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Tsunamis & Seiches

The infamous offspring of undersea earthquakes, tsunamis conjure images of towering waves Though towering waves rarely accompany tsunamis, they are an immediate secondary threat to coastal and low-lying communities following offshore or coastal earthquakes. The word "Tsunami" is Japanese for "Harbour Wave". They are often wrongly called "Tidal Waves" and have nothing to do with tides. Most are generated by shallow earthquakes in the sea or by temblors near the coast that can set off underwater landslides. Volcanic eruptions scan also cause tsunamis. The 2004 earthquake off Sumatra is the deadliest earthquake in history with more than 1,00,000 fatalities around the Indian Ocean basin. Other deadly tsunamis includes the 1755 Lisbon (60,000), 1883 Krakatoa (30,000) and 1896 Meiji Sanriku (27,000).

What causes a tsunami?
Shallow undersea earthquakes are responsible for most tsunamis though at time landslides triggered by smaller seismic events can also generated potentially lethal waves. Strong earthquakes cause a displacement of the crust. When they occur underwater, this crustal movement disturbs a large volume of water like a giant paddle and ripples spread out in all directions at speeds of 600-800 kilometres per hour, comparable to commercial aircraft. In the open ocean, they go unnoticed but once they reach shallower waters they slow down and begin to crest. The waves thus given rise to are known as "Tsunamis".  They are scientifically described as a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps and are mostly caused by earthquakes of magnitude 7.5 or greater. Small earthquakes have also been known to generate tsunami activity but the effects of these tend to be localized i.e. confined to a small region.

Date

Location

1524

Near Dabhol, Maharashtra

2 April 1762

Arakan Coast, Myanmar

16 June 1819

Allah Bund, Gujarat

31 October 1847

Kondul Island, Nicobars

31 December 1881

Car Nicobar Island

26 August 1883

Krakatau volcanic eruption

4 January 1907

Northern Sumatra

26 June 1941

Middle Andaman Island

28 November 1945

Makran coast, Balochistan

13 September 2002

Diglipur, North Andaman

26 December 2004

Sumatra-Andaman

Historical Tsunamis in south Asia
The deadliest tsunami prior to 2004, was in November 1945 which originated off the Makran coast in Pakistan and caused deaths as far as Mumbai. One of the earliest known tsunamis was experienced by the fleet of Vasco da Gama as they approached Dabhol, on the Konkan coast of Maharashtra in 1524. Most historical tsunamis that affected south Asia were basin-wide events but none approaching the scale of destruction caused by the the December 2004 Indian Ocean tsunami.

Seismic Seiches  (Pronounced: Sai-Shhh)
Long period movement of water can also be produced in lakes and reservoirs by large, usually distant, earthquakes, and sometimes by strong winds. In the late nineteenth century a Swiss professor, F.A. Forel made a systematic study of this type of a water wave, which he called a "seiche". Seiches are described as "a standing wave in a closed body of water such as a lake or bay". A seiche can be characterized as the sloshing of water in the enclosing basin. The permanent tilting of lake basins caused by nearby fault motions has produced very energetic seiches. Seiches caused by earthquakes are termed as seismic seiches, a term coined by Anders Kvale in 1955 to describe oscillations of lake levels in Norway and England caused by the M8.6 1950 Chayu earthquake. Seismic seiches have been reported after numerous earthquakes in south Asia. The M7.6 Bhuj and M7.6 Kashmir earthquakes both caused seismic seiches as far as Bangladesh while the M8.1 1934 Bihar earthquake caused a seismic seiche in Lake Vembanad, Kerala, capsizing many canoes. Great earthquakes in Sumatra have also produced noticeable seiches in part of the peninsula. The 1861 Sumatra earthquake, also caused seiches that were noticed at many locations in Bengal and Orissa, including Kolkata. The M9.1 Sumatra-Andaman earthquake in 2004 and the M8.7 Nias earthquake in 2005, also generated energetic seiches in eastern India, Bangladesh and Thailand. The only known fatality attributed to seismic seiches in south Asia occurred in West Bengal's Nadia district during the 2004 Sumatra-Andaman earthquake when an early morning bather was swept away by unusually large waves that formed in the Jalangi River near Jatrapur.

Landslide induced waves
Sometimes a landslide or an avalanche into a body of water, can also trigger local water waves. One of the most famous wave of this kind occurred on 9 July, 1958. A large earthquake struck a remote area of southern Alaska and caused a landslide into a 11 kilometre long bay known as Lituya Bay. The quake dislodged an estimated 30 million cubic metres of rock and sent it cascading into the bay, from a height of 900 metres. A massive wave of water was generated. It surged across the narrow bay and denuded nearly mountain slopes up to a height of 500 metres. 5 people were drowned but others had miraculous escapes like Bill and Vivian Swanson. They had anchored their fishing boat, the Badger, just inside Lituya bay's opening to the sea, behind a 1.5 kilometre spit called the La Chaussee Spit. After feeling the earthquake and watching the landslide they watched in terror as the wave raced towards them. Their boat was washed up onto the crest of the wave, backward, and was swept over the 140 metre wide spit. The Swansons were lucky and lived through the experience. In 1998, a strong earthquake off the coast of Papua New Guinea set off an underwater landslide that spawned a 10-metre tsunami. The wave struck the coast of West Sepik province and devastated villages near the Sissano Lagoon killing several hundreds.


Further information (External links will open in new window):
International Tsunami Warning Centre | Pacific Tsunami Warning Centre
1896 Meiji Sanriku Tsunami JST Failure Knowledge Database | Sanriku Earthquakes
1958 Lituya Bay
Tsunami USC Tsunami Research Group | Historical tsunamis in Lituya Bay
1998 Aitape Tsunami
USC Tsunami Research Group | USGS Tsunami simulation
2004 Boxing Day Tsunami NIO Dona Paula | NOAA | USGS Tsunami simulation

Page Updated: 21 Feb 2008 | Website Disclaimer

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