GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 2.1 241 simultaneously determined by discretizing the distance range into a given number of intervals and determining the frequency-dependent attenuation value for each distance bin. The equation can be written as equation (2), where k=1.2 indicates the regional attenuation models. (2) A subsequent inversion allows separating source and site effects from the residual spectral amplitudes after correcting the regional attenuation functions: (3) Here, to constrain site amplifications of all stations to one reference condition, the reference site condition is set for the station CH.LLS, which is the reference site in the network CH managed by Swiss seismological service (SED). The reference site amplification is set to e – πk 0 f with k 0 = 0.007s (Pilz et al. , 2019). After solving the constrained linear system of equations, the obtained source spectra are fit with the ω 2 -model (Brune, 1970, 1971) to estimate the corner frequency f C and seismic moment M 0 (respectively moment magnitude M W ) by the following source model, consisting of a standard ω 2 -model multiplied by an exponential factor, κ source , applied to frequencies above f k : ,           (4) where S ( f ) represents the acceleration source spectrum at the reference distance R 0 = 10 km. M . ( f ) denotes the moment-rate spectrum, R θ ∅ is the average radiation pattern of S-waves set to 0.55 (Boore and Boatwright, 1984), F=2 is the free surface factor, ρ=2.7 g/cm 3 is the density and V S = 3.3 km/s is the shear-wave velocity near the source. Stress drop Δσ was computed with the obtained corner frequency and seismic moment by the following equation (Eshelby, 1957): (5) where the rupture radius is given by (6) Results We derived spectral models for source, path, and site amplifications for southwestern Europe by analyzing events from the stable to the active part of the continent. Attenuation models have been derived for two different regions, which are the stable part (mainly Switzerland and Germany) and the active part of Europe (mainly central and northern Italy) (Fig. 2 (a) and (b)). The attenuation in region A (northern Europe) of each frequency almost decay the same as 1/R function up to 60 km. At large distances, it attenuates faster than a 1/R curve at frequencies larger than 10 Hz. The attenuation functions in region B (southern Europe) decay similarly for the entire frequency range at distances up to 50 km. At distances larger than 50 km, the attenuation functions at high frequencies attenuate faster. Southern Europe shows a stronger attenuation than northern Europe, especially at high frequencies. The attenuation-corrected spectra are decomposed into site and source contributions shown