Tsunami Characterization Studies

A B Inamdar, Centre of Studies in Resources Engineering

 

Tsunamis are sea waves generated as a result of water displacement in the sea. This could happen due to a variety of reasons: earthquakes, landslides, volcanic eruptions, explosions, impact of cosmic bodies and sometimes, or even detonation of a nuclear device. However, like earthquakes, prediction of a tsunami is complicated and difficult. But as was witnessed by the recent tsunami that originated at Sumatra on December 26, 2004, the damage and devastation by tsunamis can be far more serious than that by the earthquakes. To minimize such losses, it is important to understand how tsunamis are generated and can affect our lives.

         Tsunamis are normally a series of sea waves (not of a 'single huge wave') caused by a natural or artificial disturbance that vertically displaces the water column above it. So far, most destructive tsunamis have resulted from large earthquakes, with epicenters/fault lines near the ocean floor. Vertical displacement of the earth's crust along such fault lines can generate very powerful waves which travel with great speeds across oceans causing devastation along their path. The speed of a tsunami wave is decided by the volume of displaced water (i.e. depth of water column where it gets generated). The speed diminishes, while the wave height increases as these waves approach the coasts.

The Nature of Tsunamis
According to the continental drift/plate tectonics hypothesis, the continental plates, each about 50 mile thick, are continuously moving due to the convectional currents generated by the heat transfer processes in the earth's mantle. New crust is being formed at the sea floors where the plate boundaries diverge. These plates converge at some locations on the earth leading to friction, subduction and demolition of the plate boundaries. As a result, these plate boundaries are a constant source of earthquakes all over the earth. When these earthquakes occur beneath the sea, the water column above gets displaced. This displaced water column in turn pushes the adjoining volume of water out in the form of a wave that gives rise to a tsunami. However, the amount of vertical displacement is what determines the tsunami wave features, and not all disturbances underneath the sea necessarily lead to tsunami waves.

         Tsunamis differ from the wind generated waves in several ways. The most important difference is wind generated waves normally have a wavelength of up to 100-200 m whereas the wavelength of tsunami waves can exceed 500 kms. Tsunami waves behave as shallow water waves due to their long wavelengths and hence travel very fast in deeper waters (upto 900 kms/hr) with minor energy loss. Due to very small (sub-meter) amplitudes they are not even felt aboard ships. But as they get into shallower waters they slow down leading to increase in their wave heights. This result from piling up of energy and decrease in the distances between individual waves (called `shoaling').

         The shoreward propagating energy results in the bottom friction and turbulence resulting in heavy coastal erosion. The elevation to which such destruction can happen is termed as `run-up level' and is expressed in meters above the mean sea level. The run-up levels are determined by the obstructions on the path of the wave, like geomorphic features, thick vegetation growth as well as artificial structures. The extent of tsunami damage is decided by the 'run-up levels', which in turn are dependent on the resistance offered by the features on land, like topography and land use. Hence, tsunami characterization studies mainly include the understanding of local geomorphology and land use changes and measurement of run-up levels. ...........more on next page