What happened on Wednesday?
Back-to-back tsunami alerts were issued for Indonesia after a powerful earthquake and a series of strong aftershocks struck. So widespread was the alarm that 28 countries, including India, all along the Indian Ocean rim went into high alert in the afternoon.
The US Geological Survey said the first quake had a magnitude of 8.6 (Indian agencies put it at 8.5). Sirens sounded along coastlines and warnings spread like wildfire by mobile phone text messaging. Countless office workers in many cities, such as Calcutta and Bangalore, left their buildings while the port of Chennai closed down.
Just as the region was sighing with relief, an 8.2-magnitude aftershock followed
How did it end eventually?
No deadly waves or serious damage was reported till late Wednesday night. However, some areas close to the epicentre are remote and it could take some time to find out if there was any damage.
The watch for much of the Indian Ocean was lifted after a few hours. India withdrew the national tsunami alert in the evening
How does it compare with the December 26 earthquake of 2004?
Both earthquakes occurred under the sea off the west coast of northern Sumatra, but at different locations. The 9.1-magnitude earthquake of December 26 occurred on the boundary between the India plate and the Burma plate but the 8.6-magnitude earthquake on Wednesday occurred a significant distance away from this boundary. Both are potentially tsunamigenic
Then why was there no significant tsunami wave on Wednesday?
Preliminary observations suggest differences in the mechanisms of ground motion associated with the two earthquakes. The 2004 earthquake involved a vertical slip motion, a subduction, the Indian plate lurching beneath the Burma plate. Today's earthquake appears to have been associated with a horizontal slip motion of the Indian plate. Such an earthquake is unlikely to lead to a significant tsunami
Is there a level of magnitude beyond which we should prepare for a tsunami?
Scientists treat all undersea earthquakes beyond magnitude 6 as capable of triggering tsunami waves. Earthquakes of magnitude 8 or higher that occur in the northern Indian Ocean zone, off the coast of Sumatra, have the potential to generate widespread and destructive tsunami waves that can affect coastlines across the entire Indian Ocean basin
Which factor triggers a tsunami and determines the height of the waves?
Tsunami waves are triggered when an undersea earthquake displaces a large volume of water. The type of ground motion during the earthquake determines how much water is displaced — a vertical slip motion is likely to displace a huge volume of water, while horizontal slips are less likely to cause large waves. The undersea terrain along the movement to the coast can also influence the tsunami waves
How high were the waves on Wednesday?
Nicobar reported a 30cm wave while parts of the east coast may have received a 10cm wave. Neither warrants any cause for alarm. Contrast this with 2004: the waves then had climbed to 25 metres in Indonesia and 2.5 metres in Chennai
Why was the Chennai port shut and ships asked to move out to high seas?
The tsunami waves, which move at incredible speeds of up to 600kmph in the high sea, gain height only as they approach the coast. The wave height is not significant at great distances from the port and ships are likely to be safer there than while berthed at a port. A really big tsunami wave could even lift entire ships and carry them onto the land
What precautions should citizens take when alerts are issued?
Tsunami alerts are intended to get people away from the coast. The advisory typically asks people to move away from the coast to higher ground, as high and as far away as possible from the sea. What distance is safe will depend on the size of the tsunami waves. During the 2004 tsunami, the water ingressed 200 metres to 590 metres at sites in Chennai
At the end of the day, did everyone overreact?
No. Although some responses may have looked a bit unwarranted in hindsight, what a large populace faced on Wednesday was the unfathomed power of Nature. It is always better to be prepared than be taken by surprise as had happened in 2004.
Besides, Wednesday’s events threw up a chance to test crisis-management systems in a real-life situation. The systems appeared to have clicked into place on time, according to preliminary assessments. “Our records indicate that all the national meteorological services in the countries at risk by this tsunami received the warnings in under five minutes,” said Maryam Golnaraghi, the head of the World Meteorological Organisation’s disaster risk reduction programme.
Compiled by GS Mudur, Meghdeep Bhattacharyya. Additional reporting by AP & Reuters