Is Barak Valley quake-prone, Sunday tremor raises question

SILCHAR, Sept 19: Barak Valley, along with the entire Northeast and North Bengal, was rocked by an earthquake in the evening of Sunday, which was the sixth in thelast 24 months, causing serious concern among people of this region. The tremor was felt at 6.10p.m., flowed by aftershocks. The occurrence has raised relevant questions about safety in the zone.
Fortunately the tremor that originated in Nepal-Sikkim borderand measured 6.8 on richter scale,caused no damage or loss of life here. In the wake of the Cachar tremor in 1984 on March 31 whichhit
Sonai block, killing 8 persons in the area, a team of experts from Regional Research Laboratory, Jorhat, and ONGC geologists from Nazira and Dehradun carried out aninvestigations to find out the causes of earthquake, pin point its epicentre and all other structural causes.
According to their finding, Cachar tremor of 1984 was the result of the bending of a block of earth's 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, causing the tremor.
Their observation is that earthquakes generally occur in regions of marked instability of the earth's crust, such as geologically young mountains. Such a region is zone of the Himalayas. It is therefore not surprising that Barak Valley along with entire Assam is an extended part of the Himalayas and therefore frequently shaken by earthquakes.
The Himalayas are still growing and expanding and this 'continental drift' accounts for the drifting of the entire land mass of the Northeast region from 4 to 5 cm every year. This makes the region, the findings say, seismically active. National Geophysical Research Institute of Hyderabad, in course of their study, identifies two earthquake zones or seismic belts in the world– Trans Atlantic and Circum Pacific – which unite at the corner of Indo-Myanmar, making it the mostunstable region in the world.
The first movement of a major earthquake is followed usually for days or months by a succession of aftershocks. In the second Assam quake of 1819, the aftershocks continued for a period of 10 years. The Assam quake of June 12, 1897 is perhaps one of the most sever inthe world to occur.
Though earthquakes are unpredictable in general, 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, birds and squirrels before their area was hit by tremor. Geologists of ONGC in course of their investigations of the area discovered that the earth developed many linear cracks, squeezing out deep grey coloured sand with some clay and hot water.
The eruptions through cracks were in the form of geysars of strong yellowish tinge along with specks in rainbow colours, discovered in water, which is believed to contain some ferrogeneous contents. Their studies have also revealed that a seismic belt termed as 'Haflong Thrust' runs through the Borail and North Cachar Hills. As Barak Valley is a part of the Arakan basin, tectonic movement results in folding or faulting of the zone which makes it quite prone to tremor.
Earthquake history of Barak Valley
Year Date Epicenter Richter Scale
1869 January 10 Cachar 7.5
1924 January 30 25*N/93*E 6
1926 October 23 25*N/93*E 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 Bangladesh Border 5.7
2001 February 27 26.7*N/90.8*E 4.7
2004 December 9 Meghalaya border 4.6
2005 February 15 Bangladesh Border 5.2
2009 August 11 14.1*N/93*E Andaman 7.8
2009 August 12 Indo-Myanmar 5.6
2009 August 19 26.6*N/92.5*E,Sonitpur 4.9
2009 August 31 25.4*N/94.8*E
Indo-Myanmar 5.3
2009 September 3 Manipur E/S of Imphal 5.9 (SentinelAssam)