Tsunami "Wave Train"Tsunami "Wave Train"Many people have the mistaken belief that tsunamis are single Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated many tsunami events the shoreline is pounded by repeated large waves.large waves.

1.1.Tsunami can be Tsunami can be generated when the see when the see floor abruptly deforms and vertically floor abruptly deforms and vertically displaces the overlying water. displaces the overlying water.

2.2.Tectonic earthquakes are a particular Tectonic earthquakes are a particular kind of earthquake that are associated kind of earthquake that are associated with the earth’s crustal deformation, with the earth’s crustal deformation, when these earthquakes occur beneath when these earthquakes occur beneath the see, the water above the deformed the see, the water above the deformed area is displayed from its equilibrium area is displayed from its equilibrium position. position.

3.3.Waves are formed as the displaced Waves are formed as the displaced water mass, which acts under the water mass, which acts under the influence of gravity, attempts to regain influence of gravity, attempts to regain its equilibrium its equilibrium

• Ocean waves are normally Ocean waves are normally divided into 3 groups, divided into 3 groups, characterized by depth:characterized by depth:

• •• Deep water Deep water • •• Intermediate water Intermediate water • •• Shallow water Shallow water • Even though a tsunami is Even though a tsunami is

generated in deep water generated in deep water (around 4000 m below (around 4000 m below mean sea level), tsunami mean sea level), tsunami waves are considered waves are considered shallow-water waves. As shallow-water waves. As the tsunami wave the tsunami wave approaches the shallow approaches the shallow waters of shore, its time waters of shore, its time period remains the same, period remains the same, but its wavelength but its wavelength decreases rapidly, thus decreases rapidly, thus causing the water to pile causing the water to pile up to form tremendous up to form tremendous crests, in an effect known crests, in an effect known as "shoaling".as "shoaling".

Most tsunamis are caused by earthquakes Most tsunamis are caused by earthquakes generated in a generated in a

seduction zone, an area where an oceanic seduction zone, an area where an oceanic plate is being plate is being

forced down into the mantle by plate tectonic forced down into the mantle by plate tectonic forces. The forces. The

friction between the sub ducting plate and friction between the sub ducting plate and the overriding the overriding

plate is enormous. This friction prevents a plate is enormous. This friction prevents a slow and steady slow and steady

rate of seduction and instead the two plates rate of seduction and instead the two plates become become

"stuck". "stuck".

Subduction Zones are Potential Tsunami Locations

As the stuck plate continues to descend into As the stuck plate continues to descend into the mantle the mantle

the motion causes a slow distortion of the the motion causes a slow distortion of the overriding plage. overriding plage.

The result is an accumulation of energy very The result is an accumulation of energy very similar to the similar to the

energy stored in a compressed spring. Energy energy stored in a compressed spring. Energy can can

accumulate in the overriding plate over a long accumulate in the overriding plate over a long period of period of

time - decades or even centuries.time - decades or even centuries.

Accumulated Seismic Energy

Tsunamis occur most frequently in the Tsunamis occur most frequently in the Pacific Ocean, but are , but are

a global phenomenon; they are possible a global phenomenon; they are possible wherever large bodies of wherever large bodies of

water are found, including inland lakes, where water are found, including inland lakes, where they can be caused they can be caused

by landslides. by landslides. Japan is a nation with the most recorded Japan is a nation with the most recorded

tsunamis in the tsunamis in the world. The earliest recorded disaster being that world. The earliest recorded disaster being that

of the 684 A.D. of the 684 A.D.

Tsunami is one of the earth’s disaster. It was a Japanese word Tsunami is one of the earth’s disaster. It was a Japanese word meaning “harbor wave,” used as the scientific term for a class meaning “harbor wave,” used as the scientific term for a class of abnormal sea wave that can cause catastrophic damage when of abnormal sea wave that can cause catastrophic damage when it hits a coastline. it hits a coastline. Tsunamis can be generated by an undersea earthquake, Tsunamis can be generated by an undersea earthquake, an undersea landslide, the eruption of an undersea volcano, an undersea landslide, the eruption of an undersea volcano, or by the force of an asteroid crashing into the ocean. or by the force of an asteroid crashing into the ocean.