GNGTS 2013 - Atti del 32° Convegno Nazionale

Attenuation tomography of Friuli Venezia Giulia Italian region S. Gentili, F. Gentile Centro di Ricerche Sismologiche, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Udine, Italy Introduction. The Friuli Venezia Giulia Italian region and the western Slovenia are very peculiar from the tomographic point of view, due to the large lateral variation of velocity and Vp/Vs ratio, that is related to the high level of fracturing and, on the east, to the inhomogeneity of the medium. In this work we carry out for the first time the attenuation tomography of Friuli Venezia Giulia Italian region and western Slovenia. In particular, we analyze the 3D distribution of the frequency independent part of the S-waves quality factor, starting from a dataset of high frequency attenuation parameter k. The spectral decay parameter k was estimated using data from small-to-moderate earthquakes recorded by the NEI (northeastern Italy) seismic network managed by the National Institute of Oceanography and Experimental Geophysics (Istituto Nazionale di Oceanografia e Geofisica Sperimentale, OGS – http://www.crs.inogs.it/ ). The next section describes the relation between attenuation parameter and quality factor. In the following, the method adopted for k decay parameter estimate is described. Subsequently, the seismicity distribution (historical and instrumental) and details of the application of method tomographic inversion to Friuli Venezia Giulia region are shown. Finally, the obtained results are compared with both the previous k attenuation studies and the tomographic studies on Vp Vs velocities in the same area. Attenuation parameter vs. quality factor. The attenuation of seismic waves which propagate through the Earth can strongly influence the ground motion recorded at a site thus modifying the energy content of the signal radiated from the source. The attenuation is modelled in literature either using the quality factor Q , a dimensionless parameter introduced to quantify the fractional energy ( E ) loss per cycle of oscillation as Q = 2 π E / ΔE (Aki and Richards, 1980), or using the attenuation parameter k (e.g. Anderson and Hough, 1984). The attenuation due to high frequency spectral decay of acceleration amplitude Fourier spectrum has been modelled as (Cormier 1982): (1) A 0 depends on the source and the geometrical spreading and f is the frequency. In this model t* is defined as : (2) where Q is the quality factor and V is the average velocity of the waves. Anderson and Hough (1984) modelled high frequency acceleration spectrum as: (3) Hough and Anderson (1988) modelled k as: (4) where Q I is the frequency-independent part of Q , and Q is parameterized as: (5) Q D and Q I are respectively the frequency dependent and independent part of Q. Q I (z) means that in their simplified model Q I depends only on the depth (plane and parallel layers) but can be generalized. Hough and Anderson (1988) noted that the model they used for k* is 69 GNGTS 2013 S essione 1.1