What is a tsunami

Tsunami (read tsuːˈnɑːmi/) is a long, high sea wave that is generated when the sea floor unexpectedly deforms and disfigures vertically as an effect of disturbance that occurs beneath the sea floor.

Indonesia is known as the scene of the biggest tsunami in the world, caused by Mount Krakatoa eruption on August 27, 1883. The death toll from the calamity in the Sunda Strait is around 36,000 people. Around 165 towns and villages spread on the coast where hit by tsunami waves as high as 40 meters. In addition to devastating tsunamis caused by volcanic eruptions, Indonesia coastline can be affected by tsunamis that are caused by tectonic earthquakes in the sea floor. At the ocean floor around the country three tectonic plates meet: the Eurasian, Indo-Australian, and Pacific. Disturbance created by those plates shifting often cause earthquakes and volcanic eruptions that have earned the area the name of the ring of fire. The geography itself is dominated by volcanoes that are formed due to subduction zones between the Eurasian plate and the Indo-Australian plate.

Tsunamis in Indonesia

  • There have been 174 tsunamis recorded in Indonesia since 16th century.
  • There have been 32 tsunamis recorded in The Moluccas sea area since 16th century.
  • 28 of the Moluccan tsunamis have been caused by earthquakes.
  • A tsunami caused by Aceh earthquake in 2004 has claimed 200,000 lives.
  • Around 28% (150 of 530) of Indonesian cities are located in tsunami prone areas

Tsunami: step-by-step

Source: pubs.usgs.gov



  • Vertical Slice Through a Subduction Zone

    Step 1

    One of the many tectonic plates that make up Earth’s outer shell descends, or “subducts,” under an adjacent plate. This kind of boundary between plates is called a “subduction zone.” When the plates move suddenly in an area where they are usually stuck, an earthquake happens.

  • Between Earthquakes

    Step 2

    Stuck to the subducting plate, the overriding plate gets squeezed. Its leading edge is dragged down, while an area behind bulges upward. This movement goes on for decades or centuries, slowly building up stress.

  • During an Earthquake

    Step 3

    An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward, raising the sea floor and the water above it. This uplift starts a tsunami. Meanwhile, the bulge behind the leading edge collapses, thinning the plate and lowering coastal areas.

  • Minutes later

    Step 4

    Part of the tsunami races toward nearby land, growing taller as it comes in to shore. Another part heads across the ocean toward distant shores.

  • Tsunami
    reaches the coast
    in ~30min




Interesting facts about tsunamis


Tsunami in East Pacific

  • Tectonic plate shifts occur relatively far from the coastline.
  • Population is concentrated further from the coastline.
  • Tsunami wave travel times are long (around 3 hours)

This make it easier to warn residents at risk on time and give them enough time to evacuate.

Tsunami in Indonesia

  • Tectonic plate shifts occur relatively close to the coastline.
  • Population is concentrated closer to the shore.
  • Nearly flat coast topography increases the risk of devastating events even further.
  • The average tsunami wave travel times is only 30 minutes

This means advanced early warning systems and disaster mitigation management are required to avoid casualties.


Wave origin and height

A tsunami is a series of ocean waves caused by an earthquake, a landslide, a volcanic eruption, a meteorite impact, or in some cases it is unknown. These waves can reach heights of over 30 metres (~100 ft).


Where do they happen?

About 80% of tsunamis happen within the Pacific Ocean’s “Ring of Fire”.


Tsunami wave speed

Tsunamis can travel at speeds of about 500 miles or 805 kilometres an hour, almost as fast as a jet plane.


What happens when a tsunami approaches coastline

Deep in the ocean, tsunamis can be only around 1 metre tall and travel very quickly. The speed of a tsunami wave decreases as it’s approaching the coast with shallow water. However the wave height grows significantly.


Surviving in the water

If caught by a tsunami wave, it is usually safer not to swim, but to grab a floating object (a door, a tree trunk, if it floats, you can use it as a raft) and allow the current to carry you.


Energy

Tsunamis retain their energy, meaning they can travel across entire oceans with limited energy loss.


Warnings

Scientists can accurately estimate the time when a tsunami will arrive almost anywhere around the world based on calculations using the depth of the water, distances from one place to another, and the time that an earthquake or another event occurred.


Destructive power

In 2004, the Indian Ocean tsunami was caused by an earthquake with the energy of 23,000 atomic bombs. After the earthquake, killer waves radiating from the epicentre slammed into the coastline of 11 countries. The final death toll was 283,000.

Next: What do to if a tsunami alert is issued