A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 metres) onto land. These walls of water can cause widespread destruction when they crash ashore. Tsunamis can race across the sea at up to 500 miles (805 kilometres) an hour—about as fast as a jet airplane. These awe-inspiring waves are typically caused by large undersea earthquakes at tectonic plate boundaries (Fig. 1). When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.
Tsunami is a combination of Japanese terms ‘tsu’ (harbour) and ‘nami’ (wave) believed to be coined by fishermen who witnessed devastated coasts upon their return to port although they were unaware of any large wave in the open waters – tsunami remains unnoticed at the sea surface forming only a passing ‘hump’ in the ocean.
A tsunami’s appearance and behaviour is dependent on several local factors. Two of the most important factors are the topography of the sea floor and the actual shape of the shoreline. Even small earthquakes, otherwise ineffective, may trigger a submarine landslide quite capable of generating a tsunami. However, the presence of an offshore coral reef can dissipate the energy of a tsunami.
National Tsunami Earth Warning Centre, Control Room, INCOIS, Hyderabad
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Large vertical movements of the earth’s crust can occur at plate boundaries. Denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis. The approximately 1,000 mile long tsunami on December 26th, 2004 off the coast of Sumatra showed a wall of water travelling at some 500 miles an hour westward from Sumatra towards Sri Lanka and India. The waves were formed as the displaced water mass moves under the influence of gravity to regain its equilibrium and radiates across the ocean like ripples in a pond.
The international tsunami prediction for the Pacific Ocean and early warning is overseen by the US Pacific Tsunami Warning Centre (PTWC) which was established in 1949 and is operated by National Oceanic and Atmospheric Administration (NOAA). The Centre has issued a total of 20 warnings since it was first established. The warning systems consist of two components – a network of sensors and a communication infrastructure for timely alarms. Many areas around the Pacific – Japan, Hawaii, Alaska and the Pacific coasts of South America have tsunami warning systems and evacuation procedures.
Recognising the imperative to put in place an early warning system for the mitigation of oceanogenic disasters that cause severe threat to nearly 400 million of the Indian population that live in the coastal belt, the Ministry of Earth Sciences (MoES) has established the National Early Warning Centre in collaboration with the Department of Science and Technology (DST), Department of Space (DOS) and the Council of Scientific and Industrial Research (CSIR). The National Tsunami Early Warning Centre has been set up at the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad. The system in place now is based on near-real time determination of earthquake parameters in the two known tsunamigenic zones of Indian Ocean region, using a network of land based seismic stations. Also established is a comprehensive real time ocean observational network comprising bottom pressure recorders around the two tsunamigenic zones, tide gauges, radar-based coastal monitoring stations etc. The National Early Warning Centre generates and disseminates timely advisories to the Ministry of Home Affairs for further dissemination to the public. In addition, messages are also sent to authorised officials. In case of confirmed warnings, the National Early Warning Centre is equipped with necessary facilities to disseminate the advisories directly to the administrators, media and public. The cyclone warning network of IMD and electronic ocean information boards of INCOIS can also be effectively used for dissemination of warnings directly to the public.