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.
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.