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What hazards are caused by earthquakes?

Earthquakes become hazardous when they interact with people. There are typically around 100 earthquakes with the potential to impact human societies each year.

Ground shaking and displacement

Ground shaking and displacement is the horizontal and vertical movement of the ground. The severity of ground movement depends on a number of factors. These include:

  • the magnitude of the earthquake
  • geology
  • distance from the epicentre (the point on the Earth’s surface above the focus)

Settlements that are close to the epicentre of a high magnitude earthquake and the geology is unconsolidated with a high water content will experience greater ground shaking. In 1985 Mexico City, where these conditions exist, was hit by a devastating earthquake.

Buildings can better withstand the verticle movements associated with earthquakes better than the horizontal ones. Their stability is affected by the swaying. Ground movements cause the displacement of rocks along fault lines. This can lead to the destruction of pipelines, railway tracks, buildings and roads.

Displacement of the surface can also affect natural features such as diverting streams and rivers and affecting groundwater in aquifers leading to serious implications for public water supplies.

Liquefaction

Where earthquakes occur where the geology has a high water content and consists of fine-grained sands, alluvium or is reclaimed from the sea the seismic waves cause the materials to behave like liquids. This leads to the surface losing its strength and buildings and roads collapse and sink as their foundations fail. Liquefaction was a major problem caused by the Sulawesi earthquake in Indonesia in 2018.

The video below shows the impact of liquifaction caused by the Sulawesi earthquake.

Tsunamis

Earthquakes occurring underwater can cause the seabed to rise, leading to the displacement of water, producing powerful waves which spread out from the epicentre. Where the ocean is deep these waves, which are low in height (<1m) and have a very wide wavelength (up to 200km) they can go undetected. However, wave height increases rapidly when it approaches shallow water along the coast. Water in front of the wave is pulled back out to sea in a process known as drawdown. This is a clear indicator of an approaching tsunami. Finally, the tsunami approaches the coast as a wall of water. The recent Sulawesi earthquake in Indonesia led to a tsunami almost as high as 6m. In this case, it is believed the tsunami was formed by an underwater landslide caused by the earthquake. The 2004 Boxing Day earthquake, caused by an upwards thrust of a tectonic plate led to a tsunami, up to 30m high, that delivered 1000 tonnes of water per metre of shoreline.

 

Landslides and avalanches

Earthquakes in mountainous regions often cause landslides and avalanches. Steep, unstable slopes are notoriously unstable and vulnerable to landslides. Their vulnerability is increased by deforestation and heavy monsoon rains so that even small earthquakes can cause landslides.  The 2015 Nepalese earthquake caused widespread landslides. Avalanches on Mount Everest caused by the tremor killed at least 18 climbers and injured more than 60.

It is common for transport routes in mountainous regions are blocked by landslides. Rivers can also be blocked by landslides, creating a temporary dam. When these fail it can lead to widespread flooding downstream. This occurred in several locations in Kashmir in 2005 and Sichuan in 2008.

Reservoirs are often constructed in upland areas. If an earthquake causes a landslide in such an area it can lead to the displacement of water and the waves generated could weaken the dam and overtop it, causing flooding. A 100m wave was generated on the Vaiont Reservoir when a hillside collapsed in Italy 1963. Almost 3000 people we drowned then the wave swept over the dam and down the Piave river valley.

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