Tsunamis – Aspects and Features

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1755 Lisbon, Portugal – Earthquake and Tsunami[11]
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Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide, and for this reason they are often referred to as tidal waves.

The physical characteristics of tsunamis reported by NOAA-NGDC (National Geophysical Data Center) include distance of the tsunami run-up from source, wave height, and earthquake magnitude:[1]

  • The median distance from source was 119 km (mean 810 km; range 7-10,621 km);
  • The median wave height was 6.7m (mean 13.0m; range 1.8 -67.1m);
  • The majority of the tsunamis reported were due to earthquakes (95.5%), with small minorities resulting from landslides (3.0%), volcanoes (0.8%), and meteorological events (0.8%);
  • Median magnitude for earthquake generated tsunamis was 8.1 (mean 8.1; range 6.3-9.5).
  • About 80% of tsunamis occur in the Pacific Ocean, but they are possible wherever there are large bodies of water, including lakes.[2]

Causes of Tsunamis

The principal generation mechanism (or cause) of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to:[3]

Tsunamis are most commonly generated by earthquakes in marine and coastal regions. Major tsunamis are produced by large (greater than 7 on the Richer scale), shallow focus (< 30km depth in the earth) earthquakes associated with the movement of oceanic and continental plates. They frequently occur in the Pacific, where dense oceanic plates slide under the lighter continental plates. When these plates fracture they provide a vertical movement of the seafloor that allows a quick and efficient transfer of energy from the solid earth to the ocean.

Because earth movements associated with large earthquakes are thousand of square kilometers in area, any vertical movement of the seafloor immediately changes the sea-surface. The resulting tsunami propagates as a set of waves whose energy is concentrated at wavelengths corresponding to the earth movements (~100 km), at wave heights determined by vertical displacement (~1m), and at wave directions determined by the adjacent coastline geometry. Because each earthquake is unique, every tsunami has unique wavelengths, wave heights, and directionality.

When a powerful earthquake (magnitude 9.3) struck the coastal region of Indonesia in 2004, the movement of the seafloor produced a tsunami in excess of 30 meters (100 feet) along the adjacent coastline killing more than 240,000 people. From this source the tsunami radiated outward and within 2 hours had claimed 58,000 lives in Thailand, Sri Lanka, and India.[4]

Underwater landslides associated with smaller earthquakes are also capable of generating destructive tsunamis.

The tsunami that devastated the northwestern coast of Papua New Guinea on July 17, 1998, was generated by an earthquake that registered 7.0 on the Richter scale that apparently triggered a large underwater landslide. Three waves measuring more than 7 meter high struck a 10-kilometer stretch of coastline within ten minutes of the earthquake/slump. Three coastal villages were swept completely clean by the deadly attack leaving nothing but sand and 2,200 people dead.[5]

Other large-scale disturbances of the sea-surface that can generate tsunamis are explosive volcanoes.

The eruption of the volcano Krakatoa in the East Indies occurred on Aug. 27, 1883. During its final cataclysmic eruption Krakatau, which had been an island of about 30 square kilometers (12 square miles), nearly vanished. A total of 25 cubic kilometers (6 cubic miles) of material, including both new magma and bits of the old island, were blown into the stratosphere, the eruption column reaching more than 30 kilometers (20 miles) high. A huge tsunami rushed from the exploding island and crashed against the nearby shorelines killing tens of thousands. At no time in history had more people been killed by a volcano.[6]

Some meteorological conditions, especially deep depressions such as tropical cyclones, can generate a type of storm surge called a meteotsunami which raises water heights above normal levels, often suddenly at the shoreline.[]

Storm complexes, like derechos, can trigger tsunamis, although it is extremely rare. On June 13, 2013, a rare and powerful derecho, which had just caused widespread damage over the northeastern U.S., generated a tsunami-like surge on the northeastern United States coast. The greatest ocean heave was recorded at Newport, R.I., where it reached just under a foot above sea level.[8]

Perhaps the least likely source of tsunamis has the potential for causing the greatest tsunami: that of a meteor or comet impact.

While many previously scoffed at the idea of an extra-terrestrial object hitting the earth, that has recently changed. Models have recently shown that an asteroid hitting the ocean can cause a large tsunami that would inflict catastrophic damage to coastal cities even at great distances. Currently estimates are that an asteroid-induced tsunami exceeding 100 meters in height along the entire Atlantic coast line probably occurs once every few thousand years.

There is evidence that at least one such an impact occurred in an Atlantic Ocean coastal area about 35 million years ago. A crater at Chesapeake Bay, Virginia, was caused by an asteroid or comet traveling at about 70,000 miles (113,000 kilometers) an hour, that splashed through several hundred feet of water and several thousand feet of mud 1 22 and sediment. It is thought that billions of tons of ocean water were propelled into the air as high as 30 miles and vaporized. Millions of tons of debris and rocks were also ejected into the atmosphere. The incident probably incinerated everything along the East Coast, triggered gigantic tsunamis affecting coastal areas on both sides of the Atlantic, and decimated marine life in the surrounding area.[9]

Tsunamis also occur in lakes and reservoirs through seismic local ground shaking that triggers resonant oscillations (also known as seiching); by coseismic generation of subaerial or submarine mass movements (landslides, debris flows, slumps); or by aseismic mass movements or passing weather fronts. Lake Geneva (Lac Leman), Switzerland, is the textbook case of meteo-triggered lake oscillations.[10]

Tsunamis also occur in lakes and reservoirs through seismic local ground shaking that triggers resonant oscillations (also known as seiching); by coseismic generation of subaerial or submarine mass movements (landslides, debris flows, slumps); or by aseismic mass movements or passing weather fronts. Lake Geneva (Lac Leman), Switzerland, is the textbook case of meteo-triggered lake oscillations.[10]

 


References:

  1. NOAA-NGDC (National Geophysical Data Center): http://www.ngdc.noaa.gov/hazard/tsu.shtml
  2. National Center for Biotechnology Information: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3644289/
  3. NOAA – The Tsunami Story: http://www.tsunami.noaa.gov/tsunami_story.html
  4. NOAA – The Tsunami Story: http://www.tsunami.noaa.gov/tsunami_story.html
  5. NOAA – The Tsunami Story: http://www.tsunami.noaa.gov/tsunami_story.html
  6. USGS: http://hvo.wr.usgs.gov/volcanowatch/archive/2003/03_05_22.html
  7. NOAA – The Tsunami Story: http://www.tsunami.noaa.gov/tsunami_story.html
  8. NOAA – TSUNAMI of 13 June, 2013 (Northwestern Atlantic Ocean): http://oldwcatwc.arh.noaa.gov/previous.events/06-13-13/index.php
  9. NOAA-NGDC (National Geophysical Data Center) – Tsunamis and Tsunami-Like Waves of the Eastern United States: http://www.ngdc.noaa.gov/hazard/data/publications/ref0541_lockridge.pdf
  10. FEMA – Tsunami Hazards: http://www.fema.gov/media-library-data/20130726-1541-20490-8257/frm_p1tsun.txt
  11. Image Source: http://www.ngdc.noaa.gov/hazard/tsu_travel_time_events.shtml [Accessed: October 12, 2014]

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