Tsunami: A new name in earthquake disaster !
J. N. Singh, IAS
Member Secretary
Gujarat Council of Science City
For thousands of fisher folk, who had gone like every morning into the sea, it was again the same story - suddenly being caught in a phenomenon, tsunami, which struck India for the first time in recorded history.
The country was still coming to grips with the nature and scale of the disaster. The waves have now claimed almost 4,000. And there's no news yet of 45,000 people in Nicobar and Greater Nicobar where a quake of 7.5 magnitude hit early on Sunday morning.
The trigger for the tsunami - a destructive wave train created by an undersea disturbance - was an 8.9 magnitude earthquake, just off the northern tip of Indonesia's Sumatra island.
Every year, disasters cause the death of a million people and leave several millions more homeless. Economic losses caused by natural disasters have tripled in the past 30 years. Working together continually as a community can help reduce the impact of disasters.
A hazard is a phenomenon or a process, either natural or human made, that can endanger a group of people, their belongings and their environment, if they do not take precautions.
There are different types of hazards. Some are natural while others are caused by human beings, such as so-called industrial or technological hazards (explosions, fires, toxic chemical spillages). Wars and terrorism are also hazards caused by human beings.
The feature describes the science of tsunami and its subsequent effects to our natural weather and climatic conditions.
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. Tsunamis are large water waves, typically generated by seismic activity, that have historically caused significant damage to coastal communities throughout the world.
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.
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 characterized as shallow-water waves, with long periods and wavelengths. The wind-generated swell at Sumatra beach, for example, spawned by a storm out in the Indian ocean 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 wavelengths, 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.
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.
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.
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?" 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.
Earthquakes, floods, hurricanes, volcanic eruptions and landslides all these are natural phenomena that have occurred throughout the history of humankind. However, rapid population growth, environmental pollution and degradation, and increased poverty, have all contributed to turning these natural phenomena into disasters that cause enormous losses in human lives, infrastructure, and material belongings.
The Gujarat Science City working under the aegis of Department of Science and Technology, Govt of Gujarat is emerging as a large-scale science popularization platform in the country. The Gujarat Science City is developing a unique pavilion "Planet Earth" that aims to provide the educational community and children with an innovative and interactive tool for risk management.
The Planet Earth Pavilion in the Gujarat Science City is going to be a unique pavilion for educating the visitors about our own planet Earth and its rich biotic and abiotic resources, natural catastrophes and technological advances we can achieve sustainable development.
Shri Narendra Modi, Hon'ble Chief Minister of Gujarat performed the ground braking ceremony on 29th October 2004. The construction work has already been started and to be completed by January 2006. The Asian Development Bank has providing the financial assistance through the Gujarat State Disaster Management Authority (GSDMA) for the development of the above pavilion.
Reference: University of Washington's Department of Earth and Space Sciences
Fig 1: In the figure the waves are greatly exaggerated compared to water depth! In the open ocean, the waves are at most, several meters high spread over many tens to hundreds of kilometers in length.
Fig 2--Split: Within several minutes of the earthquake, the initial tsunami (Fig 1) is split into a tsunami that travels out to the
deep ocean (distant tsunami) and another tsunami that travels towards the nearby coast (local tsunami).

Fig 3--Amplification: Several things happen as the local tsunami travels over the continental slope. Most obvious is that the amplitude increases. In addition, the wavelength decreases. This results in steepening of the leading wave--an important control of wave runup at the coast.

Fig 4--Runup: As the tsunami wave travels from the deep-water, continental slope region to the near-shore region, tsunami runup occurs. Runup is a measurement of the height of the water onshore observed above a reference sea level.

|