Tell you all information about Tsunami. For example : What is a tsunami ? What does 'tsunami' mean ? How do tsunamis differ from other water waves ? How do earthquakes generate tsunamis ? How do landslides, volcanic eruptions, and meteorites generate tsunamis ? What happens to a tsunami as it approaches land ? What happens when a tsunami encounters land ?

The Physics of Tsunamis
The mechanisms of tsunami generation and propagation

What is a tsunami?

A tsunami ( pronounced tsoo-nah-mee ) is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

What does "tsunami" mean?

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.

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. 

This animation (2.3 MB), produced by Professor Nobuo Shuto of the Disaster Control Research Center, Tohoku University, Japan, shows the propagation of the earthquake-generated 1960 Chilean tsunami across the Pacific. Note the vastness of the area across which the tsunami travels - Japan, which is over 17,000 km away from the tsunami's source off the coast of Chile, lost 200 lives to this tsunami. Also note how the wave crests bend as the tsunami travels - this is called refraction. Wave refraction is caused by segments of the wave moving at different speeds as the water depth along the crest varies. Please note that the vertical scale has been exagaerated in this animation - tsunamis are only about a meter high at the most in the open ocean. (The QuickTime movie presented here was digitized from a video tape produced from the original computer-generated animation.).

How do earthquakes generate tsunamis?

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

This simulation (2 MB) of the 1993 Hokkaido earthquake-generated tsunami, developed by Takeyuki Takahashi of the Disaster Control Research Center, Tohoku University, Japan, shows the initial water-surface profile over the source area and the subsequent wave propagation away from the source. Areas in blue represent a water surface that is lower than the mean water level, while areas in red represent an elevated water surface. The initial water-surface profile, as shown in this image, reflects a large, long uplifted area of the sea floor lying to the west (left) of Okushiri Island, with a much smaller subsided area immediately adjacent to the southwest corner of Okushiri.

How do landslides, volcanic eruptions
and cosmic collisions generate tsunamis?

A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.

This image shows Lituya Bay, Alaska, after a huge, landslide-generated tsunami occurred on July 9, 1958. The earthquake-induced rockslide, shown in upper right-hand corner of this image, generated a 525 m splash-up immediately across the bay, and razed trees along the bay and across LaChausse Spit before leaving the bay and dissipating in the open waters of the Gulf of Alaska. Source: Lander, and P. Lockridge

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, we've learned in the "What happens to a tsunami as it approaches land?" section that 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. 

This numerical simulation (2.6 MB), produced by Professor Nobuo Shuto of the Disaster Control Research Center, Tohoku University, Japan, shows the 1923 Kanto tsunami attacking a Japanese village. A longer version (6.2 MB) of the this animation is also available. Note that the structures in this model are rigid - in a real-life tsunami, coastal structures often are destroyed. (The QuickTime movie presented here was digitized from a video tape produced from the original computer-generated animation.).

For more information about Tsunami, please visit the following websites :

• http://pubs.usgs.gov/circ/c1187/
  Surviving a Tsunami - Lessons from Chile, Hawaii, and Japan
  ( If you read nothing else, read this ).
  This is a USGS publication ( available online ) that details how to survive a tsunami. It was based on personal accounts related to the tsunami generated by the 1960 magnitude-9.5 earthquake in Chile.
  
• http://www.pmel.noaa.gov/tsunami-hazard/links.htm
  The National Tsunami Hazard Mitigation Program
  Lists many links to tsunami websites, most of which are technical.
  
• http://www.tsunami.org/
  Pacific Tsunami Museum site
  Includes answers to frequently asked questions, links, and information related to Pacific Ocean tsunamis.
  
• http://cwis.usc.edu/dept/tsunamis/index.html
  The USC (University of Southern California) tsunami research group
  Has photos from several recent tsunamis and related tsunami information

For more knowledge about Tsunami, please click at the following websites. Their original information is now somewhat out of date, and in fact the site is currently undergoing overhaul.

General Tsunami Information

• Alaska Tsunami Warning Center : Near real-time tsunami informational bulletins
• The Physics of Tsunamis : How is a tsunami generated and how does it propagate?
• A Survey of Great Tsunamis : How have tsunamis affected humans?
• The Tsunami Warning System : How are people in coastal areas warned about tsunamis?
• Tsunami Hazard Mitigation : How can you protect yourself from a tsunami?

Tsunami Survey and Research Information

• Recent Tsunami Events : Multimedia documentations of recent, significant tsunamis
• Tsunami Research : Information about ongoing tsunami research

Background on the Development of Tsunami

• Acknowledgments : The people and organizations who have made this project possible
• Motivation : The impetus for this project
• Design : Document design considerations