Nature & Distribution of Seismicity

• The Richter scale is used to measure the magnitude of an earthquake based on its intensity.
• Primary seismic waves travel faster than secondary seismic waves, so they arrive first at any given point on Earth's surface.
• What is Seismicity?

  The worldwide or local distribution of earthquakes in space, time and magnitude.
• What is the Epicentre?

  The point on the surface directly above the focus. This is where the earthquake is felt and experienced the most as the quakes are the most intense.
• What is the Focus?

  The focus is the point at which the earthquake starts below the Earth's surface: the energy released by the earthquake travels out from the focus.
• Majority of earthquakes occur along plate boundaries (95%)
• Earthquakes can occur on any type of plate margin - constructive, destructive, collision and conservative
• Earthquakes that occur away from faults / plate margins are instead associated with the reactivation of old fault lines.
• Give an example of a earthquake that occured at an old fault line?

  In 2002, West Birmingham a 4.8 earthquake occurred which was thought to be caused by movements on a old fault line (the Malvern lineament)
• Human causes of earthquakes include:

  1. Fracking
  2. Mining
  3. Reservoir Construction
• The most powerful earthquakes are associated with destructive margins. (convergent)
• At conservative margins the boundary is marked by a fault, movement along this fault produces the earthquake (locked together causing stress and pressure to build) . The most famous of these is the San Andreas Fault in California which is between the North American and Pacific plate.
• An earthquake is the sudden, violent shaking of the ground


  • When tectonic plates move, they can become locked together causing stress and pressure to build
  • Eventually, the stress becomes so great that the rocks fracture and the pressure is suddenly released
  • This causes intense ground shaking for seconds to minutes
• • Most 0-70km focus depths are found at plate boundaries. When the focus is deeper the earthquakes are found slightly further away from the plate boundary but still in very close proximity. The depth is dependable on the type of tectonic plate.
• Depending on the depth of the focus the effects on the surface vary:

  • Shallow focus (0-70 km deep): These tend to cause the greatest damage and account for 75% of all earthquakes energy released
  • Intermediate focus (70-300km deep)
  • Deep focus (300-700km deep)
• Label which is which:

  A) Epicenter

  B) Focus

  
• Name the 4 types of shock waves:

  1. P (primary) Waves
  2. S (secondary) Waves
  3. L (love) Waves
  4. R (Rayleigh) Waves
• What type of shock waves are the P (primary) and S (secondary) waves?

  Body waves
• What type of shock wave is the fastest out of the 4?

  P (primary) waves
• Which wave reaches the surface first?

  The P (primary) wave
• Which type of wave is the most damaging?

  L (love) waves
• Label the different shock waves:

  A) Primary

  B) Love

  C) Secondary

  D) Rayleigh

  
• All the characteristics of P (primary) waves:

  • Body wave
  • Fastest wave (same speed as the speed of light)
  • High frequency (cause backwards and forwards shaking)
  • Travel through mantle + core to the opposite side of the earth
  • Least damaging wave
• All the characteristics of S (secondary) waves:

  • Body wave
  • Half as fast P waves (reach the surface after P waves)
  • High frequency (shakes like a skipping rope)
  • Can travel through the mantle but not the core
  • More damaging then P waves
• Name all the characteristics of L (love) waves:

  • Surface wave
  • Slowest wave
  • Cause a side to side motion
  • Originate from the epicenter (not the focus)
  • Most damaging
• List the characteristics of R (Rayleigh) waves:

  Surface wave

  Complicated low frequency rolling motions (waves go up and down)

  Redactive from the epicenter
• The nature of an earthquake and its magnitude is affected by 3 main factors. Which are...?

  1. The Margin Type
  2. Rate of Movement
  3. The Depth of the Focus