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Tsunami - A horrible disaster

by Sreekumar B.A on 12/27/2004 | Comments | Viewed 8212 time(s) | Full Page View

What is a tsunami?

A tsunami (pronounced su-nah-me) is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

What does "tsunami" mean?

Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.

Chennai's famous Marina Beach swept by raging waves. (AFP Photo)

The sea water logged at a road near Marina Beech, Chennai after tidal waves hit the coastal area of Tamil Nadu on Sunday. (PTI Photo)

People walk through debris of their houses destroyed in tidal waves on the coastal areas in Colombo, Sri Lanka on Sunday. Massive sea waves, apparently triggered by oceanic earthquakes, crashed into coastal villages over a wide area of Sri Lanka on Sunday, killing at least 500 people and displacing 1,00,000 others. (AP Photo)

How do tsunamis differ from other water waves?

Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterised as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth. Let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses. The earthquake-generated 1960 Chilean tsunami, for instance, travelled across over 17,000 km across the Pacific to hit Japan. The wave crests bend as the tsunami travels —- this is called refraction. Wave refraction is caused by segments of the wave moving at different speeds as the water depth along the crest varies.

How do earthquakes generate tsunamis?

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created. Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

Wailing relatives in front of the dead bodies of their loved ones in Chennai on Sunday. Tidal waves in the city claimed many lives following the massive earthquake in Indonesia. (PTI Photo)

People look at the vehicles and other debris lying on the Marina Beach in Chennai on Sunday after it was hit by tidal waves following the massive earthquake in Indonesia triggering tidal waves and flooding parts of coastal India. (PTI Photo)

European tourists walk the beach of Kata assessing damages done by the tidal wave that hit Kata beach in Phuket, Thailand on Sunday. One of the most-powerful earthquakes in years triggered massive tidal waves that slammed into coastlines across Asia on Sunday, killing more than 700 people in Sri Lanka, Indonesia, India, Malaysia and Thailand. Tourists, fishermen, homes and cars were swept away by walls of water unleashed by the 8.9-magnitude, centered of the west coast of the Indonesian island of Sumatra, officials said. (AP Photo)

How do landslides, volcanic eruptions, and cosmic collisions generate tsunamis?

A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsuna-mis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area. 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. If you read the "How do tsunamis differ from other water waves?" section, you discovered that a tsunami travels at a speed that is related to the water depth — hence, as the water depth decreases, the tsunami slows.

The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.

What happens when a tsunami encounters land?

As a tsunami approaches shore, we've learned in the "What happens to a tsunami as it approaches land?" section that it begins to slow and grow in height. Just like other water waves, tsunamis begin to lose energy as they rush onshore — part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsuna-mis may reach a maximum vertical height onshore above sea level, often called a run-up height, of 10, 20, and even 30 meters.

People look at the vehicles and other debris lying on the Marina Beach in Chennai on Sunday after it was hit by tidal waves following the massive earthquake in Indonesia. (PTI Photo)