WHAT IS TSUNAMI?
A tsunami (from Japanese: 津波, "harbour wave";[1] English pronunciation: /suːˈnɑːmi/ soo-NAH-mee or /tsuːˈnɑːmi/[3]), sometimes incorrectly referred to as a tidal wave,[4][5][6] also known as a seismic sea wave, is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides, glacier calvings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami.[7] Unlike normal ocean waves, which are generated by wind, or tides, which are generated by the gravitational pull of the Moon and the Sun, a tsunami is generated by the displacement of water.
Tsunami waves do not resemble normal undersea currents or sea waves because their wavelength is far longer.[8] Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide.[9] For this reason, it is often referred to as a "tidal wave", although this usage is not favoured by the scientific community because it might give the false impression of a causal relationship between tides and tsunamis.[10] Tsunamis generally consist of a series of waves, with periods ranging from minutes to hours, arriving in a so-called "internal wave train".[11] Wave heights of tens of metres can be generated by large events. Although the impact of tsunamis is limited to coastal areas, their destructive power can be enormous, and they can affect entire ocean basins. The 2004 Indian Ocean tsunami was among the deadliest natural disasters in human history, with at least 230,000 people killed or missing in 14 countries bordering the Indian Ocean.
The Ancient Greek historian Thucydides suggested in his 5th century BC History of the Peloponnesian War that tsunamis were related to submarine earthquakes,[12][13] but the understanding of tsunamis remained slim until the 20th century and much remains unknown. Major areas of current research include determining why some large earthquakes do not generate tsunamis while other smaller ones do; accurately forecasting the passage of tsunamis across the oceans; and forecasting how tsunami waves interact with shorelines.
Causes of Tsunami
A tsunami is a series of large waves generated by an abrupt movement on the ocean floor that can result from an earthquake, an underwater landslide, a volcanic eruption or - very rarely - a large meteorite strike.
However, powerful undersea earthquakes are responsible for most tsunamis. Seismologists say only earthquakes measuring greater than 7.0 on the Richter scale can produce a major tsunami.
Most earthquakes that generate tsunamis - including Friday's jolt off Japan's eastern coast - occur in areas called subduction zones, where pieces of the Earth's crust press against each other. Subduction means that one tectonic plate slides beneath another and sinks deep into the Earth's mantle.
The friction between two slow-moving plates of the Earth's crust creates vast amounts of seismic energy which is released in the form of an earthquake. When a strong undersea earthquake strikes a relatively short distance below the sea floor, it abruptly pushes up one of the immense plates of the Earth's crust. That suddenly displaces an enormous amount of ocean water which becomes a tsunami, spreading outward in every direction from the epicenter of an earthquake - like ripples on a pond, only on a much larger scale.
Tsunamis generated in the open ocean appear to be only small waves, but they can grow rapidly in size as they reach shallow water before crashing into seacoast settlements. Waves up to nine meters high have been recorded on numerous occasions, and tsunamis up to 30 meters high are believed to have occurred in the past.
Damage is usually worst in areas closest to the undersea quake, often because the fast-moving waves will hit land so quickly. Tsunamis radiate out from the site of an undersea quake nearly as fast as a jet airliner can travel, but because the expanse of the Pacific is so wide, alert systems are in place to sound a warning to those in the path of an advancing wave.
The term tsunami comes from two Japanese words that mean "harbor" and "wave."
However, powerful undersea earthquakes are responsible for most tsunamis. Seismologists say only earthquakes measuring greater than 7.0 on the Richter scale can produce a major tsunami.
Most earthquakes that generate tsunamis - including Friday's jolt off Japan's eastern coast - occur in areas called subduction zones, where pieces of the Earth's crust press against each other. Subduction means that one tectonic plate slides beneath another and sinks deep into the Earth's mantle.
The friction between two slow-moving plates of the Earth's crust creates vast amounts of seismic energy which is released in the form of an earthquake. When a strong undersea earthquake strikes a relatively short distance below the sea floor, it abruptly pushes up one of the immense plates of the Earth's crust. That suddenly displaces an enormous amount of ocean water which becomes a tsunami, spreading outward in every direction from the epicenter of an earthquake - like ripples on a pond, only on a much larger scale.
Tsunamis generated in the open ocean appear to be only small waves, but they can grow rapidly in size as they reach shallow water before crashing into seacoast settlements. Waves up to nine meters high have been recorded on numerous occasions, and tsunamis up to 30 meters high are believed to have occurred in the past.
Damage is usually worst in areas closest to the undersea quake, often because the fast-moving waves will hit land so quickly. Tsunamis radiate out from the site of an undersea quake nearly as fast as a jet airliner can travel, but because the expanse of the Pacific is so wide, alert systems are in place to sound a warning to those in the path of an advancing wave.
The term tsunami comes from two Japanese words that mean "harbor" and "wave."
Kinds of Tsunami
Local Tsunami
A local tsunami is a tsunami that causes damage in relatively close proximity to the tsunami-causing event. Specifically, the underwater event -- usually an earthquake -- that produces a local tsunami happens within 100 km, which is a little over 60 miles, of the land damage that results. These tsunamis can be devastating because the time between the underwater event and the arrival of the tsunami can be under an hour -- and sometimes less than 10 minutes. This does not provide sufficient time for comprehensive evacuations.
Regional Tsunami
A regional tsunami is one that causes damage from 100 km to 1,000 km from the underwater event that causes the tsunami. In some cases, more contained damages occur outside the 1,000 km perimeter. Regional tsunamis provide slightly more warning time than local tsunamis, making landfall between one and three hours of the event that causes them. Within the 1,000 km area, just one to three hours may not provide enough time for people to evacuate safely.
Distant Tsunami
A distant tsunami, also called a tele-tsunami or ocean-wide tsunami -- originates with an exceptionally powerful and destructive event more than 1,000 km away from landfall. Though a distant tsunami may first appear like a local tsunami, it travels across wide swathes of ocean basin. There is more time to evacuate and escape a distant tsunami, but it also covers a larger mass of land and tends to cause extensive and widespread destruction.
The Distant Tsunami of 2004
The most devastating tsunami on historical record occurred in the Indian Ocean on December 26, 2004. On that day, a massive underwater earthquake -- measuring at 9.1 out of 10 on the Richter scale -- occurred off the northern coast of Sumatra, Indonesia. The resulting distant tsunami hit the coasts of Indonesia, Thailand, Malaysia, India, Sri Lanka, Myanmar, the Maldives, and even countries in East Africa. At least a quarter of a million people died, with the worst damage concentrated in India, Indonesia, Sri Lanka,Thailand and the Maldives.
Comments
Post a Comment