GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 14 THE 2019-2020 REPEATED SMALL SWARMS IN THE BENEVENTO AREA, SOUTHERN APENNINES, ITALY M.G. Ciaccio, L. Improta, A. Marchetti, A. Nardi, BSI Working Group * Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Between November 2019 and November 2020, four seismic swarms occurred 7 km to the SW of the Benevento town in the Sannio region, one of the most seismic active areas in the Southern Apennines (Italy). These swarms were nearly collocated with the largest magnitude seismic event occurring on December 16, Mw3.9. According to the Database Macrosismico Italiano, DBM15 (Locati et al ., 2021), the seismic hi- story of Benevento shows strong effects caused by several earthquakes that produced numerous causalities and severe damage, in particular the 1688 and 1702 earthquakes (local Intensity=9) and the 1456 earthquake (local Intensity=8.5). Significant local site effects also contributed to the high seismic hazard and controlled the distribution of the damages and casualties during the 1688 Mw6.7 destructive event (Improta et al ., 2005; Di Giulio et al ., 2008). Besides, detailed sce- narios studies carried out in the framework of a multidisciplinary project coordinated by the Na- tional Group for Earthquakes Defence (GNDT; see Traiano Project, 2002) pointed to the very high vulnerability of the building heritage of the town that would experience widespread collapses in case of a future like-1688 earthquake.  In the last decades, the background seismicity of the Sannio area has occurred as sparse events or low magnitude swarms (in Fig. 1, we show the seismicity recorded by the Rete Sismi- ca Nazionale, http://cnt.rm.ingv.it/ before November 2019): considering the recurrence time of large historical earthquakes, this region soffers one of the highest seismogenic potentials of the Italian country. Of relevance, no background seismicity was recorded in the area located to the south of Benevento before 2019-2020, representing an evident seismic gap.  The 2019-2020 Benevento seismicity comprises of nearly 150 events with Ml ranging betwe- en 0.9 and 3.9 (8 earthquakes with M≥3.0, Tab. 1). We manually picked the first P- and S-wave arrival times, assigning a weighting factor inversely proportional to the uncertainty in the pick. First, we locate earthquakes using a multi-parameter procedure (Ciaccio et al ., 2021): to lo- cate seismic events, we choose the linearized approach by Lahr,1999 (the Hypoellipse code), but exploring the hypocenter solutions space by changing three keys a priori conditions that typically strongly influence the solution convergence in the linearized approach: the starting trial depth, the weight function of arrival times with distance, and the travel time residual cut. Then we computed relative locations by applying a double-difference technique (HypoDD; Waldhauser and Ellsworth, 2000) that minimizes the residuals between observed and theoretical travel time differences (or double differences) for pairs of earthquakes at each station. We use a 1-D velocity model defined explicitly for the area. The deterministic model is con- strained for the first 4 Km by a sonic velocity log from a nearby oil exploration well, and for the deep part from previous tomographic studies (Improta et al ., 2014). The seismicity is distributed in the map (Fig. 2-A, Fig. 3, different colors denote different clu- sters) in a relatively narrow WNW-ESE direction for a near 6 km-long distance, showing a mi- gration moving towards NW and SE over time. Fig. 2-B shows aftershocks projected onto the N30°-oriented section. The alignment of seismicity delineates a well defined sub-vertical plane * Bollettino Sismico Italiano Working Group : Mariucci M.T., Pinzi S., Pizzino L., Sciarra A., Smedile A., Spadoni S., Arcoraci L., Baccheschi P., Battelli A., Battelli P., Berardi M., Cantucci B., Castellano C., Castello B., Cheloni D., Colini L., Di Maro R., Frepoli A., Latorre D., Lisi A., Lombardi A.M., Margheriti L., Mele M. F., Melorio C., Miconi L., Modica G., Monna S., Montuori C., Moretti M., Pagliuca N.M., Pastori M., Pintore S., Rossi A., Sciarra A., Scognamiglio L., Sgroi T., Tardini R., Thermes C., Tozzi R.