The recent 6.9-magnitude earthquake in Sikkim, which claimed more than 100 lives, was different from most temblors that had rocked the Himalayan region.

It did not occur on the well-known thrust faults formed due to under-thrusting of the Indian plate below the Eurasian plate as in 95 per cent of the major earthquakes in the Himalayan region. Instead, it occurred between two transverse faults represented by Tista and Gangtok lineaments where two segments of Himalayas have moved in a horizontal direction.

Although the transverse faults were also the result of under-thrusting of the Indian plate, this was the first time that a major earthquake had occurred along them in Sikkim region, according to R.K. Chadha, Chief Scientist, CSIR-National Geophysical Research Institute (NGRI) which deputed a team of five scientists to the affected region following the earthquake to carry out field investigations. The team has set up a network of four broadband seismic stations. The earthquake with its epicentre located 68 km of northwest of Gangtok occurred at a depth of 20 km beneath the Himalayas.

Seismically very active

With the Himalayan region being one of the most seismically active places in the world, the recent earthquake, however did not come as a surprise. Most other major earthquakes above magnitude (M) 8.0 in Shillong (1987), Kangra (1905), Bihar-Nepal border (1934), Arunachal Pradesh (1950) and many others between M 6 and 7 were caused by faults formed by the under-thrusting of Indian plate below the Eurasian plate. The Sikkim region experienced relatively moderate seismicity over the past 35 years with temblors of M 5 or greater occurring within 100 km of the epicentre of the September 18 event.

Large area impact

Dr. Chadha said another interesting feature of the recent earthquake was that it was felt over a large area, including Delhi and other major cities such as Lucknow, Patna and Ranchi along the Indo-Gangetic plains. With most of the region lying in the soft sediments in the Indo-Gangetic plains and the Brahmaputra Valley, the earthquake was felt strongly as the seismic waves get amplified in soft sediments when compared to hard rock regions. As experienced earlier, such temblors would cause damage in faraway regions too due the amplification process. The amplification phenomenon again showed that major cities located in the Indo-Gangetic plains face greater earthquake hazard than other regions.