GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.1 73 PRELIMINARY ESTIMATION OF Q β IN THE GARGANO PROMONTORY (SOUTHERN ITALY) S. Lucente 1 , T. Ninivaggi 1 , S. de Lorenzo 1 , E. Del Pezzo 2 , M. Filippucci 1 , A. Tallarico 1 1 Department of Earth and Geo-Environmental Science, University of Bari Aldo Moro (UniBa) 2 National Institute of Geophysics and Volcanology (INGV), Osservatorio Vesuviano We have examined 190 small earthquakes with magnitude ranging from 1.0 to 2.8 Ml, occurred in the Gargano Promontory (Apulia region, Italy) and recorded by OTRIONS local seismic network and by the INGV national seismic network, to estimate the quality factor of S waves ( Q β ) for the Gargano area. Q β has been estimated using the coda normalization method proposed by Aki (1980): (1) where A β ( r,f ) the S wave amplitude spectrum recorded at a distance r from the earthquake and at a given frequency f , A c ( r,f,t L ) the amplitude spectrum of the coda waves, sampled around the time elapsed from the origin time t L , a is a constant, V β is the S wave velocity. Spectral ratios in eq. (1) were computed taking t L =30 s, considering different frequency bands in the range [3,12] Hz and applying a 15% cosine taper window to minimize aliasing effect. The plot of the first member of eq. (1) versus distance is shown for all the considered frequencies in Fig. 1. The slope of the regression line allows the inference of Q β . In Table 1 the estimation of Q β and its error is reported. Tab. 1 - Preliminary estimations of Q β parameter at different frequencies in the Gargano area computed using V β = 3.86 km/s. Frequency Q β preliminary estimation 3 Hz 118.64 ± 22 4 Hz 130.95 ± 24 5 Hz 166.64 ± 29 6 Hz 182.14 ± 32 8 Hz 319.48 ± 52 10 Hz 420.39 ± 75 12 Hz 556.82 ± 100 The estimation of Q β will allow us to separate the intrinsic and scattering attenuation in the Gargano area using the formulation proposed by Wennerberg (1993) and using the estimates of Q -coda recently obtained by Filippucci et al. (2021). These Q β results were compared with those obtained by De Lorenzo et al . (2013) for the Umbria-Marche region. Results at all frequencies indicate that Q β estimated for the Gargano Promontory is higher than that estimated for central Italy. This difference can be ascribed to a different rheological behavior of the lithosphere in these two distinct geodynamic frameworks. References Aki K.; 1980: Attenuation of shear-waves in the lithosphere for frequencies from 0.05 to 25 Hz . Phys. Earth Planet. Inter. 21, pp. 50-60. De Lorenzo S., Del Pezzo E. and Bianco F.; 2013: Q c , Q β , Q i and Q s attenuation parameters in the Umbria-Marche (Italy) region . Physics of the Earth and Planetary Interiors, 218, pp. 19-30.