2.3.1. Spatial Density of Primary Vibration Directions

The daily primary vibration directions (ϕ*m*1) were calculated for all stations and the great circles corresponding to all ϕ*m*<sup>1</sup> were generated in the three DF bands. Any point on a great circle can be considered as a possible energy source of the corresponding station in the corresponding day and DF band. In order to find the possible source areas, spatial density of the points sampled from the great circles with a sampling interval of 100 m were calculated, normalized, and plotted as a color gradient map for each DF band. The relative magnitude of the spatial density reflects the possibility of the area to be an energy source, i.e., the larger the density, the higher the possibility.
