Himal da, its all due to tectonic activities going over there. Earlier only San Andreas was thought to be responsible for all the tectonic activites but now they have found several fault systems over there which are responsible for this type of earthquake. But I am really worried about the accumulating energy in Nepal Himalaya due to prolonging seismic gap. The big earthquake ( about 8 in Richter Scale) we are expecting in Nepal will be bigger if it takes more time to occur. I am not much worried about the earthquake itself  because it has to come to release the accumulated energy due to the movment of Indian Plate towards China. But haphazard construction of buildings in Kathmandu Valley without following any construction code will cause big disaster of both life and property. As Kathmandu Valley still consists of unconsolidated soft sediments there is also risk of liquefaction. Department of Mines and Geology has already published engineering geological map of Kathmandu Valley where it has been clearly shown which parts are hazardous and which parts are safe for the construction but it seems nobody is looking for this map, nobody is following the building construction code and nobody is using earthquake enigeering technique. I don't know what will happen when big one will come.