Inundation of sea water in Andaman and Nicobar Islands and parts of Tamil Nadu coast during 2004 Sumatra tsunami

Earthquakes of large magnitude generally above M 7.0, occurring in shallow depths of seabed fault zones, cause rapid and vertical displacement of sea water. This results in the generation of tsunamis. The tsunamis in turn generate a series of deep ocean waves that have larger wavelengths, greater amplitude, period and velocity compared to normal waves. These waves with tremendous energy propagate in the deep sea at a speed of 750–900 km/h. When they encounter shallow waters of islands and landmass, their velocity decreases but change in total energy of the tsunami remains constant. At locations like open coast beaches, bays and harbours, due to shoaling effect, the height of the waves increases causing rise in sea level even to an extent of 30 m at the coast. Thereafter, the sea water penetrates the coast with high speed and causes extensive inundation, which is called run-up. Run-up is usually expressed in metres above normal tide or mean sea level. Figure 1 shows a schematic representation of different aspects of inundation, including measurement of maximum run-up height. Run-up values can be used for determining the extent of vulnerability of human settlement, in coastal villages or towns and therefore they are useful in coastal land use planning. Run-up levels from the same tsunami within a coast or