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Keywords:

  • sustained hydrothermal tremor;
  • Campi Flegrei Caldera;
  • polarization analysis;
  • tidal modulation;
  • synchronization

[1] Long continuous seismic data recorded at five broadband seismic stations during 2006 at Campi Flegrei caldera have been analyzed. Introducing a coarse-grained method, we evaluate the time evolution of amplitude and polarization of the seismic noise in the frequency band common to long-period events. The series are modulated on tidal time scales: the root-mean square is basically dominated by solar contribution, while the azimuth of the polarization vector shows lunar diurnal and semidiurnal constituents. In addition, we find that in the frequency band common to long-period events the azimuths are polarized toward a specific area, suggesting that these persistent oscillations can be induced by the activity of the shallow geothermal reservoir.