The Tsunami Story
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| Figure 1. Click to see an animation of a tsunami generated by an earthquake. | |
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Tsunami is a set of ocean waves caused by any large, abrupt disturbance
of the sea-surface. If the disturbance is close to the coastline,
local tsunamis can demolish coastal communities within minutes.
A very large disturbance can cause local devastation AND export
tsunami destruction thousands of miles away. The word tsunami is
a Japanese word, represented by two characters: tsu, meaning, "harbor",
and nami meaning, "wave". Tsunamis rank high on the scale of natural
disasters. Since 1850 alone, tsunamis have been responsible for
the loss of over 420,000 lives and billions of dollars of damage
to coastal structures and habitats. Most of these casualties were
caused by local tsunamis that occur about once per year somewhere
in the world. For example, the December 26, 2004, tsunami killed
about 130,000 people close to the earthquake and about 58,000 people
on distant shores. Predicting when and where the next tsunami will
strike is currently impossible. Once the tsunami is generated, forecasting
tsunami arrival and impact is possible through modeling and measurement
technologies.
Generation. Tsunamis are most commonly
generated by earthquakes in marine and coastal regions. Major tsunamis
are produced by large (greater than 7 on the Richer scale), shallow
focus (< 30km depth in the earth) earthquakes associated with the
movement of oceanic and continental plates. They frequently occur
in the Pacific, where dense oceanic plates slide under the lighter
continental plates. When these plates fracture they provide a vertical
movement of the seafloor that allows a quick and efficient transfer
of energy from the solid earth to the ocean (try the animation in
Figure 1). When a powerful earthquake (magnitude 9.3) struck the
coastal region of Indonesia in 2004, the movement of the seafloor
produced a tsunami in excess of 30 meters (100 feet) along the adjacent
coastline killing more than 240,000 people. From this source the
tsunami radiated outward and within 2 hours had claimed 58,000 lives
in Thailand, Sri Lanka, and India.
Underwater landslides associated with smaller earthquakes are also
capable of generating destructive tsunamis. The tsunami that devastated
the northwestern coast of Papua New Guinea on July 17, 1998, was
generated by an earthquake that registered 7.0 on the Richter scale
that apparently triggered a large underwater landslide. Three waves
measuring more than 7 meter high struck a 10-kilometer stretch of
coastline within ten minutes of the earthquake/slump. Three coastal
villages were swept completely clean by the deadly attack leaving
nothing but sand and 2,200 people dead. Other large-scale disturbances
of the sea -surface that can generate tsunamis are explosive volcanoes
and asteroid impacts. The eruption of the volcano Krakatoa in the
East Indies on Aug. 27, 1883 produced a 30-meter tsunami that killed
over 36,000 people. In 1997, scientists discovered evidence of a
4km diameter asteroid that landed offshore of Chile approximately
2 million years ago that produced a huge tsunami that swept over
portions of South America and Antarctica.
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| Figure 2. Click to see the propagation of the December 24, 2004 Sumatra tsunami. | |
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Wave Propagation.Because earth movements
associated with large earthquakes are thousand of square kilometers
in area, any vertical movement of the seafloor immediately changes
the sea-surface. The resulting tsunami propagates as a set of waves
whose energy is concentrated at wavelengths corresponding to the
earth movements (~100 km), at wave heights determined by vertical
displacement (~1m), and at wave directions determined by the adjacent
coastline geometry. Because each earthquake is unique, every tsunami
has unique wavelengths, wave heights, and directionality (Figure
2 shows the propagation of the December 24, 2004 Sumatra tsunami.)
From a tsunami warning perspective, this makes the problem of forecasting
tsunamis in real time daunting.
