"The affected region is geologically considered to be a weak zone and prone to earthquakes. It is termed as a major fault zone in the Himalayan region because of the convergence of Tectonic plates there," Prashanta Bhaumik, a senior geologist at the GSI's Sikkim set-up, told PTI.
"There were earthquakes before, but this time the devastation is more because of the severity and the duration of the calamity," he said.
The earthquake measured 6.8 on the Richter scale and it lasted for a fairly longer period than earlier and its effect was felt as far as in Chennai, he said.
"Moreover, the earthquake was shallow with a depth of 10 to 20 km and naturally the earth surface was hit with a severe intensity, justifying the casualty figure," he said.
Bhaumik said the epicentre was 68 km north of Mangan, near the Indo-Nepal border, which is why the worst-affected places were those in North Sikkim like Chungtham, Lachen and Lachung and small hamlets surrounding them.
"Previously the most severe earthquake was in 1980 when the intensity was 6.1 on the Richter scale and the casualty was much lower," he said.
Bhaumik said the GSI has a permanent station at Ranipur and there are temporary stations too to monitor earthquakes and other natural disasters.
He said a team of two scientists - a seismologist and a geologist - have started work on finding out the reasons for the ''quake which has cut a wide swathe of destruction in Sikkim, West Bengal, Tibet and to a some extent in Bhutan.