We analyze broadband seismic data recorded at a single station deployed on Stromboli island from 1996 to 1999 in order to determine basic parameters of the seismic wavefield and study their relationship with volcanic activity. In addition to a characterization in the frequency domain, we apply polarization analysis and study long-term variations of these parameters possibly reflecting also changes in source properties. Our analysis is twofold. First, we focus on a meaningful reduction of the data to selected parameters, i.e., azimuth and incidence angle, which can be correlated with volcanic activity. Secondly, we investigate if there is maintenance of the correlations over long time spans. We observe statistically significant long-term changes (from several months to year) in the wavefield parameters considered. We interpret such changes as possibly being caused by the enhanced strength of a deep source, potentially by supply of fluids from a low part of the magma feeder between the end of 1997 and the first months of 1998.