Fig. 2

Output of the polarization analysis for the reference station at Gondo in 2014; the color tells how often a certain frequency occurs in the ambient vibration recording (red often, purple seldom); left ellipticity plot—affected by a dominant anthropogenic noise source around 1 Hz; middle strike plot—north direction goes from the circle center vertically up, the radius represents the frequency in Hz; there is some polarization in north–south direction for frequencies above 3 Hz and a slight polarization at around 2 Hz with an azimuth of 35°; right dip plot—no significant features visible for this station