GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 33 (about 20 ≤ H ≤ 30 km) in the north-western flank. From the initial dataset we selected about 1,600 earthquakes located in the study areas of Timpe and Pernicana faults, (red and green in Fig. 2). In order to better define seismic clusters or epicentre alignments, the selected earthquakes have been relocated using ���the tomoDDPS algorithm (Zhang et al. , 2009) and the 3D velocity model of Alparone et al. (2012). Compared to more simple algorithms, this code uses a combination of both absolute and differential arrival time readings, between couple of events of an earthquake cluster. This essential feature allows to considerably improve the relative locations. In fact, the use of the differential arrival times ensures that, for earthquake clusters with foci lying close to each other, travel time errors due to incorrect velocity models in the volume outside the cluster will essentially cancel out (see Scarfì et al. , 2009). ����������� ������� ���� ���� ������� Preliminary results show that seismic events tend to cluster around active faults mainly in the depth range 3-10 km in the Timpe zone, and between -1 and 3 km along the Pernicana fault. Geodetic data . ��������� ��� Available GPS observations collected by the Mt. Etna Permanent GPS Network, spanning the 2005.00-2013.99 time interval and covering the eastern flank of Mt. Etna (Fig. 1), have been processed using the GAMIT/GLOBK software with IGS (International GNNS Service; http://igscb.jpl.nasa.gov ) precise ephemerides. To improve the overall configuration of the network and tie the regional measurements to an external global reference frame, data coming from 10 continuously operating IGS stations were introduced in the processing (BRUS, CAGL, GENO, GRAS, GRAZ, JOZE, MATE, MEDI, NOT1 and ZIMM; http://www.epncb . oma.be) . In a first step, we used daily double differenced GPS phase observations to estimate station coordinates and Earth orientation parameters. In this step, the observations were weighted according to the elevation angle, for which a cut-off angle of 10° was chosen. In addition, we used the latest absolute receiver antenna models by the IGS and we adopted the Saastamoinen (1972) atmospheric zenith delay models, coupled with the Global Mapping Functions (Böhm et al. , 2006) for the neutral atmosphere. Then, the GAMIT solutions were used as quasi observations in a Kalman filter (GLOBK) in order to estimate a consistent set of daily coordinates (i.e. time series) for all sites involved. Each time series was analyzed for linear velocities, periodic signals and antenna jumps by using the TSVIEW software package described in Herring (2003). To account for correlated noise during the site velocity estimation, the “Realistic Sigma” method (RSM) developed by Herring (2003) was adopted. Finally, estimated geodetic velocities were referred to the “Etn@ref” reference frame (a Fig. 2 – Map and W-E cross-section of the earthquakes located by 1D velocity model in the Etna area in the period January 2005 - December 2013 (data from Gruppo Analisi Dati Sismici, 2013). Dashed lines indicate the investigated sectors and the seismogenic zones.