Based on a point of view that the near-field seismic source can't be considered as a point source, the author derived a formula for focal depth corresponding to the line and plane sources from their radiant field as follows:h=r₁√10(1₀-1_i)/(1.5m)-1
Where I₀ is the seismic intensity at epicentre, I₁, the intensity at observation point i, and r₁ the radius of the isoseismal curves defined by the intensity I₁, the index n of geometrical diffusivity within macroseismic field is a varible parameter. For n=2, it is G-R-Min formula, for n=1-2 corresponds to S=1.5-3.0 in Blake formula, n=1.78, Blake formula et al. And importantly, first gave the value of macroseismic focal depth to a standard error in the world.