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Seismic waves and earthquake mechanism

Short answer

Body waves: compressional P-waves (fastest, through solids/liquids) and shear S-waves (slower, solids only). Surface waves: Love (side-to-side) and Rayleigh (rolling), slowest and most damaging. Earthquakes originate by elastic rebound: stressed rocks rupture, slip, and release stored energy as seismic waves.

Define and describe the types of seismic waves. Write down the mechanism of earthquake origination.

Seismic waves

Seismic waves are elastic waves that carry the energy released by an earthquake through and along the Earth. They are of two main groups.

A. Body waves (travel through the interior)

  • P-waves (primary, compressional): particles vibrate parallel to the direction of travel (push-pull). Fastest (about 6-8 km/s in crust), arrive first, pass through solids, liquids and gases.
  • S-waves (secondary, shear): particles vibrate perpendicular to the direction of travel. Slower (about 3.5-4.5 km/s), arrive second, pass only through solids; they stop at the liquid outer core.

B. Surface waves (travel along the surface)

  • Love waves: horizontal side-to-side shear motion.
  • Rayleigh waves: elliptical rolling motion, like ocean waves.
  • Slowest but largest, and cause most damage.
 P: >>><<<>>><<<  (compression)
 S: ~~~~~~~~~~~~  (shear, up-down)

Mechanism of earthquake origination

Based on the elastic rebound theory:

  1. Plate movement applies stress on rocks along a fault.
  2. Friction prevents slip, so rocks deform elastically and store strain energy.
  3. When stress exceeds rock strength, the rock ruptures suddenly at the focus.
  4. The rock rebounds and the stored energy is released as seismic waves, which reach the surface first at the epicentre.
  5. Smaller aftershocks follow as the crust adjusts.

Causes are mostly tectonic; others are volcanic activity, landslides, and reservoir loading or explosions.

✓ Answer checked by Claude Opus 5.5 (high effort)

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