Tremor of anxiety


FROM THE STATESMAN

The epicentre of recent earthquakes has been traced to Sikkim, Darjeeling and the Bhutan Himalayas. Considering the possibility of great damage to life and property the administration ought to take urgent steps, says Santanu Basu

FROM a gentle ripple to a terrifying and violent movement an earthquake rocks whatever part of the globe it strikes and devastates life, habitat and property. As a matter of fact, no other calamity causes such havoc. The huge loss of life and property at Kutch in Gujarat, the devastation wrought at Latur in Maharastra and that in the Indo-Pak border are still fresh in public memory. The long stretch from Bhutan and Assam to North Bengal felt the vibrations as an earthquake shook this vast geographical zone on 21 September last.

Earthquakes are tremors in the ground, created by sudden convulsions of tectonic plates — huge slabs of rocks that lie at the subterranean parts of the planet. The tremors of 21 September were not only confined to Guwahati and Bhutan; Kolkata, too, experienced it and Malda was no exception.

This quake which had its epicentre at Mongar in Bhutan and shook North Bengal, Assam and the North-eastern states recorded 6.3 in the Richter Scale. Though severe, it, fortunately, didn’t inflict much damage in North Bengal, but wrecked Bhutan badly; some high-rises in Guwahati sustained major cracks.

A majority of buildings in Bhutan were reportedly damaged. Earthquake, however, is not a sporadic feature in this zone, it occurs on and off. In recent years, there occurred a series of such natural aberrations causing great tremor and panic among locals. In the last two months, as many as eight incidents measuring 4.6 to 6.3 in the Richter Scale were reported in North Bengal and the North-eastern states — the epicentre being Myanmar. Hence, Bhutan has become the most earthquake-prone zone in the country.

Although earthquakes occur everywhere, they are a great many “prone” zones. These zones such as Japan and California lie near the moving margins of the tectonic plates called faultlines. The crust of the earth is 40 km thick and its mantle is 1,400 km in diametre. The lithosphere and esthenosphere comprise the whole of the earth’s crust downwards and the upper portion of it measures around 100 km. The planet’s outer layer of rock varies in thickness — beneath the oceans it is about 11 km (four to seven miles) but stretches up to 70 km (43 miles) under mountain ranges. By recording the vibrations of earthquakes scientists have discovered a “yolk” of metal surrounded by an “egg-white” called mantle at the centre.

And this mantle plays an important role in generating earthquakes. The theory of plate- tectonics states that the crust stands on six continental and oceanic plates. Tectonic plates usually slide past each other, but sometimes they get stuck together. The stress on the rock builds up until they fault, that is crack. The tectonic plates then jolt past each other, sending shock waves through the ground.

This movement is usually classified under three heads — convergent when a plate enters into another; divergent when a plate separates from another; and parallel when two plates exist simultaneously. The movements of the tectonic plates in the mantle finally determine the causes and intensity of an earthquake.

The edge of the Indo-Oz plate and the Eurasian plate at the subterranean level come to the northern parts of the Himalayas and North Bengal. Geographically, the Indian territory is north-centric and tilts northwards. Besides, the Himalayas also grows by an inch (on an average scale) every year and friction by the two tectonic plates are but natural events, the impact of resultant energy can be felt at a long distance from its origin. Though on an average an inch of growth is of minuscule significance, this uncommon natural phenomenon makes vast stretches of territory fragile and earthquake-prone.

The geological effects of an earthquake can be imagised as waves in a calm pond created as a result of a stone being thrown. Once a stone is thrown concentric waves are created that approach towards the bank.

The friction originating at the inter plate in the lithosphere and ethensphere zones in the mantle reach the crust. The similarities, between the spread of concentric waves and tremors in a pond with the vibrations reaching in a straight line the crust, are very appropriate. So the straight line — if drawn — from the actual point of friction at subterranean level is how the tremors reach the crust causing damage.

The Indian Met department, depending on the frequency of tremors in the “prone” zones in the North-eastern states or the not-so-vulnerable regions, has prepared a map and classified different areas between one and five in the Richter Scale. According to such classification, the Himalayas and adjacent Kutch, Andaman and Nikobar are the most vulnerable zones in the country. Cooch Behar and the North-east, including Assam, have also been classified and bracketed five on the list. But Jalpaiguri and Sikkim are placed at category four.

In short, North Bengal should be classified as a “prone” zone. The three districts, Malda and North and South Dinajpore, however, have a distinct advantage; earthquakes are less likely to take a heavy toll in these parts. Seismologists in Bangalore believe that the landslips in the Darjeeling-Himalayas were an exception because it is impossible to ascertain whether those were triggered by earth movements or had a natural origin.

According to scientists, a majority of earthquakes with low-intensity tremors — measuring below four in the Scale — often take place that neither stir us because of distance nor inflict much damage on life and property. So the gentle ripple often remains unnoticed. Among the major quakes causing great damage during the last 50 years, some of the grievously destructive were the 30 August 1964 quake measuring 5.2 in the Richter Scale that which occurred on 29 November 1980 measuring six in the Scale and the 27 September 1988, 2 December 2001 and 14 February 2006 measuring 6.7, 5.1 and 5.3 respectively. The epicenter of these quakes has been traced at 27 degree north of latitude and 88 degree longitude. In other words, the epicenter lies at Sikkim, Darjeeling and the Bhutan Himalayas. On the other hand, the epicentre of quakes in Assam and the North- east is at the Indo-Myanmar borderline and adjoining areas.

The writer is lecturer in Political Science, Chanchal College, Malda