Abstract Earthquake magnitude calibration using hydrophone records has been carried out at Campi Flegrei caldera, an active area close to the highly populated area of Naples city, partly undersea. Definite integrals of the hydrophone records amplitude spectra, between the limits of 1 and 20 Hz, were calculated on a set of small volcano-tectonic earthquakes with moment magnitudes ranging from 1 to 3.3. The coefficients of a linear relationship between the logarithm of these integrals and the magnitude were obtained by linear optimization, thus defining a useful equation to calculate the moment magnitude from the hydrophone record spectra. This method could be easily exported to other volcanic areas, where submerged volcanoes are monitored by networks of hydrophones and seismic sensors on land. The proposed approach allows indeed magnitude measurements of small magnitude earthquakes occurring at sea, thus adding useful information to the seismicity of these volcanoes.
We present 4 years of continuous seafloor deformation measurements carried out in the Campi Flegrei caldera (Southern Italy), one of the most hazardous and populated volcanic areas in the world. The seafloor sector of the caldera has been monitored since early 2016 by the MEDUSA marine research infrastructure, consisting of four instrumented buoys installed where sea depth is less than 100 m. Each MEDUSA buoy is equipped with a cabled, seafloor module with geophysical and oceanographic sensors and a subaerial GPS station providing seafloor deformation and other environmental measures. Since April 2016, the GPS vertical displacements at the four buoys show a continuous uplift of the seafloor with cumulative measured uplift ranging between 8 and 20 cm. Despite the data being affected by environmental noise associated with sea and meteorological conditions, the horizontal GPS displacements on the buoys show a trend coherent with a radial deformation pattern. We use jointly the GPS horizontal and vertical velocities of seafloor and on-land deformations for modeling the volcanic source, finding that a spherical source fits best the GPS data. The geodetic data produced by MEDUSA has now been integrated with the data flow of other monitoring networks deployed on land at Campi Flegrei.
An innovative fiber-optic hydrophone (FOH) was developed and investigated via an experiment at sea; it is capable of operating at a very low frequency of the seismic spectrum and detecting small magnitude earthquakes. The FOH exploits an optical fiber coil wrapped around a sensitive mandrel in a Michelson interferometric configuration. The FOH operated for about seven days at a water depth of 40 m, in the Campi Flegrei volcanic area (Southern Italy), and a few meters from a well-calibrated PZT hydrophone used as a reference. Thirty-three local earthquakes occurred during the simultaneous operation of the two hydrophones, allowing a straightforward comparison of the recordings. The local earthquakes occurred at an epicentral distance less than 2.5 km from the site of recording, and were estimated to be in the range of magnitude from -0.8 to 2.7. The analysis of the recorded earthquake waveforms in the frequency and time domains allowed retrieving the response function of the FOH in the frequency range from 5 to 70 Hz. The FOH responsivity in terms of acoustic pressure reached about 230 nm/Pa and was flat in the studied frequency range. Due to the high quality of the FOH recordings, this equipment is suitable for applications addressing submarine volcanic activity and the background seismicity of active faults in the ocean.
Seafloor deformation monitoring is now performed in the marine sector of the Campi Flegrei volcanic area. MEDUSA infrastructure consists of 4 buoys at depths of 40-96m equipped with cGPS receivers, accelerometers and magnetic compasses to monitor buoy status and a seafloor module with a bottom pressure recorder. We study the seafloor deformation in the caldera. Previously we show that cGPS onland network and MEDUSA timeseries for the years 2017-2020 are in agreement with the deformation predicted by a Mogi model describing the observed deformation of an active volcano. Only for buoy A data differ significantly from model, at 6.9sigma and 23.7sigma for the horizontal speed (v) and direction. We devised a new method to reconstruct the sea bottom displacement including cGPS and compass data. The method, applied to buoy A and validated also on C, uses compass data to correct cGPS positions accounting for pole inclination. Including systematic errors, the internal consistency, within 3sigma (2sigma) for the speed (angle), between the results derived for different maximum inclinations of the buoy pole up to 3.5deg shows that the method allows to significantly reduce the impact of the pole inclination which can alter the estimation. We find good convergence of the velocity and deformation angle for increasing values of the buoy pole inclination. We found v=3.521+-0.039(stat)+-0.352(syst)cm/yr and an angle -115.159+-0.670(stat)+-7.630(syst)deg. The relative impact of potential systematics (statistical) effects increases (decreases) with cutoff. Our analysis gives v consistent with Mogi at 5.2sigma(stat) or 0.5sigma(stat and syst), and a deformation angle consistent at 4.3sigma(stat) or at 0.3sigma. The module of the vectorial difference between v from the data and Mogi diminishes by a factor 7.65+-1.23(stat) or +-5.78(stat+syst) compared with previous work. Potential improvements are discussed.
