Processes and tectonic hazards

• Generation of an earthquake

  1. Tectonic strain is built up and energy is stored in crustal rocks
  2. When the pressure exceeds the strength of the fault, the rock fractures
  3. This produces the sudden release of energy, creating seismic waves that radiate away from the point of fracture
  4. The brittle crust then rebounds on either side of the fracture (that is the ground shaking) and is how the earthquake is felt on the surface
• Hypocentre

  the point within the Earth where an earthquake rupture starts
• Focal length

  the distance between the hypocentre and the epicentre
• Epicentre

  the point on the surface of the Earth, directly above the hypocentre
• Seismometer

  • a device that measures the amount of ground shaking during an earthquake - it records the vertical and horizontal movements of the ground
  • analysing this data shows that an earthquake produces different seismic waves
• Primary waves

  • P wave
  • vibrations caused by compression that spread quickly at 8 km/s
• Secondary waves

  • S waves
  • vibrate at right angle to direction of travel and can't travel through liquids
  • they travel slower at 4 km/sec
• Love waves

  • L waves / Q waves
  • surface waves with horizontal vibration
  • have a high amplitude
• S and L waves are more destructive than P waves because they have more energy and a higher amplitude
• Liquefaction

  • secondary hazards of earthquakes
  • can cause buildings to sink, tilt or collapse
  • land adjacent to rivers and sloping ground could slide under low friction conditions across a liquified soil layer - called lateral spreading and can cause fissure and cracks in the surface
  • liquefaction can damage roads, bridges, telecommunication and services (gas, electricity, sewage)
  • short term impact is the delivery of aid
  • long term impact is rebuilding costs
• Landslides

  • secondary hazards of earthquakes
  • causes slopes to weaken and fail; because many destructive earthquakes occur in mountainous areas it can cause many landslides, rock falls and avalanches
• Soil liquefaction

  • the process by which water saturated material can temporarily lose strength and behaves like a liquid under the pressure of strong shaking
  • liquefaction occurs in saturated soils (spaces between molecules are filled with water)
  • an earthquake can cause water pressure to increase and causes particles to more more easily than normal especially in sand and silt
• Intensity

  • a measure of ground shaking
• Magnitude

  • the amount of movement or displacement in the fault
• Tsunamis

  • have a very long wavelength and isn't a single wave but a series known as a wave train (caused by sea bed displacement)
  • go unnoticed in deep water as they start low in height but rapidly grow in height as water becomes shallower
  • the amount of time between waves (wave period) is often a few minutes but in some rare cases it can be over an hour which is arguably more dangerous as people return home thinking the waves have stopped
• Global distribution of tsunamis

  • is quite predictable in terms of source areas - around 90% of all events occur in the Pacific Basin
  • most are generated at subduction zones (convergent boundary)
• Factors of tsunamis

  • duration of event
  • wave amplitude, column displacement and distance travelled
  • the physical geography of the coast eg water depth
  • degree of coastal ecosystem buffer eg mangroves and coral reefs
  • timing of event - night vs day
  • warning systems
  • degree of coastal development and proximity to coast, especially in tourist areas
• Primary hazards of volcanoes

  • pyroclastic flows
  • ash clouds
  • lava flows
  • volcanic gases
• Pyroclastic flow

  • primary hazard of volcanoes
  • responsible for most primary volcanic deaths
  • hot gases and pyroclastic material like glass shards, pumice, crystals and ash are ejected from the volcano and can be ejected from the side, close to the ground
• Tephra/ ash fall

  • ejected material that can vary in size from 'bombs' that are over 3 cm to fine dust of less than 4 mm
  • the ash and larger materials can cause building roofs to collapse as well as start fires
  • the ash can reduce visibility and affect air travel
• Lava flows

  • pose a large threat to human life if they are fast moving
  • the viscosity (runniness) of lava is determined by the amount of silicon dioxide
  • the worst lava related disaster was in Iceland in 1873 where lava erupted from a fissure for five months and around 22% of the population died in the resulting famine
• Volcanic gases

  • water vapour, sulphur dioxide, hydrogen, carbon monoxide, carbon dioxide
  • carbon dioxide has caused the most deaths as it is colourless, odourless and heavier than air so it accumulated in valleys and was undetected - killed 1700 people in Cameroon in 1986
• Lahars

  • secondary impacts of volcanoes
  • volcanic mudflows generally made of fine sand and silt material
  • the degree of hazard depends of the slope angle, volume of material and particle size
  • are associated with heavy rainfall as old tephra deposits can be mobilised into mud flows
• Jokulhlaups

  • secondary impacts of volcanoes
  • a sudden, rapid, glacial flood with ice and debris
  • a hazard to people and infrastructure and can cause landform modification through erosion and deposition