Shallow shear-wave velocity structure of Solfatara volcano (Campi Flegrei, Italy),from inversion of Rayleigh-wave dispersion curves

Abstract

In this work, we infer the 1D shear-wave velocity model at Solfatara volcano using the dispersion properties of Rayleigh waves generated by artificial explosions. The groupvelocity dispersion curves are retrieved by applying the Multiple Filter Technique to single-station recordings of air-gun sea shots. Seismic signals are filtered in different frequency bands and the dispersion curves are obtained by evaluating the arrival times of the envelope maxima of the filtered signals. Fundamental and higher modes are carefully recognized and separated by using a Phase Matched Filter. The dispersion curves obtained indicate Rayleigh-wave fundamental-mode group velocities ranging from about 0.8 to 0.6 km/s over the 2-12 Hz frequency band. These group velocity dispersion curves are then inverted to infer a shallow shear-wave velocity model down to a depth of about 250 m. The shear-wave velocities thus obtained are compatible with those derived both from cross- and down-hole measurements in neighbouring wells and from laboratory experiments. These data are eventually interpreted in the light of the geological setting of the area. Using the velocity model obtained, we calculate the theoretical ground response to a vertically-incident S-wave getting two, main amplification peaks centered at frequencies of 2.2 and 5.4 Hz. The transfer function was compared to those obtained experimentally from the application of Nakamura’s technique to microtremor data, artificial explosions and local earthquakes. Agreement among the experimental and theoretical transfer functions is observed for the amplification peak of frequency 5.4 Hz.