Seismicity

• How are earthquakes caused?

  caused by tension that builds up at plate margins. When the plates jerk past each other they send out shockwaves.
• What is the focus?

  where the shockwaves spread out from. Doesn’t have to be a single point, can be along a fault line.
• What is the epicentre?

  The point on the Earth's surface directly above the focus of an earthquake. it is where the earthquake is felt first.
• Describe the Richter scale?

  measures magnitude of earthquake. Logarithmic - means that an earthquake with a magnitude of 5 has an amplitude 10x greater than one with 4. No upper limit.
• Describe the moment magnitude scale?

  based on total amount of energy released by an earthquake. Logarithmic and no upper limit. More accurate than Richter scale.
• Describe the mercalli scale?

  measures impacts of an earthquake using observations of the event e.g reports and photos. Scale is between 1-12.
• Describe p waves?

  fastest, reach surface first. High frequency. Travel through mantle and core to opposite sides of earth.
• Describe s waves?

  half as fast as p waves. Reach surface next. High frequency but shake like skipping rope. Can travel through mantle but not core.
• Describe love waves?

  slowest but cause most damage. move from side to side. Originates from epicentre.
• Describe rayleigh waves?

  radiate from epicentre in complicated rolling motions. Move up and down.
• What are the secondary hazards of earthquakes?

  tsunamis, landslides/avalanches, soil liquefaction
• Describe tsunamis

  • displacement of water when underwater earthquakes cause seabed to move
  • the closer to land, the more powerful - waves lose energy as they travel towards land
  • travel fast in deep water
• Give an example of a tsunami?

  tohoko tsunami - waves up to 40m high - 450000 homeless
• Describe an avalanche/landslide

  • Caused by shaking of ground dislodges rock, soil or snow
  • shaking can loosen ground material - making it easier for water to infiltrate
  • weight of water may trigger a landslide after shaking has stopped
• what is an example of a landslide?

  Nepal earthquake 2015 - triggered 21000 landslides covering an area of 25000km2
• Describe soil liquefaction

  • soil is saturated - vibrations cause it to act like a liquid
  • this makes soil weaker and easier to deform so it is more likely to subside, especially if there is a heavy weight on top like a building
  • the soil must be loose, sandy and silty and requires a degree of saturation to liquefy
• What is an example of soil liquefaction?

  Christchurch, New Zealand - 80% of water and sewage system damaged from soil liquefaction
• What is the nature and magnitude of an earthquake affected by?

  margin type, depth of focus, rate of movement
• How does a constructive plate margin affect the nature and magnitude of an earthquake ?

  • shallow depth of focus
  • less powerful
  • hazard risk: occur along mid ocean ridges away from land or large populations
  • Frequency: Iceland experiences on average 87 mag 4 or above earthquakes
• How does a destructive plate boundary affect the nature and magnitude of an earthquake ?

  • Deep focus
  • very powerful
  • hazard risk: may take place by sea next to heavily populated coastal zones - tsunamis e.g Japan 2011
• How does a conservative plate boundary affect the nature and magnitude of an earthquake ?

  • shallow focus
  • infrequent
  • powerful and can be severe
• How does depth of focus affect the nature and magnitude of an earthquake?

  deep focus earthquakes tend to be higher magnitude than shallow focus earthquakes. But deep focus tend to do less damage as they have to travel further which reduces their power.
• How does rate of movement affect the nature and magnitude of earthquakes?

  tectonic plates move at different rates, between 1-15 cm each year. for example pacific plates move at a rate of 10cm per year and Antarctic 1cm a year. There’s no clear relationship between rate of movement and earthquake magnitude.
• Describe the magnitude and frequency of earthquakes globally?

  • lots of low mag earthquakes daily
  • high mag less often
• Describe the randomness of earthquakes?

  no pattern or trend
• How predictable are seismic hazards?

  • can predict areas at risk by monitoring tectonic plate movement
  • impossible to tell when an earthquake will occur and what mag
• How does geology influence the hazard risk of an earthquake?


  unconsolidated rocks such as clay and sand can amplify shockwaves and cause the collapse of buildings, liquefaction
• How does time of day influence hazard risk of earthquakes ?


  More deaths can occur if people are asleep/inside, especially when buildings are poorly built
• What is an example of the time of day influencing hazard risk?

  2023 turkey and Syria - occurred at night
• How does location of epicentre influence hazard risk of earthquakes?

  if near urban areas then more people will be affected
• How does depth of focus influence the hazard risk of an earthquake?

  shallow focused earthquakes cause more shaking
• How does population density influence hazard risk of an earthquake?

  more likely to be more deaths and damage when earthquakes strike densely populated areas
• How does the level of development influence hazard risk of an earthquake?


  rich countries can afford to mitigate the impact of earthquakes through prediction, protection and preparation
• How does prediction influence hazard risk of an earthquake?


  forecasting when and where an earthquake might strike can provide planning for people
• How does building standards influence the hazard Rick of earthquakes?

  building control and aseismic building designs can prevent building collapse and save lives