Tidal synchronization of Long Period (LP) events and hydrothermal tremor at Campi Flegrei (Italy)

Abstract

The generation of seismic signals within volcanoes is a complex phenomenon that involves the coupling between a fluid phase (magmatic and/or hydrothermal) and the vibrations of the solid rock. This mechanism caused the most relevant swarm of Long-Period (LP) events at Campi Flegrei in October 2006, during the 2004-2006 ground uplift episode. We investigated the source properties of the LPs by analyzing the temporal release of seismic energy, amplitude distribution and inter-event occurrence time. Moreover, we applied the Independent Component Analysis to identify the simpler waveforms representative of the source mechanism. On the basis of the results, we propose a conceptual model for the LP generation: the source process is triggered by a mechanism of fluid charge/discharge in the branches of a dentritic network of the hydrothermal system. Partial shunting of the fluid flow toward different conduits is likely activated by tidal stress variation, thus modulating the time pattern and the energy release of the LPs. We also recognized the influence of earth tides in the background seismic noise by analysing continuous data recorded at five broadband stations during 2006. We estimated the time evolution of amplitude and polarization of the signal in the frequency band common to the LPs. The series resulted modulated on tidal time scales: the RMS amplitude is basically dominated by solar contribution, while the azimuth of the polarization vector shows lunar diurnal and semidiurnal constituents. Moreover, in the frequency band common to the LPs the azimuths are polarized toward a specific area, suggesting that these persistent oscillations can be induced by the activity of the shallow geothermal reservoir. All these observations indicate that the interaction between fluid and solid modulated by tidal strain represents a key point for interpreting the seismic activity observed in 2006 and for better understanding the caldera dynamics.