The surface subsidence that manifests itself a few months after mineral extraction and that usually constitutes 60% to 90% of the total subsidence. It varies according to the depth and thickness of the seam, the nature of the overburden, the mining method, and the thoroughness of the filling in the mined-out areas. The primary period is followed by the secondary period, in which the surface subsides gradually for a period of many years or even decades.