Topography effects in the polarization of earthquake signals: a comparison between surface and deep recordings

Local and regional earthquakes recorded in 2007 by two seismic arrays at Gran Sasso (Italy), one located at surface and one at 1.4 km depth, have been analyzed. Polarization properties of the seismic wavefield have been estimated at both arrays on a short sliding window and in several frequency bands. Array methods were also applied, and the estimated slowness and backazimuth were used to compute the stacking of phase shifted signals in order to improve the signal to noise ratio. Results of polarization computed over single station and array stacked signals have been compared between the two arrays. A well defined polarization azimuth, roughly perpendicular to the main massif ridges, is evident at surface in a broad frequency band. This is observed for earthquake body waves and coda waves. On the contrary, underground the same effect is slightly visible only at very low frequency.