One reason behind the relief: this quake fault line pointed the other way

While experts were keeping their fingers crossed as the hours clocked by on Monday night and into early Tuesday morning, there was growing c...

While experts were keeping their fingers crossed as the hours clocked by on Monday night and into early Tuesday morning, there was growing consensus that the reason why the quake had not triggered off tsunamis was the direction of the fault line and the position of the epicentre.

By 12.30 am, scientists at the National Institute of Oceanography in Goa and the National Geophysical Research Institute (NGRI) in Hyderabad had scoured seismological data and projected that the threat was remote.

Speaking to The Indian Express, NGRI director R K Chaddha said that tonight’s epicentre was close to the ‘‘triple-junction of three tectonic plates,’’ the Indian, Australian and Burmese plates in the Sunda trench of the Indian Ocean. This was at 2.3 North latitude, 97.1 East longitude, 30 km below the ocean floor off the coast of northern Sumatra.

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Here, the fault line begins from the Andamans, Chadha said, and stretches towards Sumatra, Java and Indonesia. So at the epicentre, the fault line runs in a north-west to south-east direction. The energy, therefore, would be focused in the south-west direction from the epicentre—a direction perpendicular to that of the fault line.

‘‘This means that the wave is directed at the open Indian Ocean and because the epicentre this time is 100 km south of the December 26 quake, India is very likely to remain on the periphery,’’ Chaddha said. If the waves hit the coast, he said, the maximum height at the shoreline would be just 1 m.

Another factor that could explain the absence of a tidal wave, experts said, is the nature of displacement of the ocean floor. A tsunami is triggered by vertical displacement of the ocean floor at the epicentre. he quake occurred 30 m below the sea-bed (the same depth of the Dec 26 quake of 9 on the Richter scale). ‘‘Because no tsunami wave has been recorded,’’ Chaddha said, ‘‘there could have only been horizontal displacement of the sea-bed along the fault line.’’

This was reinforced from Indonesia. Said Dr Harsh K Gupta, secretary, Department of Ocean Development in New Delhi: ‘‘The Home Ministry issued a tsunami warning but I have just spoken to Jan Sopalehwakan, the director of the Indonesia Institute of Sciences, and he has told me that there was no tsunami effect seen till 11.30 pm IST.’’

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In Goa, at the National Institute of Oceanography, director Satish R. Shetye said: ‘‘The information available with us is from the tsunami warning issued by the West Coast and Alaska Tsunami Warning Centre of the NOAA in Hawaii. We are on alert, but according to tide-gauge data made available to us from Port Blair, there has been no change in sea-level readings even after two hours of the quake.’’