Topography effects in the polarization of coda waves: 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.