Understanding the very strong damaging Nepal/Sikkim earthquake (18/09/2011)

Earthquake information : A magnitude 6.9 earthquake hit Sikkim, India 12:40:48 UT (10:48 PM at epicenter) The depth of the hypocenter was at 20.7 km. The earthquake caused victims and huge damage in Sikkim (India), India and Nepal. It was also well felt in Bangladesh.

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Tectonic explanation of what happened

The September 18, 2011 Sikkim, India earthquake occurred near the boundary between the India and Eurasia plates, in the mountainous region of northeast India near the Nepalese border.
Initial analysis suggest the earthquake was complex, likely a result of two events occurring close together in time at depths of approximately 20 km beneath the Earth's surface.

At the latitude of the September 18 earthquake, the India plate converges with Eurasia at a rate of approximately 46 mm/yr towards the north-northeast. The broad convergence between these two plates has resulted in the uplift of the Himalayas, the world's tallest mountain range. The preliminary focal mechanism of the earthquake suggests strike slip faulting, and thus an intraplate source within the upper Eurasian plate or the underlying India plate, rather than occurring on the thrust interface plate boundary between the two.

This region has experienced relatively moderate seismicity in the past, with 18 earthquakes of M 5 or greater over the past 35 years within 100 km of the epicenter of the September 18 event.
The largest of these was a M 6.1 earthquake in November of 1980, 75 km to the southeast.

Text and images : courtesy USGS