GNGTS 2019 - Atti del 38° Convegno Nazionale

224 GNGTS 2019 S essione 1.4 of Sardinia) seismic array, together with the public data available from Istituto Nazionale di Geofisica e Vulcanologia (INGV) and Reseau sismologique et géodésique français (RESIF), provide us with continuous waveforms recorded at 20 receivers scattered across Sardinia and Corsica (Fig. 1), and enable us to investigate the Earth structure beneath this continental block. In order to characterize the lithosphere of the study area we use Rayleigh wave phase- velocity measurements and we compute interstation dispersion curves by using ambient-noise interferometry and teleseismic Earthquakes. For the majority of the stations, more than 2 years of continuous seismograms (from June 2016 to September 2018) are available. This ensures that a relatively large number of AN sources at different azimuths contribute to the waveforms (which is essential for the application of seismic interferometry) and that a sufficiently large number of earthquakes can be exploited for this study. We retrieved the Rayleigh wave dispersion curves from the vertical component of recorded noise using an automated algorithm that measures the phase velocities for a given station pair by cross-correlation in frequency domain and stacking of noise records (Kästle et al., 2016). We recover average interstation dispersion measurements for periods ranging from 2.5 s up to 20 s. In order to generate AN phase-velocity maps at different periods we adopted a linearized inversion algorithm based on the ray theory. We also retrieved the Rayleigh phase velocities from teleseismic surface waves using an open source, automated algorithm that measures the frequency-dependent interstation phase delays between all possible nearby stations by cross- correlation in time domain [Jin and Gaherty (2015)]. We obtain phase-velocity maps (together with their uncertainties) for periods ranging from 20 s to 80 s. Fig. 2 - Median (continuous lines), 10th and 90th percentiles (dashed lines) of the Rayleigh-wave phase velocities as function of period for the Sardinia-Corsica continental block (black) and for the Italian peninsula (blue, obtained by Molinari et al., 2015). The 1-D phase-velocity profiles are inverted individually for the 1-D shear-velocity structure. The generated depth-profiles are then combined to create a 3-D shear-velocity model. The inversion is performed by means of a damped iterative linearised inversion (Greve et al., 2014). A starting model is calculated from the average dispersion curve of the study area and, for each location, it is iteratively perturbed trying to minimize a misfit function that depends on 1) the misfit between the calculated and the observed phase-velocity dispersion curve, 2) the vertical smoothness of the model, and 3) the difference between the background and the inverted model. We discretized the model using layers of progressively increasing thickness with depth, without including any constraints regarding the Moho: layers 2 km thick in the first 10 km, 5 km thick from 10 to 50 km, 10 km thick from 50 to 100, and 20 km thick between 100 and 400 km. The inversion was carried out for structures as deep as 400 km in order to avoid leakage from (otherwise fixed) deeper structures into the shallow structure. Following Ma and Clayton 2014, we tested that the inversion results were independent from the chosen starting model.