Indonesia Tsunami – Causes

After a major 7.5-magnitude earthquake, tsunami hit Palu, a city in the Indonesian island of Sulawesi, recently.

What caused the 2004 tsunami?

 

  • Vertical earthquakes – Catastrophic tsunamis are often triggered by “megathrust earthquakes.”
  • These occur at subduction zones when one tectonic plate is forced under another.
  • It causes massive chunks of the earth’s crust to move vertically.
  • Such movements on the ocean’s floor cause huge volumes of water to be displaced suddenly.
  • They thus throw up giant waves that can travel very fast across great distances.
  • E.g. the December 2004 Indian Ocean tsunami.
  • It had waves up to 100 ft high which was triggered by a megathrust earthquake of 9.1-magnitude in Sumatra.

Indonesia Tsumani

What is the present Indonesia case?

 

  • ‘Horizontal’ earthquakes – The recent 7.5-magnitude quake in Indonesia was triggered by what is called a ‘strike-slip fault’.
  • In this type of quake, the earth’s movement is largely horizontal which would not normally trigger a tsunami.
  • However, it is possible for a strike-slip fault to also have some amount of vertical motion that could displace water.
  • Or the fault’s rupture zone may pass through an area where the seafloor rises or drops off.
  • In such cases, when the fault moves during the quake, it pushes seawater in front of it.
  • Notably, in Indonesia’s case, the fault’s rupture zone was estimated to be about 70 miles long, suggesting a large possibility for the above.
  • Landslide – Another possibility is that there could have been a mudslide on the ocean floor.
  • This could have displaced a lot of water and created waves, causing a cataclysmic effect on the bay.
  • Topography – The tsunami could have been impacted by Palu’s location at the end of a narrow bay.
  • The coastline and the shape of the bottom of the bay could have focused the wave energy and guided it up the bay.
  • This could have increased the wave height as it approached the shore.