GNGTS 2013 - Atti del 32° Convegno Nazionale

Ground motion attenuation with distance and the variation of excitation with magnitude are parameterized for this area to define a consistent model that describes both peak ground motion and Fourier spectra observations (Fig. 2). The regression results are plotted with color lines and the theoretical predictions as a solid black curves. Regression results for Fourier amplitude spectra and peak velocities are used to define a piecewise continuous geometrical spreading function, frequency dependent attenuation paramenter Q ( f) , and a distance- dependent duration. A general form for a predictive relationship for observed ground motion is written as: AMP ( f, r ) = EXC ( f, r ref ) + SITE ( f ) + D ( r, r ref , f ) (1) where AMP ( f, r ) represent the logarithm of peak amplitude of ground motion velocity on each filtered seismogram recorded at the hypocentral distance r , EXC ( f, r ref ) is the excitation term for the ground motion at an arbitrary reference hypocentral distance r ref , SITE ( f ) represents the distortion of the seismic spectra induced by the shallow geology at the recording site, D ( r, r ref , f ) represents an estimate of the average crustal response for the region at the hypocentral distance r , at the frequency f . It is determined as a piecewise linear function (Yazd, 1993; Anderson and Lei, 1994; Harmsen, 1997), allowing to consider complex behavior of the regional attenuation. The results of the analysis show that the regional attenuation of the ground motion can be modeled with a geometric spreading function with a 40-km crossover distance. A body-wave geometric spreading, g (r) ~ r −1 , is used at hypocentral distances ( r < 100 km). Eq. (1) is solved in time domain, from multiple narrow band-pass signals. Due to the constraints applied to the system prior to the regressions, the excitation term represents the expected peak ground motion at the reference distance, as it would be observed Fig. 2 – The regional attenuation functional D ( r, rref, f ) obtained for the Maule region from the regression on the peak amplitudes of the band-pass-filtered ground velocities (right) and of the Fourier spectral amplitudes (left) at the sampling frequencies of 0.20, 0.25, 0.30, 0.40, 0.50, 1.00, 2.00, 3.00, 4.00, 0.50, 1.00, 2.00, 3.00, 4.00, 5.00, 6.00, 8.00, 10.00, 12.00, 14.00, 16.00, 18.00, 20.00, 22.00, 24.00, 26.00, 28.00 and 30.00 Hz (color curves). The attenuation function is normalized to zero at the reference hypocentral distance of 40 km. The black curves in the background are from a attenuation model: geometrical spreadind 1/r from the source, out to 100 km, r 0.7 for larger distances; the anelastic attenuation used was the following: Q ( f )=200 f 0.5 . 90 GNGTS 2013 S essione 1.1