A powerful undersea earthquake measuring 8.5 on the Richter scale off Indonesia triggered fears of a tsunami in India and other countries in the region and kept hundreds of thousands of people in states along Indias east coast nervous for nearly two hours this afternoon. In the end,a tsunami did hit the Indian coastline,but it was barely noticeable,and too weak to cause any damage. The most the sea rose in Indian territory was 30 cm,recorded in Campbell Bay in the Nicobar Islands. In Chennai,the sea rose 10 cm,and in Vizag,8 cm. The Tsunami Warning Centre in Hyderabad issued an alert immediately after the quake struck at 2.08 pm India time today. For some areas,including the Andaman and Nicobar Islands,it issued a warning which meant that people were evacuated from coastal areas. An alert is generally issued if the tsunami waves are expected to be between 0.5 m and 2 m in height; a warning is issued if the waves are expected to be bigger. About an hour after the first tremors though,it became reasonably clear that a 2004-like disaster in which over 1,50,000 people were killed after an earthquake off Sumatra generated waves as high as 30 m was unlikely to occur. By that time,the first waves cause by the earthquake had travelled to the Nicobar Islands,the closest point in India from the epicentre,about 700 km away,and the rise in sea level was not more than 30 cm. However,the alerts were not immediately withdrawn as fears lingered of aftershocks that generally accompany earthquakes of such magnitude. Two aftershocks did follow of 6.0 magnitude at 3.07 pm,and of 8.2 at 4.30 pm but neither generated a major tsunami. Apart from several other factors,two important conditions for a major tsunami are an earthquake above 7 in magnitude,and a vertical movement of the faultline. In this case,while the magnitude was very high,the vertical movement that causes a large displacement of water was missing. There was more of lateral movement,because of which a major tsunami did not take place, said Dr Shailesh Nayak,secretary in the ministry of earth sciences,and chairman of the Hyderabad-based Indian National Centre for Ocean Information Systems (INCOIS) that houses the Tsunami Warning Centre. Once we came to know that the sea level changes in the Nicobar Islands were not significant,we were reasonably reassured that nothing big was likely to happen, he said. The alerts and warning were withdrawn later in the afternoon and,at 6.18 pm,an all clear was declared.
