GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 CARS - Catalog of Relative Seismic Locations of Italian Earthquakes occurred in the period 1981-2018 M. Michele, R. Di Stefano, L. Chiaraluce, D. Latorre, B. Castello Istituto Nazionale di Geofisica e Vulcanologia The Istituto Nazionale di Geofisica e Vulcanologia (INGV) monitors the Italian peninsula seismicity through data recorded by the Italian National Seismic Network (RSN) plus the contribution of data collected by other permanent regional networks (PRN). Seismic activity is real-time located by the INGV surveillance system and then manually revised by analysts of the Italian Seismic Bulletin (BSI) group. We present an earthquake CAtalog of Relative Seismic locations (CARS) consisting of about 310,000 events that occurred in Italy the period 1981-2018. CARS is essentially a relocation of CLASS, an earthquake catalog of Italian seismicity (from Latorre et al., 2023) consisting of homogeneous absolute locations constrained within a regional 3D velocity model. The relative locations are obtained inverting for P- and S- waves arrival times derived from data collected by RSN plus PRN for the period 1981-2008 and only by RSN for 2009-2018. For this latter period, we also integrated the absolute travel times with relative ones obtained by waveforms similarities analysis grounded in cross-correlations measurements and performed on pairs of similar events. The time domain cross‐correlation method proposed by Schaff et al., 2004 and Schaff & Waldhauser, 2005 was applied to seismograms of all pairs of events separated by 10 km or less and recorded at common stations. Seismograms were filtered in the 1–15 Hz frequency range using a four pole, zero phase band‐pass Butterworth filter. The correlation measurements were performed on a 1.0 s long window for P- and S-waves. We retain all measurements with correlation coefficients greater than 0.7 resulting in a total of ~17 million P- and ~23 million S-wave delay times. The resulting cross‐correlation delay time measurements have been combined with delay times computed from picks for event pairs. We used the outcome as input for the HypoDD code (Waldhauser and Ellsworth, 2000), that being based on the double-difference algorithm, can ingest differential arrival times. For locating the events we used 1D velocity models characterising 18 different (geologically, seismically and tectonically homogeneous) Italian macroareas (Fig. 1a - after Pastori et al. B2-2019-2021, Wp1-task4). The HypoDD code appears to be steady operational when managing a maximum number of about 15,000 events described by an average number of readings usually less than 200.