Tsunamis!

A tsunami is a series of ocean waves generated by sudden movement in the sea floor.

In the deep ocean, the tsunami wave may only be a few inches high. The tsunami wave may come gently ashore or may increase in height to become a fast moving wall of turbulent water several meters high.

The original Japanese term literally translates as "harbor wave.“

Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded.

The tsunamis in Japan tend to be regional or local tsumanis and mover very, very quickly, but are not that strong.

1700 Japanese Tsunami Location

Why do you think Japan is hit with tsunamis so

frequently (two reasons?)

What causes tsunamis?Many different events can generate a tsunami. The more common ones are:Underwater earthquakesUnderwater volcanoesDetonation of nuclear bombs under the sea

Landslides (that slide into the sea)

Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions.

Casualties can be high because the waves move faster than humans can run.

Aftermath of the Indonesian Tsunami

How does a tsunami form? Tsunamis occur mostly at convergent

(subduction) plate boundaries.

It is very rare for a tsunami to occur at a divergent plate boundary (seafloor spreading) because these do not disturb the water above the plate boundary.

Most tsunamis occur because of an earthquake on a subduction zone.

This sudden movement of water upward (because of the earthquake) results in the water immediately trying to return to the ocean floor (because of gravity).

The downward movement of the water (because of gravity) back into the ocean (to a state of equilibrium) generates a fast-moving ocean wave outward from the epicenter. This is called a tsunami.  

How fast do tsunamis move?Tsunamis travel with great speed through the ocean, with initial speed approaching 500 miles per hour.  

As a tsunami moves over a continental shelf, it slows dramatically. This slowing and the shallowness of the water causes the wave height to abruptly rise from one or two feet to 10 to 30 feet in height.  

Once the tsunami comes ashore, it can surge far inland causing great destruction in a coastal zone.  

This is referred to as "runup".   Once the surge loses momentum, gravity will draw the water back into the ocean, called "backwash".   This can be as dangerous as the initial surge runup.  

History of Tsunamis! The Greek historian Thucydides was the

first to relate tsunamis to submarine earthquakes, but scientists did not really begin to understand tsunamis until the 20th century. Research is still on-going about tsunamis.

May early geological, geographical and oceanographic texts refer to tsunamis as "seismic sea waves."

Waves!Waves (in general) have several parts. They have:A crest: the highest part of the wave.A trough: the lowest part of the wave.

A wavelength: the distance between one crest to the next or the distance between one trough to the next.

An amplitude: the distance between one crest and one trough.

The Tsunami Wave! Tsunamis have a small amplitude (wave height)

offshore and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea.

The very long wavelengths means they are always in contact with the ocean floor.   This causes them to gradually slow down as they travel across the ocean. 

  While the tsunami is moving toward the shore, they

usually form a very, very small wave (no higher than 12 inches above the normal wave weight).

Wave amplitude of the Indonesian Tsunami

As the tsunami approaches shallower water, the tsunami grows in height!

How a tsunami is generated by subduction!

Stranded Boat

Tele-tsunamis! A teletsunami occurs when an extremely

powerful (subduction) earthquake generates a powerful tsunami that can cross an entire ocean!!!

Three teletsunamis have been recorded in recent history: Valdivia, Chile earthquake (1960) (9.5) Prince William Sound, Alaska earthquake

(1964) (9.2) Sumatra, Indonesia earthquake (2004) (9.1-

9.3)

Tsunami Size!

The size of the tsunami wave can be small (less than a centimeter) to gigantic (over 100 feet high!)

Indonesian Tsunami

The Largest Tsunami in the World

Occurred when a 9.3 earthquake struck off the coast on Sumatra, Indonesia. The waves were over 100 feet tall and over 230,000 people were killed in Thailand, Sri Lanka, Indonesia, and India.

Propagation of the Indonesian Tsunami

Indonesian Tsunami (December 26, 2004)

National Oceanic and Atmospheric Administration (NOAA) In the US, NOAA is responsible for tsunami

preparedness. There are 2 levels of warning: Tsunami Warning - Indicates that a tsunami

is imminent and that coastal locations in the warned area should prepare for flooding. The initial warning is typically based on seismic information alone.

Tsunami Watch - An alert issued to areas outside the warned area. The area included in the watch is based on the magnitude of the earthquake.

Tsunami Warning Systems NOAA established an early warning

tsunami system called DART II. DART stands for: Deep ocean Assessment and Reporting of

Tsunamis

Watch the animation and then tell me what the different parts of this system are!

http://nctr.pmel.noaa.gov/Dart/Jpg/DART-II_05x.swf