Geography Prelims cum Mains

The destructive power of the Indonesia tsunami

The News

  • The death toll in Indonesia rose to 844, after a powerful earthquake of 7.5 magnitude and tsunami waves struck the island of Sulawesi, that destroyed the Indonesian city of Palu.
  • Waves rose 20 feet, which has surprised some scientists who said they wouldn’t normally associate such high levels of destruction with the kind of earthquake that preceded the tsunami.

 

What are the different possibilities for such a huge wave Tsunami ?

  • ‘Vertical’ earthquakes
    • Catastrophic tsunamis are often triggered by ‘megathrust earthquakes’, which occur at subduction zones when one tectonic plate is forced under another, causing 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, and throw up giant waves that can travel very fast across great distances.
    • The December 2004 Indian Ocean tsunami, which had waves up to 100 ft high and killed nearly a quarter million people, was triggered by a megathrust earthquake of 9.1-magnitude in Sumatra. 
  • Megathrust earthquakes
    • Megathrust earthquakes occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another.
    • These interplate earthquakes are the planet’s most powerful, with moment magnitudes (Mw) that can exceed 9.0.
    • Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
    • No other type of known terrestrial source of tectonic activity has produced earthquakes of this scale.

 

  • ‘Horizontal’ earthquakes
    • Friday’s 7.5-magnitude quake was triggered by what is called a ‘strike-slip fault’, in which the earth’s movement is largely horizontal.
    • This 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, which in this case was estimated to be about 70 miles long, may pass through an area where the seafloor rises or drops off, so that when the fault moves during the quake, it pushes seawater in front of it.

 

  •  Strike-slip
    • Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally.
    • If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.

  • Landslide
    • There is yet another possibility — that the quake set off a landslide on the ocean floor that displaced a lot of water and created waves.
    • Several such events occurred during the 9.2-magnitude earthquake in Alaska in 1964

 

  • Topography
    • The tsunami could have been impacted by Palu’s location at the end of a narrow bay.
    • The coastline and the contours of the bottom of the bay could have focused the wave energy and guided it up the bay, increasing the wave height as it approached the shore.

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