GNGTS 2014 - Atti del 33° Convegno Nazionale

• Zone 1: The eastern part of the study area is the Kobarid region (western Slovenia) and it is characterized low QS values, corresponding to large attenuation, in good agreement with the results of Gentili and Franceschina (2011) on the high frequency attenuation parameter k. Low QS values are associated with large amount of fracturing and variable pore pressure as suggested by the highly variable VP/VS pattern (Bressan et al., 2009, 2012) and low heat flow (30-40 mWm-2 - Lapanje et al., 2011) characterizing the area. The zone is composed by high shear modulus rocks [1.8 to 2.5 x 1010 Nm-2: Bressan et al. (2012)] of prevailing platform and dolomitic limestones (Bressan et al. , 2009), locally high- fractured, corresponding to the source zone of the magnitude 5.6 and 5.2 1998 and 2004 earthquakes, the most relevant events occurred in the study area in the last 30 years (Gentili and Bressan, 2008). • Zone 2: The central part of the study area shows marked lateral variation of Q S at 6 – 9 km depth, that we attributed to the variation of mechanical properties of the geologic units. We relate high Q S values to Triassic and dolomitic limestones, characterized by high shear modulus and variable V P /V S values; low Q S values, viceversa, can be connected with less competent rocks, composed by alternation of mixed carbonatic-clastic limestons, dolomitic limestones and chert limestones of Triassic-Jurassic age �������� (Bressan et al. , 2012)�. • Zone 3: Thewestern part of the study area shows a zone of lowattenuation (high Q S ) partially overlapping a zone of high attenuation (low Q S ). The high Q S structure is characterized by higher shear modulus and on average lower V P /V S values (Bressan et al. , 2012) with respect to the low Q S zone. Also in this case, we attributed the variation of attenuation to the variation of the mechanical properties of the geologic units. High Q S images are attributed to dolomitic limestones with variable density of cracks. Low Q S images are associated with Triassic limestones and to Paleozoic sandstones and limestones. Finally, we emphasize that the large variability in mechanical characters of the crust are related to the complex deformation pattern of the area, resulting from several tectonic phases with different orientation of the stress tensor. Acknowledgements. We would like to thank Gianni Bressan for his valuable discussions and suggestions. Thanks are due also to M. Živčič of the Geophysical Survey of Slovenia (Ministry of the Environment and Spatial Planning) for kindly providing the waveforms of 2004 aftershocks recorded by locally temporary stations. The local seismic network is managed by the Dipartimento Centro di Ricerche Sismologiche of Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS) and financially supported by the Civil Protection of the Regione Autonoma Friuli Venezia Giulia, and of the Regione Veneto, and by the Geological Survey of Provincia Autonoma di Trento. We thank the technical staff of the Department Centro di Ricerche Sismologiche (OGS) for data acquisition and processing. References Aki K. and Richards, P.G.; 1980: Quantitative Seismology, Theory and Methods . Vol. 1, W.H. Freeman and Company, San Francisco, CA. Anderson J.G. and Hough S.; 1984: A model for the shape of Fourier amplitude spectrum of acceleration at high frequencies . Bull. seism. Soc. Am., 74 , 1969–1994. Bianco F., Del Pezzo E., Malagnini L., Di Luccio F. and Akinci A.; 2005: Separation of depth-dependent intrinsic and scattering seismic attenuation in the northeastern sector of the Italian Peninsula . Geophys. J. Int., 161 , 130–142. Bressan, G., Gentile, G. F., Perniola B. &Urban S.; 2009: The 1998 and 2004 Bovec-Krn (Slovenia) seismic sequences: aftershock pattern, focal mechanisms and static stress changes . Geophys. J. Int., 179 , 231–253. Bressan G., Gentile G. F. , Tondi R., De Franco R. and Urban S.; 2012: Sequential Integrated Inversion of tomographic images and gravity data: an application to the Friuli area (north-eastern Italy) . Bollettino di Geofisica Teorica ed Applicata, 53 , 2, 191-212. Carroll D. L.; 2001: ���������� ������� ��������� �������� ��� ��������� ����������� ������� GAFORTRAN. Genetic algorithm approach for parameter estimation. Urbana. http://cuaerospace. com/carroll/ga.html Chen T. and Clayton R. W.; 2012: Structure of central and southern Mexico from velocity and attenuation tomography . J. Geophys. Res., 117 , B09302, doi:10.1029/2012J2IB009233. De Siena, L., Del Pezzo, E. and Bianco, F.; 2010: Seismic attenuation imaging of Campi Flegrei: Evidence of gas reservoirs, hydrothermal basins, and feeding systems , �� �������� ���� J. Geophys. Res. , 115 , B09312, doi:10.1029/2009JB006938. 150 GNGTS 2014 S essione 1.2