What is a Tsunami?

Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths
of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they
build up to higher and higher heights as the depth of the ocean decreases. The speed of tsunami waves
depends on ocean depth rather than the distance from the source of the wave. Tsunami waves may
travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters. While
tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides
have little to do with these giant waves.
Tsunami (pron: 'soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. The
phenomenon is usually associated with earthquakes, landslides or volcanic eruptions in, or adjacent to
oceans, and results in sudden movement of the water column. Until recently tsunami were called tidal
waves, even though the event has nothing to do with tides.
A tsunami is different from a wind generated surface wave on the ocean. The passage of a tsunami
involves the movement of water from the surface to the seafloor which means its speed is controlled by
water depth. Consequently, as the wave approaches land and reaches increasingly shallow water it
slows. However, the water column still in deeper water is moving slightly faster and catches up,
resulting in the wave bunching up and becoming much higher. A tsunami is often a series of waves and
the first may not necessarily be the largest.
When a tsunami travels over a long and gradual slope, it allows time for the tsunami to grow in wave
height. This is called shoaling and typically occurs in shallow water less than 100m. Successive peaks
can be anywhere from five to 90 minutes apart. In the open ocean, even the largest tsunami are
relatively small with wave heights of less than one metre. The shoaling effect can increase this wave
height to a degree such that the tsunami could potentially reach an onshore height of up to 30 metres
above sea level. However, depending on the nature of the tsunami and the nearshore surroundings, the
tsunami may create only barely noticeable ripples.
Interesting fact: Tsunami can travel at speeds up to 950km/h in deep water which can be represented by
the speed of a passenger jet.

More facts
A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over
100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they
crash ashore.
These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate
boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water
above it and launches the rolling waves that will become a tsunami.
Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically
active area where tectonic shifts make volcanoes and earthquakes common.
Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be
launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging
into an ocean.
Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet
airplane. At that pace they can cross the entire expanse of the Pacific Ocean in less than a day. And
their long wavelengths mean they lose very little energy along the way.
In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and
enter shallower water they slow down and begin to grow in energy and height. The tops of the waves
move faster than their bottoms do, which causes them to rise precipitously.
A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it
produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This
retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its
enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon
can save lives.
A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be
compounded as successive waves reach shore. People experiencing a tsunami should remember that the
danger may not have passed with the first wave and should await official word that it is safe to return to
vulnerable locations.
Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly
surging tide that inundates coastal areas.
The best defense against any tsunami is early warning that allows people to seek higher ground. The
Pacific Tsunami Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a web
of seismic equipment and water level gauges to identify tsunamis at sea. Similar systems are proposed
to protect coastal areas worldwide.
When considering how tsunami waves propagate, or travel across the ocean, it is important to
understand wave behavior. Before discussing tsunami waves, this unit defines what a wave is and
describes wave characteristics. Comparing wind-generated waves and tsunami waves is useful for
understanding the force, scope and potential danger of large tsunamis.
Using data from past tsunami events and known wave characteristics, scientists have developed models
for calculating tsunami travel times to deliver warnings to communities that may be impacted by a
tsunami. These models make use of complex data based on the size and location of an earthquake, the
depth of the ocean as determined by bathymetric measurements, the distance to a given location, the
shape of the coastline in impact zones, and past run-up heights.
Tsunami warning center scientists have developed models to predict the tsunami travel times for certain
high risk locations. When an earthquake of magnitude 7.5 or higher is generated along a coastal area,
warning centers may be able to warn communities of an impending tsunami and give a time estimate of
when the first wave will arrive.

How to Prepare for a Tsunami
Tsunamis are a series of waves caused by a massive disturbance of water. In general, tsunamis are not
particularly threatening, as they constantly happen every day around the world, often in the middle of
the ocean. In fact, most tsunamis don't reach much higher than regular ocean waves on the beach. But
in some cases, the tsunami will develop into potentially destructive waves. If you live in a coastal area,
it's imperative that you know what to do should this situation arise.