GNGTS 2014 - Atti del 33° Convegno Nazionale

96 GNGTS 2014 S essione 1.1 The geographic distribution of the recording stations is irregular with high density located in the Piedmont FVG zone (the interstation distance is 10 km) and a lower density in Veneto Plain (the interstation distance is 50 km). The different stations density influences strongly the events recognition and, therefore, the magnitude completeness in various areas; however the selection of a minimum number of stations as function of maximum source-receiver takes a consistent improvement in the percentage of recognized earthquakes. The grid association utilizes the IASPEI velocity model (Kennett, 1991) on all four grids (‘Local 1’, ‘Local 2’, ‘Regional’ and ‘Teleseismic’); in this step we can not use the Friuli model (Bressan, 2005) on the local grid because the adoption of different velocity models in different grids can take to skews in the computation of the origin times generating false double events in case of earthquakes occurred close to the grids border. The GENLOC algorithm performs inversion from a starting location done by grid associator; the GENLOC searches a local minimum close to the starting location but, if it is totally wrong, the inversion can not adjust this result. The adoption of a local velocity model improves the accuracy of the locations and it reduces the related uncertainties. In this procedure we must utilize two different grids for local earthquakes because the irregular stations distribution complicates the identification of the microseismicity in some zones of Veneto; the adoption of two detections with different SNR thresholds and different parameters with the association of the S phases in ‘Local 2’ configuration (Tabs. 1 and 2) facilitate the recognition of the weakest events also in areas where the stations distribution is not very dense. Our procedure is tested on the signals acquired in 2011 and it recognizes and locates 80% events in addition to the earthquakes of the published catalogue; all events with M L ≥��� ��� 1.0 are correctly recognized and located by our automatic procedure while the false earthquakes are about 18% of all located events. In FVG and in western Slovenia the capability of detecting the microseismicity range increases strongly and many events of low magnitude are located on the principal seismogenic structures of the area; on the other side the new catalogue has a smaller number of new events in Veneto with the locations that can be influenced by relevant uncertainties especially in zone where there are few instruments. The low spatial homogeneity of the recording stations can take to these discrepancies and to different values of completeness in various NEI zones; however future development of the network will improve the homogeneity of the catalogue increasing the number of detected events in the microseismicity range in the entire monitored area. Acknowledgments. We are grateful to the technical staff of the OGS Centro di Ricerche Sismologiche (CRS) for their efforts in the management of the information technology infrastructures and in the maintenance of the stations. References Bragato P.L., Di Bartolomeo P., Pesaresi D., Plasencia Linares M. and Saraò A.; 2011a: Acquiring, filing, analyzing and exchanging seismic data in real time at the Seismological Research Center of the OGS in Italy . Ann. Geophys., 54 , 67-75. Bragato P.L., Sugan M., Augliera P., Massa M., Vuan A. and Saraò A.: 2011b: Moho reflection effects in the Po Plain (Northern Italy) observed from instrumental and intensity data . ����� �������� ���� �� Bull. Seismol. Soc. Am ., 101 , 2142-2152. Bressan G.: 2005: Modelli di velocita’ 1D dell’Italia Nord-Orientale . �������� ������ ������� ��� �� Internal Report 2005/20 CRS 5. Garbin M. and Priolo E.: 2013: Seismic event recognition in the Trentino area (Italy): performance analysis of a new semiautomatic system. Seismol. Res. Lett ., 84 , 65-74. Geiger L.: 1910: Herbestimmung bei erdbeden aus den ankunftzeitzen . K. Gessel. Wiss. Goett, 4 , 331–339. Gentili S., Sugan M., Peruzza L. and Schorlemmer D.: 2011: Probabilistic completeness assessment of the past 30 years of seismic monitoring in Northeastern Italy . Phys. Earth Planet. Inter ., 186 , 81–96. Kennett B.L.N.: 1991: IASPEI 1991 Seismological Tables . Bibliotech, Canberra, Australia, 167 pp. Klein F.W.: 1978: Hypocenter location program HYPOINVERSE. Part 1 . User’s guide to versions 1,2,3 and 4 . US Geological Survey, Open-File Report 78-694, 113 pp. Lee W.H.K. and Lahr J.C.: 1975: HYPO71 (revised): a computer program for determining hypocenter, magnitude and first motion pattern of local earthquakes . U.S. Geological Survey, Open file report, 75-311, Menlo Park, CA, USA, 113 pp.