"There is nothing surprising in this earthquake as the region north of Sikkim, which forms the outliers of Tibetan tectonics, is known for moderate earthquakes in the past," C.P. Rajendran at the Indian Institute of Science here told IANS.

But what makes it different is its "unusually greater magnitude". These earthquakes are different in the sense they are along the somewhat north-south structures trending transverse to the east-west Himalayan axis, Rajendran said. They are different from the usual Himalayan thrust earthquakes that are caused by the collision of the Indian plate with the Eurasian plate.

Vineet Gahaulat at the National Geophysical Research Institute (NGRI) in Hyderabad said: "The (Sikkim) earthquake highlights the presence of the role of transverse features in the Himalayas."

"This 6.9 magnitude earthquake possibly occurred on a northwest-southeast trending almost vertical fault through strike slip motion." "This is what we generally do not expect in the Himalayas," Gahaulat told IANS. "We expect large magnitude earthquakes on the detachment having thrust motion on gently dipping planes - like the 1999 Chamoli and 1991 Uttarkashi earthquakes."

Gahaulat said the role of transverse features in segmenting the Himalayan arc and accommodating some of the convergence of the India-Eurasia plates have been talked about earlier.  

"But this one (Sikkim quake) makes it clearer as this is possibly the largest magnitude earthquake of this type in the Himalayas."  Gahaulat is also surprised at the fewer number of aftershocks - only two to three aftershocks of magnitude greater than 4.5. "So where are all the aftershocks gone? Are they yet to occur in the following days," he asked.

According to Gahaulat, "we need to be careful" if, in the coming days, the aftershocks occur southeast of the main shock epicentre - the way two aftershocks have occurred.  

"The region where the main shock occurred has very low population density, but further southeast, population density is higher," he cautioned. "The occurrence of this earthquake does not lower down the threat from the great earthquake which we expect in the Himalayas," the NGRI scientist warned.  

The one (or more) which is expected will be a thrust type earthquake on the detachment which will be much more devastating than this one, he said.  "We need to treat each and every earthquake in a special way, you just need to turn it around and it will tell a somewhat different story."

Rajendran, however, does not think this particular event in Sikkim can be treated as a precursory signal to any major earthquake that may occur in future along the Himalayan arc.

"Having said that, the fact remains that some historically earthquake-deficient parts of the Himalayas has the potential to generate large earthquakes any time, irrespective of the present earthquake," he cautioned.