What is Unique about the Sikkim Earthquake?

Up till now, it has been reported in the media that the 6.9-magnitude earthquake in Sikkim occurred on the well-known thrust faults formed due to under-thrusting of the Indian plate below the Eurasian plate. This is the case with majority (95%) of the major earthquakes in the Himalayan region. (Subduction)

However, as per a news article published in “The Hindu”, R.K. Chadha, Chief Scientist, CSIR-National Geophysical Research Institute (NGRI) says that the Sikkim Earthquake occurred between two transverse faults represented by Tista and Gangtok lineaments where two segments of Himalayas have moved in a horizontal direction.

What is transverse fault?

We know that the lithospheric plates grind and slide along one another. The processes of rock crowding together or pulling apart along these fracture lines is known as faulting. The movement along various kinds of faults causes earthquakes, the emergence of new landforms, and other consequences. In the case of Sikkim Earthquake, though 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 due to this particular region as per the scientists.

The Earthquake is unique because 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.

In comparison to other earthquakes, recent Sikkim Earthquake was felt over a larger area. Why?

This is 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. The reason is that 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. ^{(Includes inputs from The Hindu)}