Based on the analysis of the geological tectonics setting and deep-seated geophysical characteristics, the causative structure of the Yongdeng M_S 5.8 earthquake on July 22, 1995 has been determined by the synthetical study of the faults in the earthquake area, focal mechanism solution, focal depths, meizoseismal region direction, post-earthquake distribution, earthquake fissures and ground motion direction. The tectonodynamic mechanism forming this earthquake has also been approached. It is considered that the earthquake results from the reactivation of the Yaojie-Zhoujiatai buried fault under the direct action of local tectonic stress.