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Seismologists find UP quake extraordinary

Pradip K Bagchi in New Delhi

Seismologists are concerned over the frequency at which the earth has quaked in the Himalayan region.

Prior to last night's earthquake in Uttar Pradesh's Chamoli district, which measured 6.8 on the Richter scale, Uttarkashi experienced a devastating quake on October 20, 1991. It measured 6.6, and led to massive loss of life and property.

''In seismically active areas, a quake measuring more than 6.5 may occur once in 20 years, and that measuring six once in 10 years,'' said Dr H N Srivastava, emeritus scientist in the Council of Scientific and Industrial Research.

According to a statistical analysis which Dr Srivastava and CSIR Director (seismology) R S Dattatreya carried out in 1986, a quake measuring more than seven on the Richter scale may occur in a particular area once in 40 years and more than eight only once in 200 years.

''In a terrain like the Himalayan region, the intensity decreases at a much faster rate with distance than in the plains. The rapid attenuation at low frequency implies that only high rock-filled dams should be constructed in these areas,'' Dr Srivastava said.

What makes the Himalayan region quake-prone is the north and northeastern movement of the Indian Plate at the rate of about 5 centimetres per year and its collision with the Eurasian Plate, which is relatively stationary. The movement gives rise to accumulation of stress. As it exceeds the bearing capacity of the rocks, quakes occur due to rock-slippage.

There are two fault lines passing across the Himalayan region - the main central thrust and main boundary fault. Chamoli and Uttarkashi fall on the MCT while the MBF passes through Shimla-Dehra Dun-Almora. The entire area falling on or between the MCT and MBF is seismological very active.

On the basis of seismic activity, India has been divided into five zones. Chamoli district falls under zone V, the most seismically active region where earthquakes of magnitude 8 or more could occur, said Dr Bhattacharya, deputy director general (seismology) in the meteorological department.

India's seismic map shows the entire north-eastern region, the Andaman and Nicobar Islands, Kutch in Gujarat, Kangra in Himachal Pradesh and areas around Jammu falling under zone V.

A recent study by Dr Srivastava and Dr Bhattacharya shows that the occurrence of quakes in the Himalayan region is 'very chaotic' and could not be predicted on a long-term basis.

"A detailed study is required to know how the slip occurred in Chamoli and the exact fault line,'' said Dr Bhattacharya, who plans to visit the affected area. He described Monday's quake as 'significant', but added that the loss of life and property was comparatively less as the area has lower density of population.

Earthquakes are classified as 'very great' (magnitude of 8 or more on the Richter scale), 'great' (7 to 7.9), 'moderate' (5 to 6.9) and 'slight' (up to 4.9).

UNI

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