Coastal Zones

Biography

Biography: Dr Thiruvenkatasamy

Abstract

Tsunami is a very rare transient seismic event with long return period. The return periods of destructive tsunamis could be of the order of century or more. For country like India, the word tsunami was almost unknown to the public in general and very few earth scientists were aware of the word tsunami. When the 26 December 2004 Sumatran earthquake caused destructive tsunami, Indians became aware of this natural happening. Tsunami is product of large magnitude earthquake (usually M > 7.5). India had been experiencing very large magnitude earthquakes up to Magnitude of 8.7 (the 12 June 1987 Assam Earthquake). But most of these earthquake are confined to Himalayan regions. In absence of any historical records and references about tsunami on eastern side of Indian coast, most of the observed tsunami data were interpreted based on the available literature from the littoral countries of Pacific. Receding of sea few minutes before the advent of tsunami was unknown to India coastal population. On 26 December 2004, at some locations, the sea had receded by a noticeable distance of about 1000 to 1500 meters. Everybody watched this receding and not knowing the precursory tsunami signal in this process, no administrative action could be initiated. This tsunami had taken a very heavy death toll of about 2, 50,000 lives in several Asian countries. After the tsunami the authors tried to understand why there was excessive and extensive damage at locations near Nagapattanam and Chennai. Similar destructive picture was observed on the other side on Thailand Coast. . At these places the height of tsunami waves was up to 7.0 meters. While at distances of about 100 km on either sides of coast in India and Thailand the tsunami heights were about one meter of less and there was no damage. It is important to know the scientific reason for this uneven distribution of destruction by tsunami waves. The Cocos Island located at similar distance from the rupture had only 42 cms of tsunami. During the Sumatran earthquake, the ground rupture was of about 1200 km and it took about 7 minutes and 28 seconds to complete the rupturing process. It was seen that the locations which were at right angles to the rupture suffered the highest tsunami waves. However, the energy and amplitudes of the waves is different in different directions. Nagapattanam of Indian East coast and Thailand coast were hit by high tsunami, while Cocos Island and locations in Sri Lanka and Nagapattanam are at same distance from the ruptured fault. The seismic waves at the hypocenter travel in 4π directions while the tsunami waves travel in 2π directions. It is observed that the waves travelling at right angles to the direction of rupture have maximum energy and amplitude. Waves travelling in the direction of rupture have minimum energy and amplitude. Taking the value of highest tsunami height of 7.0 m at Nagapattanam and rupture direction, tsunami wave heights at different locations on East Indian Coast have been calculated. These values are useful for developmental and expansion activities at various ports, harbors on coast. Further there are two major delta region viz Godavari Delta and Krishna Delta the tsunami height values would also be useful for these delta region. It is proposed to discuss the likely impact of tsunami waves at various ports. The most important finding the study has given is that the Kolkata (Calcutta) and Bangla Desh would not face any destructive tsunamis.