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Saturday, September 5, 2009

Is Barak Valley seismically prone to earthquake?

SILCHAR, Sept 4: Barak Valley along with the entire Northeast has been hit by an earthquake for the fourth time in the last 23 days, causing serious concern among the people of this region. According to many, four tremors in such a short time, the last to happen was at 1.30 am today, cannot but evoke the relevant question : how safe is our zone?

Quite significantly, the recent tremors with their origin along the Indo-Myanmar border did not cross the danger mark of 5.6 on Richter scale, causing no damage or loss of life. In the wake of Cachar tremor of 1984 on March 31, which hit Sonai block, a team of experts from Regional Research Laboratory of Jorhat and ONGC geologists from Nazira and Dehradun carried on their studies and investigations in order to find out the causes of earthquake, locate its epicentre and all other structural causes. 10 people died during the 1984 tremor.

According to their investigation, the 1984 Cachar tremor was the result of bending of a block of earth crust by stresses operating horizontally which reduced its elastic and plastic limit and suddenly yielded to fracture, displacement or faulting. The sudden shifting furnished the impulse which sent out vibrations or waves into surrounding areas of the earth, leading to the tremor.
Their observation is that earthquakes generally occur in regions of marked instability of the earths crust, such as geologically young mountains. Such a region is the zone of the Himalayas. It is therefore not surprising that Barak Valley and entire Asom which are an extended part of the Himalayas and are about 20 million years among the youngest mountains of the world are frequently shaken by earthquakes.

The Himalayas are still growing and expanding and this continental drift accounts for the drifting of entire land mass of the Northeast region from four to five cm every year. This makes the region, the findings say, seismically active. National Geophysical Research Institute of Hyderabad identifies two earthquake zones or seismic belts in the world- Trans Atlantic and Circum Pacific which unites at the corner of Indo-Myanmar border, making it the most unstable region in the world.

The first movement of a major earthquake is usually followed for days and even months by a succession of aftershocks. In the second Asom quake of 1819, the after shocks continued for a period of over 10 years. The 1897 Asom quake on June 12 is perhaps one of the most severe to occur in the world.

Though earthquakes in general are unpredictable, it is interesting to note that during the tremor of 1984, the villagers of Sonai in Cachar had in particular noticed unusual behaviour among bats and squirrels before their area was hit by the tremor. Geologists of ONGC in course of their investigations of the area discovered that the earth developed many linear cracks, squeezing out deep and grey coloured sand with some clay and hot water.

The eruptions through cracks were in the form of geysers. Strong yellowish tinge along with specks in rainbow colour discovered in water is believed to contain some ferrugineous contents.
Their studies have also revealed that a seismic belt termed as ‘Haflong Thrust’ runs through the Borail and North Cachar Hills. ‘Haflong Thrust’ forms part of the land set image of aerial survey of the ONGC.

As Barak Valley is part of the Arakan basin, tectonic movement results in folding or faulting of the zone which makes it quite susceptible to tremor.

Earthquake History of Barak Valley
Year Date Epicentre Richter scale
1869 January 10 Cachar 7.5
1924 January 30 25^ N 93E 6
1926 October 23 25^ N 93E 5.6
1957 December 12 24.5^ N 93^E 5.6
1984 December 31 Sonaimukh 6
1996 November 19 Cachar 4.8
1997 May 8 Bangla border 5.7
2001 February 27 26.7^ N 90.8^ E 4.7
2004 December 9 M’laya border 4.6
2005 February 15 Bangladesh 5.2

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