Indonesia Tsunami - Causes

Why in news?

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.

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.

Source: Indian Express