Tsunami  >> About Tsunami


Facets

Definition

What is a Tsunami

It is a wave train, or series of waves, generated in a body of water by an spontaneous disturbance that verticially displaces the water column.

What are the factors that give rise or result in tsunamis

Earthquakes, landslides, volcanic eruptions, explosions, impact of cosmic bodies such as meteorites, detonation of nuclear devices near the sea. 

When the destructive tsunamis are generated

Due to large shallow-focus earthquakes with an epicenter or fault line near or in the ocean.

Speed of tsunami waves

Very High in the oceans and reduced along the shore.

Height of tsunami waves

Lower in the oceans.  Higher as it approaches the coast or shore.

Tsunami is a

Japanese Word

Meaning of tsunami in English

Harbour Wave (Tsu – Harbour and Nami – wave)

General Public refer tsunami as

Tidal waves

Scientific Community refer tsunami as

Seismic waves

Can tsunami be caused by a non-seismic event

Yes.  It can be caused due to a landslide or meteorite impact

What should be magnitude of earthquake at its epicenter to generate a tsunami

More than 7.0 on Richter Scale.

In which Ocean the tsunamis are commonly generated

Pacific Ocean

Countries most prone to tsunamis in Pacific Ocean

Japan, South America, Papua New Guinea, Solomon Islands and Indonesia, Mexico and Central America and some parts Alaska in USA.

Countries most prone to tsunamis in Indian Ocean

Indonesia, Thailand, India, Sri Lanka, Pakistan, Iran, Malaysia, Myanmar, Maldives, Somalia, Bangladesh, Kenya, Madagascar, Mauritius, Oman, Reunion Island (France), Seychelles, South Africa and Australia.

What is the wavelength of a tsunami

More than 500 km.

Speed of the tsunami is directly proportional to 

Depth of the ocean.

What provides the force to allow a tsunami to travel a long distance

i) Slope of the sea surface, which creates a horizontal pressure force and ii) Piling up (or lowering of sea surface) as water moves with different speeds in the direction that the wave form is moving.   When these two processes have the right relationship in time, they create propagating or long distance waves.

Whether tsunamis can be felt aboard the ships in deep oceans or can be seen from the air in the open ocean

No.

Which can detect the changes in deep ocean

Radar Satellites.

What is called the wavelength of a tsunami

Distance from crest to crest

What is called the amplitude of a tsunami

Height from crest to trough

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 and travels at a speed that is related to the water depth  and hence as the water depth decreases, the tsunami slows down.

What happens when a tsunami encounters land

As a tsunami approaches shore or land it begins to slow and grow in height.

What is called ‘ run-up height “ of a tsunami

It is a maximum vertical height onshore above Mean Sea level with a maximum of 10 meters.  It is the vertical distance between the maximum height reached by the water on shore and the mean-sea-level surface. 

Run-ups from the same tsunami is same or different and if so the reasons.

Run-ups from the same tsunami can be variable because of the influence of the geomorphology (shape) of the coastline.  Therefore, in coastal area the waves can be large and violent with damaging activity, while in another area without being violent can cause extensive flooding with rise in water level to a few meters.

Extent of the damage due to tsunami depends on

Run up height, velocity of water, local topography and land utility pattern (say settlement, agriculture, forestry etc.,)

Whether estuary helps in inundation

Yes.   An estuary with a good depth profile helps the tsunami waves to cause inundation upto 2 to 5 km inland.

Which part of tsunami will arrive first

Trough.  Example : When the tsunami originate from an oceanic earthquake associated with land subsidence or sinking, which causes the water column to drop down at the earthquake site.  

What is the sign or a common precursor  for a tsunami for the laymen and onlookers at the coast.

Unusual low tide and retreading of sea water from the shore when large portion of sea floor becomes exposed.

Difference between tsunami wave and other water waves

i) Wind or other water waves: They are shallow water waves characterized by their long periods and wavelengths.  The time period of two successive waves is about 10-20 seconds and a wavelength (distance between two successive waves) of 100-200 m.

ii) Tsunami Waves: They have wavelength of 500 km and period of ten minutes to two hours and because of their long wave length they behave as shallow water waves.

Whether the tsunamis generated by falling cosmic bodies or debris on the sea surface affect the coast.

Generally speaking, tsunamis generated from these mechanisms, unlike the earthquake triggered, dissipate quickly and rarely affect coastlines distant from the source area.

Forecasting a tsunami

It requires adequate understanding of the phenomenon, good and expeditious collection of earthquake and sea level data, and accurate and expeditions assessment and interpretation of this data, since Tsunami Warning System works in a real time frame of short duration, often with inadequate data and information.  Further, problems of communications and lack of sufficient station density, often complicate the process.

What is a plate with reference to earth’s landmass

A plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and ocean lithosphere.  These plates are about 50 miles thick, are not anchored to a particular place;  they move relative to one another at an average speed of a few inches a year.  The earthquakes and volcanic eruptions occur when these plates collide at their boundaries.

What are three types of plate movements

i) Convergent, ii) Divergent and iii) Transform.

How tsunamis can be generated

When the sea floor abruptly deforms and vertically displaces the overlying water.   The size of the tsunami wave depends upon the quantum of deformation of the sea floor and also depends on the vertical displacement of water.  More the vertical displacement, greater will be the size of the waves.

Large Vertical Movements occur at

Plate boundaries

The size of the tsunami can be also due to

Earthquake’s magnitude, depth, fault characteristics and coincident slumping of sediments or secondary faulting.