Moment magnitude (Mw) from hydrophone records of low energy volcanic quakes

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.