GNGTS 2013 - Atti del 32° Convegno Nazionale

at a site representative of the average site response of the network. For reproducing D ( r,r ref , f ) we use the crustal attenuation parameter Q ( f ) = 200 f 0.5 (2) The quality factor given in Eq. (2), and coupled with a simple 1/ r geometrical spreading, is consistent with the results described by Garcia et al. (2004), who found: Q ( f ) = 251 f 0.58 . Also two parameters are used to predict shapes and levels of the seismic spectra, the stress drop Δσ, and a high-frequency attenuation parameter κ = 0.030. For example, comparisons can be done among different zone in which this kind of studies have been conducted. It has been found that the western Alps (Morasca et al. , 2006), eastern Alps (Malagnini et al. , 2002), Southern Appenines, central Italy (Malagnini et al. , 2000a), eastern Sicily (Scognaniglio et al. , 2005), Central Europe (Malagnini et al. , 2000b; Bay et al. , 2003), Utah e Yellowstone (Jeon et al. , 2004), and Chile region (this work) have different characteristics for the attenuation parameters because the combination of the geometrical spreading function and the parameter Q ( f ) is strictly related to the crustal characteristics. Fig. 3 compares the observed and predicted excitation of filtered velocity spectra obtained from the Chile region. The black lines indicate the regression result, and the red thick lines denote the theoretical excitations computed. After trying to fit using a costant stress drop, we then programmed a frequency dependent stress drop scaling Δσ = M 0 f c 3 (3) with the constrain that M 0 f c p = costant. We found: logΔσ = (1- 3/p ) log M 0 + log ( k -3/p ) (4) where log M 0 = 1.5 ( M w + 6.03). By referring everything to D s ref = 10 MPa and M wref =3, we can compute stress drop that changes with M w : logΔσ = (1- 3/p ) ( M w - M wref ) + logΔσ ref (5) Fig. 3 – Filtered ground velocity excitation terms relative to the aftershocks recorded during the Maule seismic sequence in 2010 (black lines). Red thick lines indicate the theoretical prediction at the indicated levels of moment magnitude. 91 GNGTS 2013 S essione 1.1