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Tsunamis

The word is Japanese and means "harbor wave," because of the devastating effects these waves have had on low-lying Japanese coastal communities. 

 

Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave."

 

In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact. Tsunamis are often incorrectly referred to as tidal waves, but a tsunami is actually a series of waves that can travel at speeds averaging 450 (and up to 600) miles per hour in the open ocean. Sometimes reaching heights of 40 meters (120 ft.) or more, tsunamis are the most dramatic and destructive of waves. Underwater disturbances, such as volcanoes, earthquakes and landslides, are the cause of these monster waves. The larger the disturbance, the larger the tsunami will be.

In the open ocean, tsunamis may be hard to spot. Long wavelengths can hide the size of the wave, but just like other kinds of waves, changes occur when the wave enters shallow water. The wavelength shortens, and the height increases. The strength of the disturbance, the distance the wave travels and the shape of the coastline combined determine the tsunami's height, and ultimately, its destructiveness.

EARTHQUAKES ----------LANDSLIDES------------VOLCANOES

All three can cause a tsunami!

Source: Office of Naval Research

Graphic: Office of Naval Research

(not to scale)

http://www.onr.navy.mil/focus/ocean/motion/waves3.htm

How do tsunamis differ from other water waves?


Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.
As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.

What happens to a tsunami as it approaches land?


As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. If you read the "How do tsunamis differ from other water waves?" section, you discovered that a tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.

What happens when a tsunami encounters land?


As a tsunami approaches shore it begins to slow and grow in height. Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a runup height, of 10, 20, and even 30 meters.

 

FREQUENTLY ASKED QUESTIONS ABOUT TSUNAMIS

Credit: NOAA

National Weather Service International Tsunami Information Center

http://www.noaa.gov/tsunamis.html

 

 What is a tsunami?

The phenomenon we call tsunami is a series of large waves of extremely long wavelength and period usually generated by a violent, impulsive undersea disturbance or activity near the coast or in the ocean.  When a sudden displacement of a large volume of water occurs, or if the sea floor is suddenly raised or dropped by an earthquake, big tsunami waves can be formed by forces of gravity.   The waves travel out of the area of origin and can be extremely dangerous and damaging when they reach the shore.   The word tsunami (pronounced tsoo-nah'-mee) is composed of the Japanese words "tsu" (which means harbor) and "nami" (which means "wave").  Often the term, "seismic or tidal sea wave" is used to describe the same phenomenon, however the terms are misleading, because tsunami waves can be generated by other, non seismic disturbances such as volcanic eruptions or underwater landslides, and have physical characteristics different of tidal waves.   The tsunami waves are completely unrelated to the astronomical tides - which are caused by the extraterrestrial, gravitational influences of the moon, sun, and the planets.  Thus, the Japanese word "tsunami", meaning "harbor wave" is the correct, official and all-inclusive term.   It has been internationally adopted because it covers all forms of impulsive wave generation

 How do earthquakes generate tsunamis?

By far, the most destructive tsunamis are generated from large, shallow earthquakes with an epicenter or fault line near or on the ocean floor.  These usually occur in regions of the earth characterized by tectonic subduction along tectonic plate boundaries.  The high seismicity of such regions is caused by the collision of tectonic plates.  When these plates move past each other, they cause large earthquakes, which tilt, offset, or displace large areas of the ocean floor from a few kilometers to as much as a 1,000 km or more.  The sudden vertical displacements over such large areas, disturb the ocean's surface, displace water, and generate destructive tsunami waves.  The waves can travel great distances from the source region, spreading destruction along their path.  For example, the Great 1960 Chilean tsunami was generated by a magnitude 8.3 earthquake that had a rupture zone of over 1,000 km.   Its waves were destructive not only in Chile, but also as far away as Hawaii, Japan and elsewhere in the Pacific.  It should be noted that not all earthquakes generate tsunamis.  Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.



How do volcanic eruptions generate tsunamis?

 

Although relatively infrequent, violent volcanic eruptions represent also impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area. According to this mechanism, waves may be generated by the sudden displacement of water caused by a volcanic explosion, by a volcano's slope failure, or more likely by a phreatomagmatic explosion and collapse/engulfment of the volcanic magmatic chambers.

 

How do submarine landslides, rock falls and underwater slumps generate tsunamis?
Less frequently, tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps.   Such events may be caused impulsively from the instability and sudden failure of submarine slopes, which are sometimes triggered by the ground motions of a strong earthquake.  For example in the 1980's, earth moving and construction work of an airport runway along the coast of Southern France, triggered an underwater landslide, which generated destructive tsunami waves in the harbor of Thebes.   Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation.  For example, many scientists believe that the 1998 tsunami , which killed thousands of people and destroyed coastal villages along the northern coast of Papua-New Guinea, was generated by a large underwater slump of sediments, triggered by an earthquake.  In general, the energy of tsunami waves generated from landslides or rock falls is rapidly dissipated as they travel away from the source and across the ocean, or within an enclosed or semi-enclosed body of water - such as a lake or a fjord.   However, It should be noted, that the largest tsunami wave ever observed anywhere in the world was caused by a rock fall in Lituya Bay, Alaska on July 9, 1958.  Triggered by an earthquake along the Fairweather fault, an approximately 40 million cubic meter rock fall at the head of the bay generated a wave, which reached the incredible height of 520-meter wave ( 1,720 feet) on the opposite side of the inlet.  A initial huge solitary wave of about 180 meters (600 feet) raced at about 160 kilometers per hour (100 mph) within the bay debarking trees along its path.  However, the tsunami's energy and height diminished rapidly away from the source area and, once in the open ocean, it was hardly recorded by tide gauge stations.


How does tsunami energy travel across the ocean and how far can tsunamis waves reach?

Once a tsunami has been generated, its energy is distributed throughout the water column, regardless of the ocean's depth.  A tsunami is made up of a series of very long waves.  The waves will travel outward on the surface of the ocean in all directions away from the source area, much like the ripples caused by throwing a rock into a pond.   The wavelength of the tsunami waves and their period will depend on the generating mechanism and the dimensions of the source event.   If the tsunami is generated from a large earthquake over a large area, its initial wavelength and period will be greater.   If the tsunami is caused by a local landslide, both its initial wavelength and period will be shorter.  The period of the tsunami waves may range from 5 to 90 minutes.  The wave crests of a tsunami can be a thousand km long, and from a few to a hundred kilometers or more apart as they travel across the ocean.  On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers.   In the deep ocean, the height of the tsunami from trough to crest may be only a few centimeters to a meter or more - again depending on the generating source.  Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process.  The deeper the water, the greater the speed of tsunami waves will be.   For example, at the deepest ocean depths the tsunami wave speed will be as much as 800 km/hr, about the same as that of a jet aircraft.   Since the average depth of the Pacific ocean is 4000 m (14,000 feet) , tsunami wave speed will average about 200 m/s or over 700 km/hr (500 mph).   At such high speeds, a tsunami generated in Aleutian Islands may reach Hawaii in less than four and a half hours.  In 1960, great tsunami waves generated in Chile reached Japan, more than 16,800 km away in less than 24 hours, killing hundreds of people.

Credit: The BBC, Office of Naval Research, USGS, The University of Washington, The Guardian,NOAA,National Geographic