GNGTS 2018 - 37° Convegno Nazionale

36 GNGTS 2018 S essione 1.1 FIRST-MOTION FOCAL MECHANISM SOLUTIONS OF ITALIAN EARTHQUAKES (M ≥ 4.0) FROM JANUARY 2015 TO AUGUST 2018 OUTSIDE THE AMATRICE- VISSO-NORCIA SEISMIC SEQUENCE ZONE (ITALIAN SEISMIC BULLETIN) M.G. Ciaccio, Gruppo di Lavoro Bollettino Sismico Italiano Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy The ItalianSeismicBulletin. The ItalianSeismicBulletin (BSI) has beenproduced for decades by Istituto Nazionale di Geofisica e Vulcanologia (INGV), but from 2015 the BSI is publishing every 4 months and is available at cnt.rm.ingv.it/bsi ; the contents of the 4 months are described in a technical document ( http://webservices.rm.ingv.it/bsi//2017/20170101_20170430__INGV.pdf ) . The BSI locations (1985-2018) are available in the database ISIDe (ISide Working Group, 2016): since January 2015, BSI analysts elaborate events with ML ≥ 3.5, i.e. “strong events”, quickly in the days immediately following their occurrence, and particular attention was paid to the analysis of the earthquakes with magnitudes M≥4.0, determining new location and focal mechanism using first polarity techniques (Hypoellipse code; Lahr, 1999; FPFIT code; Reasenberg and Oppenheimer, 1985). The Amatrice-Visso-Norcia seismic sequence (AVNss) started on August 24th, 2016 and it includes the strongest earthquake occurred over the last 30 years in Italy. The BSI group dedicated a particular analysis strategy for the AVNss earthquakes with specific rules, while the earthquakes outside the box (with min/max latitude: 42.2/43.2 - and min/max longitude: 12,4/14,1) were published in the regular 4 months releases. In this work, we present the focal mechanisms of the earthquakes with magnitudes M≥4.0 occurred outside the AVNss zone: a database of relocated events with focal mechanisms determined will be released as catalog in the BSI web page. We separately analyzed the events with M ≥ 3.5 occurred during the AVNss seismic sequence to prepare a special issue of the BSI. The catalog of first motion focal mechanisms: earthquakes M ≥ 4.0 occurred between January 2015 - August 2018 outside the AVNss zone. A catalog of regional earthquake focal mechanisms can be used to improve understanding of the contemporary seismotectonic processes and can contribute to an evaluation of the potential seismic hazard. In the Italian region, during the period January 2015-August 2018, 111 earthquakes (M ≥ 4.0) occurred; between them, 35 seismic events occurred outside the AVN zone (Tab. 1), 13 earthquakes with hypocenters deeper than 200 km depth. We present the focal mechanism solutions generated with the computer program FPFIT and we compare them with the TDMT (http://cnt.rm.ingv.it/tdmt , Scognamiglio et al., 2009) and RCMT solutions available (http://autorcmt.bo.ingv.it/quicks.html, Pondrelli et al. , 2011 and references therein). This allows us to assess the goodness of the cataloged focal mechanism and, at the same time, to try to understand what further information can give us first-motion polarities respect to the analysis adopting the long-period full waveform inversion code. Indeed, while the moment tensor solution provides a good indication of the general style of deformation occurring in an area, careful consideration of the first motion solution is necessary for detailed study of source mechanics. FPFIT reads the hypocenter summary from Hypoellipse program (Lahr, 1999) and computes double-couple fault plane solutions from P-wave first motion data using a grid search method. We used the dataset collected for the BSI: the analysts assign weights to the first P-arrival times because the quality of the first P-onset varies from station to station. The weight ranges from 0 to 4, high weight represents low quality. We used polarities, picked and reviewed by data analysts, with weight 0 or 1, and we required a minimum number of observations of 8 P- polarities per event. We present the solutions for M ≥ 4.0 but we are working to collect a complete catalog from M ≥ 3.5.