Hazards Posed by Earthquakes

• Ground shaking and displacement


  The vertical and horizontal movement of the ground during an earthquake
• Severity of ground shaking and displacement

  • Dependent on the magnitude of the earthquake
  • Dependent on distance from the epicentre
  • Dependent on local geology
• Locations close to the epicentre of a high magnitude earthquake with unconsolidated surface layers and high water content will experience more shaking
• Example of severe ground shaking and displacement

  • In 1985 Mexico City where these conditions exist was hit by a large earthquake killing around 10,000 people
• Buildings can better withstand the vertical movements associated with earthquakes better than the horizontal ones
• It is the swinging of structures that is so dangerous to stability
• Ground movement can cause

  1. Displacement of rocks along fault lines

  2. Destruction of pipelines, railway tracks, buildings and roads
• Displacement of the surface

  Can disrupt the natural drainage, diverting streams and rivers and affecting the movement of groundwater
• Disruption to natural drainage can lead to serious implications for public water supplies, irrigation and agriculture
• Liquefaction:

  ·         Liquefaction is when the ground loses its strength and behaves like a liquid.

  ·         It occurs when an earthquake strikes an area with surface materials of fine grained sands; alluvium and where the geology has a high water content.

  ·         Liquefaction can cause buildings to collapse and sink as their foundations fail.

  ·         Liquefaction was a major issue in the 1995 Kobe, Japan earthquake and the Sulawesi earthquake (Indonesia) in 2018.
• Earthquakes in mountainous regions

  Often cause landslides and avalanches
• Causes of landslides and avalanches

  • Ground shaking
  • Liquefaction
• Deforestation and heavy monsoon rains

  Increase vulnerability to landslides even from small quakes
• Landslides and avalanches caused by the 2015 Nepalese earthquake


  • Widespread landslides in Kathmandu and Langtang valleys
  • Avalanches on Mt Everest killed at least 18 climbers and injured more than 60
• Impacts of landslides


  • Block transport routes in mountainous regions where accessibility is already limited
  • Rivers can be blocked, creating a temporary dam (quake lake)
  • When quake lakes fail it can lead to widespread flooding downstream
• Earthquake causes a landslide in an area with a reservoir


  Can lead to the displacement of water and the waves generated could weaken the dam and overtop it, causing flooding
• ·       Landslides and Avalanches -   A 100m wave was generated on the Vaiont Reservoir when a hillside collapsed in Italy in 1963. Almost 3000 people drowned when the wave swept over the dam and down into the Piave river valley.

   

  Avalanche – A mass of snow, rock, ice and soil that tumbles down a mountain.

  Landslide – The movement of rock, debris and earth down a slope.

   
• Tsunamis caused by underwater earthquakes

  1. Seabed rises vertically

  2. Displaces water above

  3. Produces powerful waves spreading out at high velocity from epicentre
• Tsunami waves


  • Low height (≤1m)
  • Very long wavelength (up to 200km)
  • Can pass underneath a ship out at sea undetected
• Tsunami waves approach the shore and enter shallow water


  Wave height increases greatly due to increased friction
• Before the tsunami wave breaks

  1. Water in front of the wave is pulled back out to sea (drawdown)

  2. Clear indicator of an approaching tsunami
• Tsunami


  • Rushes in as a wall of water that can exceed 25m in height
• The tsunami generated by the earthquake in Sumatra in December 2004 caused a wave up to 30m high that delivered 100 tonnes of water per metre of shoreline
• The local height of the tsunamis is affected by the shape of the seabed and coastline
• Depending on the relief of the coastline tsunamis can travel varying distances inland
• Tsunamis caused by underwater landslides

  1. Large volume of rock shaken and slides downslope

  2. Water is dragged behind it from all sides and collides in the centre

  3. Can generate a tsunami wave which rotates outward
• The resulting waves from an underwater landslide may not have the power to cross oceans but the local affects can be devastating
• In 2018 an earthquake in Sulawesi is believed to have caused an underwater landslide that caused a tsunami as high as 6m
• Because such events are local, warning times are short, making them particularly hazardous
• Mapping the seabed and identifying at risk places could reduce the risk (California)