North-East may be rocked by a 'mega-quake'

North-Eastern India is likely to be rocked by a mega-earthquake measuring nearly 7.5 on the Richter Scale within a decade.

Geophysicist Dr J G Negi said the North-East had been earlier rocked by a mega-quake of 8.4 on the Richter Scale in the last decade of the 19th century followed by an equally massive quake on August 15, 1950. The return period of earthquakes in this region appeared to be around 50 years and therefore, he said, there was a distinct possibility of another by the year 2010 or before.

Dr Negi warned that while the energy released in the Latur earthquake (6.3 on the Richter Scale) in 1993 was equivalent to only one hydrogen bomb, the possible quake in the North-East could be as powerful as 50 hydrogen bombs. However, the loss of life and property depended on the population of the area where the quake's epicentre was located, he said.

The more the time gap between a previous and the next quake, Dr Negi said the more destructive the latter would be as energy accumulation was proportionate to the magnitude of the quake. One likely hot-spot for the next quake in the North-East could be the Shillong plateau where the folded Himalayas were quite prone to seismic activity.

The next quake in the North-East could be even more destructive, measuring about 8.5 on the Richter Scale, the same as the last century's, if its time gap compared to the 1950 quake was greater, he added.

Dr Negi said the movement of continental plates, called the plate tectonic movement, was the major reason for this natural disaster, especially in the case of the Indian peninsular shield which had many rift zones due to presence of hidden faults in the earth's crust.

Some of the rift zones in the country were located in the faults of the valleys of Koyna, Kurudwadi, Godavari, Krishna, Narmada, Tapti, Mahanadi, Damodar and Sone river systems. Most of these were earthquake-prone and quakes have already rocked these areas in the past. The Damodar rift had caused an earthquake in 1737 in which more than 300,000 people were killed, Dr Negi said.

He pointed out that any of these rifts could be activated in the future due to the continuous spreading of the Indian Ocean floor from the Carlsberg ridge and the plate tectonic forces pushing India into Eurasia towards China. In other words, India is being pushed towards China, the reason why the Himalayas were formed as folded mountains. Due to this movement, the Himalayas, especially in the North-East, were highly earthquake-prone, he added.

The earth, he said, was made of continental plates which, in turn, were made of sub-plates, whose continuous movement results in friction due to their rubbing together along the rifts and valleys of weak zones that run several kms deep into the earth.

Dr Negi said the quakes in the Himalayan range could, therefore, be due to India being pushed into China. On the other hand, the quakes in the rest of the country, especially in the brittle peninsular area, are due mainly due to the rubbing together of land masses against each other along the rift zones. Both these processes were inter-linked, he added.

He said the exact movements and chronology of plate tectonic forces were yet to be studied in detail. When would a particular plate or sub-plate move is difficult to say. That is why it was not yet possible scientifically to predict the exact date and place of a likely earthquake, especially in the case of the Himalayas, he said.

In the case of the rift systems, however, swarms of tremors usually come as a warning prior to an earthquake, through which the plates engage themselves in crystal adjustment through release of energy. For instance, the quakes at Koyna (1967), Kurudwadi (1993) and Jabalpur were all preceded by such tremors. Even Khandwa, also on the Narmada banks, had received such swarms in 1993 as a warning to a possible quake in future, he added.