The Campi Flegrei (Southern Italy) is one of the most active calderas in the World. With several hundred thousand people living within its borders, this places the area in a high risk category should there be an eruption. The seismological monitoring system in the Campi Flegrei is based on eight short-period seismic stations and one broad-band digital station. While all of the seismic stations are located on land, part of the seismic activity occurs in the under-sea area of Pozzuoli Gulf (Campi Flegrei), where there are no seismic stations. This gap in the data coverage produces a biased and incomplete image of the volcanic area. We carried out an experiment in Pozzuoli Gulf with the installation of two broad-band seismic stations on the sea floor with remote and continuous data acquisition for a duration of 31 days between January and March, 2005. Using the data acquired, we have computed the power spectral density (PSD) to characterize the background seismic noise, and to evaluate the true noise variation, we have generated the seismic noise probability density functions from the computed PSD curves. The results of our analysis show that the broad-band seismic noise is high when compared with the Peterson noise model (land model), but for periods less than 0.3 s the seismic noise on the sea floor is lower than the recordings on land over the same period range. The last bradyseismic crisis (1982-1984) highlights the importance of this frequency range, as most of the spectral content was recorded in this range. Finally, we evaluate the detection threshold of a new seismic station located on the sea floor of Campi Flegrei caldera considering the characteristics of the local seismicity.

 

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