Because Himalayan region lie along boundaries of the tectonic plates.
Chief tectonic sub-regions of India are the mighty Himalayas along the north. The Himalayas consist primarily of sediments accumulated over long geological time in the Tethys. The Indo- Gangetic basin with deep alluvium is a great depression caused by the load of the Himalayas on the continent.
What is tectonic plate?
the lithosphere of the earth is divided into a small number of plates which float on and travel independently over the mantle and much of the earth’s seismic activity occurs at the boundaries of these plates.
Some of the damaging and recent earthquakes are listed in Table 1. Most earthquakes occur along the Himalayan plate boundary (these are inter-plate earthquakes).