Earthquakes

• Earthquakes


  The release of stress that has built up within the earth's crust caused by tension, compression or the shearing of rocks
• Earthquake focus


  The location where the stress is suddenly released
• Epicentre


  The point on the earth's surface immediately above the earthquake focus
• Seismicity


  • The earth shaking, strongly linked to tectonic theory
  • 5% of earthquakes do not happen near plate boundaries
  • Most earthquakes are usually unfelt
• Seismic shockwaves


  1. Primary/pressure (P) waves

  2. Secondary (S) waves

  3. Surface (L) waves

  4. Rayleigh (R) waves
• Primary/pressure (P) waves


  The fastest waves, reach the surface first, very loud, travel through both the mantle and the core, compressional waves that vibrate in the direction in which they travel (longitudinal)
• Secondary (S) waves


  Half the speed of P waves, travel through the mantle but not the core, responsible for the shaking of the ground in an earthquake, vibrate at right angles to the direction in which they travel (transverse), cannot pass through liquid
• Surface (L) waves


  The slowest waves, cause the least damage as they are not very strong, travel through the outer crust only
• Rayleigh (R) waves


  Move in a rolling motion, very low frequency, rotate from the epicentre
• Depth of focus


  • Shallow focus (surface down to about 70km)
  • Deep focus (70-700km deep)
• Shallow focus earthquakes


  • Occur in cold, brittle rock resulting from the fracturing of rock due to stress within the crust
  • Very common, with many releasing only low levels of energy, although other high-energy shallow quakes are capable of causing severe impacts – less distance over which energy can dissipate
• Deep focus earthquakes


  • Poorly understood
  • With increasing depth pressure and temperatures increase to very high levels
  • Minerals change type and volume, which may contribute to a release of energy