use are very sensitive. They can detect motions that are much smaller than the thickness of a sheet of paper or as tall as a room. We can detect ground motion in Missouri caused by increased surf activity as a hurricane or large storm system approaches the eastern coast of the lower 48 states.

The IRIS consortium produces animations of ground motions recorded on EarthScope's Transportable Array, which has been deployed in the conterminous U.S. over the last five or so years. The network is relatively uniform making it possible to see the waves travel across it.

The image below is one frame of the animation for the 2008 Wells, Nevada earthquake. Each small red, blue, or white circle represents a seismometer. The color of the cricle indicates whether the ground has displaced upward (red) or downward (blue) relative to its normal position (white). The large circular pattern surrounds the earthquake (small yellow circle) and is cause by waves that have propagated outward from the earthquake. Click on the image to open a window with the full animation.

Wells, NV Earthquake Image

Visit the IRIS ground-motion-animation website to view seismic ground-motion animations from many recent, large earthquakes; click here: http://www.iris.edu/dms/products/usarraygmv/. Robert Woodward of IRIS has developed a great tutorial on the animations that you can access here. And you can read more about the Earthscope project at http://earthscope.org

For more information on waves, please see the list of Seismology Texts or the list of popular-science books on earthquake science.