Nepal earthquake of 25 April 2015
A brief, very simply explanation.

The earthquake that struck Nepal on 25 April 2015 occurred in a region with a history of catastrophic tremors.

The Indian tectonic plate is slowly moving at an average rate of 45 mm/year underneath the Eurasian plate. Such movement is never smooth and sometimes the plates get temporarily stuck. Eventually the force producing the movement of the Indian plate increases the pressure on the snagging point, or points, and the plate can jolt. This jolt sends out shock waves through the neighbouring rock which causes the earthquake. The bigger the jolt the stronger the earthquake.

More detail:

From the news services and the US Geological Service


  the USGS wrote in its summary, the quake "occurred as the result of thrust faulting on or near the main frontal thrust between the subducting India plate and the overriding Eurasia plate to the north. At the location of this earthquake, approximately 80 km to the northeast of the Nepalese capital of Katmandu, the India plate is converging with Eurasia at a rate of 45 mm/yr towards the north-northeast, driving the uplift of the Himalayan mountain range. The preliminary location, size and focal mechanism of the April 25 earthquake are consistent with its occurrence on the main subduction thrust interface between the India and Eurasia plates."
  
  According to the USGS, a quake in the region in 1934 is believed to have killed roughly 10,600 people. Another in 1988 killed 1,500.
  But the experts said big quakes in the area are not necessary common.
  "Although a major plate boundary with a history of large-to-great sized earthquakes, large earthquakes on the Himalayan thrust are rare in the documented historical era," the UGS wrote. "Just four events of magnitude 6 or larger have occurred ... over the past century."