Attenuation and velocity structure in the area of Pozzuoli-Solfatara (Campi Flegrei, Italy) for the estimate of local site response

Abstract

In the present work I infer the 1D shear-wave velocity model in the volcanic area of Pozzuoli-Solfatara using the dispersion properties of both Rayleigh waves generated by artificial explosions and microtremor. The group-velocity dispersion curves are retrieved from application of the Multiple Filter Technique (MFT) 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 (PMF). The obtained dispersion curves indicate Rayleigh-wave fundamental-mode group velocities ranging from about 0.8 to 0.6 km/sec over the 1-12 Hz frequency band. I also propose a new approach based on the autoregressive analysis, to recover group velocity dispersion. I first present a numerical example on a synthetic test signal and then I apply the technique to the data recorded in Solfatara, in order to compare the obtained results with those inferred from the MF analysis Moreover, I analyse ambient noise data recorded at a dense array, by using Aki’s correlation technique (SAC) and an extended version of this method (ESAC) The obtained phase velocities range from 1.5 km/s to 0.3 km/s over the 1-10 Hz frequency band. The group velocity dispersion curves are then inverted to infer a shallow shear-wave velocity model down to a depth of about 250 m, for the area of Pozzuoli-Solfatara. The shear-wave velocities thus obtained are compatible with those derived both from cross- and down-hole measurements in neighbour wells and from laboratory experiments. These data are eventually interpreted in the light of the geological setting of the area. I perform an attenuation study on array recordings of the signals generated by the shots. The  attenuation curve was retrieved by analysing the amplitude spectral decay of Rayleigh waves with the distance, in different frequency bands. The  attenuation curve was then inverted to infer the shallow Q inverse model. Using the obtained velocity and attenuation model, I calculate the theoretical ground response to a vertically-incident SH wave obtaining two main amplification peaks centered at frequencies of 2.1 and 5.4 Hz. The transfer function was compared with those obtained experimentally from the application of Nakamura’s technique to microtremor data, artificial explosions and local earthquakes. Agreement among the transfer functions is observed only for the amplification peak of frequency 5.4 Hz. Finally, as a complementary contribution that might be used for the assessment of seismic risk in the investigated area, I evaluate the peak ground acceleration (PGA) for the whole Campi Flegrei caldera and locally for the Pozzuoli-Solfatara area, by performing stochastic simulations of ground motion, partially constrained by the previously described results. Two different methods (random vibration theory (RVT) and ground motion generated from a Gaussian distribution (GMG)) are used, providing the PGA values of 0.04 g and 0.097 g for Campi Flegrei and Pozzuoli-Solfatara, respectively.