Shaking of ground caused mostly by gradual built up, and sudden release of energy due to tectonic movements is referred to as earthquake. Energy released, or magnitude of an earthquake is measured by Richter Scale, while the impact, or intensity of an earthquake is measured by Mercalli Scale.
The place below the ground surface where the rock rupture or from where the earthquake initiates is called hypocenter or focus. Location over ground surface vertically above the hypocenter is called epicentre of the earthquake, and an earthquake is generally identified by the place in the proximity of the epicentre; Latur Earthquake, Gorkha Earthquake, Bhuj Earthquake.
Ongoing subduction of the India Plate beneath the Eurasian Plate causes built up of strain making the Himalayan region seismically vulnerable. In the previous 120 years the Himalayan region has witnessed 06 major earthquakes; 8.0 magnitude Shillong Earthquake of June 12, 1897, 7.8 magnitude Kangara Earthquake of April 4, 1905, 8.2 magnitude Bihar–Nepal Earthquake of January 15, 1934, 8.6 magnitude Assam now Arunachal Earthquake of August 15, 1950, 7.6 magnitude Kashmir Earthquake of October 8, 2005, and 7.8 magnitude Gorkha Earthquake of April 25, 2015.
Areas between the epicenters of these great earthquakes that have not witnessed major earth shaking for a long time are identified as seismic gap zones and the state of Uttarakhand falls in one such zone. Seismic gap zones are identified as being potential areas for housing future major earthquakes.