Аннотация:The study of intra-earth processes requires a comprehensive approach, which should bebased on the understanding different nature and origin depth of terrestrial processes. In this regard,the problem of the relationship between seismicity and heat flow in the lithosphere is of interest. We analyzed seismic observation data in Bulgaria from 1981 to 2018 using statistical methods. A significant correlation was found between the heat flow and the earthquakes magnitude, as well as between the thickness of thermal lithosphere and the magnitude. To assess the released friction heat and hydrodynamic processes during the earth’s crust movements, it is necessary to develop theoretical models based on a comprehensive analysis of the geological structure and geophysical fields in seismically active regions.