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Facts about Seismic Waves for Kids

Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer.

  • Velocity tends to increase with depth, and ranges from approximately 2 to 8 km/s in the Earth’s crust up to 13 km/s in the deep mantle.
  • Earthquakes create various types of waves with different velocities; when reaching seismic observatories, their different travel time help scientists to locate the source of the earthquake hypocenter.
  • In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth’s interior, and man made vibrations are often generated to investigate shallow, subsurface structures.
  • Other modes of wave propagation exist than those described in this article, but they are of comparatively minor importance for earth-borne waves, although they are important in the case of asteroseismology.
  • Body waves travel through the interior of the Earth.
  • They create raypaths refracted by the varying density and modulus (stiffness) of the Earth’s interior.
  • Primary waves Primary waves (P-waves) are compressional waves that are longitudinal in nature.
  • These waves arrive at seismograph stations after the faster moving P waves during an earthquake and displace the ground perpendicular to the direction of propagation.
  • S waves are slower than P waves, and speeds are typically around 60% of that of P waves in any given material.
  • Rayleigh waves Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water.
  • Love waves Love waves are horizontally polarized shear waves (SH waves), existing only in the presence of a semi-infinite medium overlain by an upper layer of finite thickness.
  • The equation for Stoneley waves was first given by Dr. Robert Stoneley (1894 – 1976), Emeritus Professor of Seismology, Cambridge.
  • An earthquake occurs, seismographs near the epicenter are able to record both P and S waves, but those at a greater distance no longer detect the high frequencies of the first S wave.
  • Since shear waves cannot pass through liquids, this phenomenon was original evidence for the now well-established observation that the Earth has a liquid outer core, as demonstrated by Richard Dixon Oldham.
  • The P-wave then travels through the outer core, the inner core, the outer core, and the mantle.
  • In the case of earthquakes that have occurred at global distances, four or more geographically diverse observing stations recording P-wave arrivals permits the computation of a unique time and location on the planet for the event.
  • Residuals of 0.5 second or less are typical for distant events, residuals of 0.1-0.2s typical for local events, meaning most reported P arrivals fit the computed hypocenter that well.