Tsunami | Ten most devastating tsunamis | Few interesting Facts

TSUNAMI

A tsunami refers to a series of high ocean waves that cause water surges with heights of about 100 feet or 30.5 meters to reach the land. These enormous ocean waves bring water sufficient enough to cause widespread damage to onshore.

What Causes a Tsunami?

Tsunami is generally caused as a result of strong earthquakes occurring under the sea/ocean tectonic plate boundaries. The ocean waves/surface near the plate boundary begins to rise or fall all of a sudden. This produces gigantic rolling waves that rapidly advance towards the shore, becoming a tsunami.

Tectonic plate movement under the ocean leading to the occurrence of an earthquake.
The strain produced causes the over-riding plate to bulge, causing tectonic uplift.
As a result of subsidence or sudden sinking of the ground surface, a large amount of energy is released.
The energy released then forms the enormous Tsunami waves. Image Source: Wikipedia, – U.S. Geological Survey, Circular 1187 (http://pubs.usgs.gov/circ/c1187/illustrations/BlockDigrm_1.ai)
Vertical Slice Through a Subduction Zone
Public Domain

More than 80 percent of the tsunamis occur in the“Ring of Fire” region of the Pacific Ocean. This region is known for being a geologically active area that experiences frequent tectonic shifts making volcanoes, calamity and earthquakes common.

Apart from the movement of tectonic plates, underwater landslides or volcanic eruptions might also give birth to it. Thousands of years ago, frequent meteorite plunging into an ocean also gave rise to it.  

Tsunamis advance across the sea towards the land with about 805 kilometers or 500 miles an hour. With such a pace, it can cross the entire Pacific Ocean within a day. The length of it’s waves is very long. Therefore the amount of energy loss along the way is very little.

Tsunami waves tend to appear only a few feet high in the deep ocean. But as the waves advance towards the shore or enter shallower water regions, they tend to slow down their speed and increase height and energy. The upper part of the waves moves at a faster pace as compared to lower parts, which results in the waves rising precipitously.

What happens when Tsunamis hits the land?

A tsunami’s trough i.e., the lower part lying below the wave’s crest, generally approaches the shore first. Once the wave approaches the shore, it creates a vacuum effect due to which the coastal water is sucked seaward, exposing the sea floors and harbors. The seawater retreating provides a vital warning signal before a tsunami because, within a matter of minutes, the wave’s crest carrying a vast volume of water hits the shore. Hence, recognizing such sea activities is extremely important.

A tsunami is generally made up of a wave train i.e., a series of waves. Therefore, the extent of destruction it can cause depends on the speed, frequency, and height of the successive waves reaching the shore. Even after the first big wave passes, the tsunami might not be over, and there may be chances of subsequent waves striking the vulnerable areas afterward.

Some of it do not occur in the form of gigantic waves hitting the shore, but as rapidly surging tides that inundate coastal areas.

The best way to defend against it, is early warning and evacuation of the people residing in the vulnerable areas. Once the warning signs are detected, people are moved to higher ground for protection. The Pacific Tsunami Warning System is an organization built by 26 nations (having the headquarters in Hawaii) to maintain a series of water level gauges and other seismic equipment types to detect tsunamis at sea. Such organizations are responsible for governing the signs of tsunami occurrences all over the world.

What should you do during tsunamis?

This usually start with the occurrence of an earthquake first. So,

  • Run to an open ground without buildings and trees nearby.
  • If there is no proximate open ground, then take shelter under a strong metal table.
  • Do not start running because of panic and avoid using elevators

Once the shaking stops,

  • Be alert and wait for tsunami warnings from the authorities.
  • Move to a safe, high-level land if possible.
  • Follow the authorities for proper evacuation routes.
  • If you are in a water body, try to hold onto some floatable objects (tree trunk, raft, etc.).
  • Avoid using bridges and moving through waterlogged areas.
  • Do not stand near electrical poles.
  • Keep your phones with yourselves to contact the authorities in case you require some assistance.
  • Do not go near any water body after the first wave.

Ten most devastating tsunamis:

  1. 1. Sumatra, Indonesia – 26 December 2004

Earthquake Magnitude: 9.1 

Region of occurrence: Sumatra coast, at a depth of 30 km.

Fault zone width: 1300 km.

Estimated damage worth: US$10 billion

Life loss: Around 230,000 

  1. 2. North Pacific Coast, Japan – 11 March 2011

Earthquake Magnitude: 9.0

Region of occurrence: east coast of Japan, depths of 24.4km.

Fault zone width: 800km 

Estimated damage worth: US$235 billion

Life loss: Around 18,000 people

3. Lisbon, Portugal – 1 November 1755

Earthquake Magnitude: 8.5

Region of occurrence: West coast of Portugal and Southern Spain, depth of 30 m.

Life loss: Around 60,000

4. Krakatau, Indonesia – 27 August 1883

Volcano eruption: Krakatau caldera volcano

Region of occurrence: Anjer and Merak

Wave height: 37 m

Life loss: Around 40,000 people

5. Ensenada Sea, Japan – 20 September 1498

Earthquake Magnitude: 8.3

Region of occurrence: Coasts of Kii, Mikawa, Surugu, Izu, and Sagami.Fault 

Life loss: Around 31,000 people

6. Nankaido, Japan – 28 October 1707

Earthquake Magnitude: 8.4 

Region of occurrence: Pacific coasts of Kyushyu, Shikoku, and Honshin. Osaka

Wave height: 25 m

Life loss: Around 30,000 people

7. Sanriku, Japan – 15 June 1896

Earthquake Magnitude: 7.6

Region of occurrence: coast of Sanriku and Shirahama, Japan. 

Wave height: 38.2 m

Life loss: Around 22,000 people

8. Northern Chile – 13 August 1868

Earthquake Magnitude: 8.5

Region of occurrence: Coast of Chile (previously Arica, Peru)

Wave height: 21 Km.

Estimated damage worth: US$300 million

Life loss: Around 25,000 people

 9. Ryuku Islands, Japan – 24 April 1771

Earthquake Magnitude: 7.4 

Region of occurrence: Ishigaki and Miyako Islands 

Wave height: 11 to 15m

Life loss: Around 12,000 people

10. Ise Bay, Japan – 18 January 1586

Earthquake Magnitude: 8.2

Region of occurrence: Ise Bay and Nagahama town, Japan

Wave height: 6m

Life loss: Around 8000 people.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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