The 2012-2015 Seismicity of the Sannio-Irpinia Border Region (Southern Italy) Ada De Matteo (1), Bruno Massa (1,2), Girolamo Milano (2), and Luca D’Auria (2,3)
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The 2012-2015 seismicity of the Sannio-Irpinia border region (Southern Italy) Ada De Matteo (1), Bruno Massa (1,2), Girolamo Milano (2), and Luca D’Auria (2,3) (1) Universita` degli Studi del Sannio, Dipartimento di Scienze e Tecnologie, Benevento, Italy ([email protected]); (2) Istituto Nazionale di Geofisica e Vulcanologia, sezione di Napoli Osservatorio Vesuviano, Napoli, Italy ([email protected]); (3) Consiglio Nazionale delle Ricerche, Istituto per il Rilevamento Elettromagnetico dell'Ambiente 13°30' 14°00' 14°30' 15°00' 15°30' 16°00' 16°30' 17°00' 43°00' 43°00' 1. INTRODUCTION N The Sannio and Irpinia seismogenic regions fall in the axial-frontal sector of the Southern Apennine belt (Fig. 1). These regions 42°30' 42°30' have been struck by several historical destructive earthquakes as the most recent 1962 and 1980. Looking at the border between these two areas historical destructive earthquakes, with Io≥ IX MCS, occurred in 1456, 1702, 1732. Although the location, geometry and kinematics of the causative faults of these earthquakes are still matter of debate, at present almost three main 42°00' 42°00' hypotheses of active faults related to these strong earthquakes have been proposed: the Miranda-Apice, the Pago Veiano- 9 10 Montaguto and the Mirabella Eclano-Monteverde (Fig.1; DISS Working Group, 2015). These three structures would cross the Isernia Sannio-Irpinia seismogenic border. 41°30' Campobasso 41°30' Foggia 1 2 8 Ariano Irpino Benevento 3 41°00' Caserta 41°00' 2. DATA AND RESULTS 4 7 6 5 In order to characterize the seismicity of the Sannio-Irpinia border, we studied the present-day seismicity 40°30' 40°30' recorded by the Italian National Seismic Network of the INGV (Fig. 1). In detail the analysis has been mainly devoted to a short time interval across 2012-2015 years, due to the occurence of two low energy seismic swarms. In order to obtain accurate hypocentral distributions and reliable fault plane solutions we 40°00' 40°00' collected the wave forms of all the seismic events occurred in the study area to perform a re-picking of P- and S- phases and to obtain a reliable hypocentral location. The re-picking allows also to collect P-wave 0 10 20 40 60Km 39°30' 39°30' 13°30' 14°00' 14°30' 15°00' 15°30' 16°00' 16°30' 17°00' dataset to compute fault plane solutions. Figure 1 – Spatial distribution of the seismicity occurred between 2000-2015 in the Central The relocated events (ML≤3) clearly show an hypocentral distribution aligned along the axis of the Southern Appenine chain (data from ISIDE catalogue). The seismogenic sources reported in the Apennine fold-and-thrust belt, with hypocentral depth ranging from 5 to 25 km. A prevalence of oblique- DISS catalogue (DISS Working Group, 2015) are also shown with green boxes (1 Miranda- Apice; 2 Pago Veiano-Montaguto; 3 Mirabella Eclano-Monteverde; 4 Andretta-Filano; 5 Conza slip kinematics is observed for isolated events. The December 2014 swarm (ML<3), with hypocenter depth della Campania-Tolve; 6 Irpinia-Agri Valley; 7 Rapolla-Spinazzola; 8 Castellucci dei Sauri-Trani; 9 Ripabottoni-San Severo; 10 San Marco in Lamis-Mattinata). The area delimited by a blue box between 9 and 14 km, is characterized by normal dip-slip kinematics with NW-SE striking nodal planes. represents our study area (Fig. 2). Yellow stars represent the epicentres of the main events of the Agust 21th 1962 Ariano Irpino Earthquake. The July 2015 swarm (ML<3.5; depth between 19 and 23 km) shows strike-slip kinematics (Fig. 2). Longitude Depth (km) 15˚00' 15˚20' 15˚40' 0 10 20 30 41˚45' km 14°45' 15°00' 15°30' 15°45' 41°45' 41°45' 0 10 20 9 10 CIGN N 41˚30' A 2015 0 10 20 Km 41°30' 41°30' GATE Foggia MOCO 41˚15' a 1 8 PSB1 2 Latitude BIOG Ariano SGTA Irpino 2014 Benevento MRB1 41˚00' 3 41°00' CAFE 41°00' 4 7 SNAL M VULT 0-1 5 1-2 6 A' 2-3 40˚45' 3-4 40°45' MCRV MRLC 40°45' 0 magnitude 14°45' 15°00' 15°30' 15°45' 10 1 2 20 3 Depth (km) Depth Figure 2 – Epicentral distribution of the best relocated events occurred between 2012-2015 in the study area (see Fig.1). The dimension of 30 the circles is proportional to the magnitude of the events. The trinalges represents seismic stations of the INGV national network running in the area. The seismogenic sources reported in the DISS catalogue (DISS Working Group, 2015) are represented by green boxes (1 Miranda- 0 Apice; 2 Pago Veiano-Montaguto; 3 Mirabella Eclano-Monteverde; 4 Andretta-Filano; 5 Conza della Campania-Tolve; 6 Irpinia-Agri Valley; 7 Rapolla-Spinazzola; 8 Castellucci dei Sauri-Trani; 9 Ripabottoni-San Severo; 10 San Marco in Lamis-Mattinata). Orange circles A A' highlight the seismic swarms of June 2015 and December 2014. The fault plane solutions of the events with at least 10 P-wave polarities are 10 also shown. b 20 3. COMMENTS (km) Depth 30 The study area represents the 0 20 40 60 80 100 border between the Sannio and Distance (km) Irpinia seismogenic regions. For Figure 3 – a) N-S and W-E hypocentral distribution of relocated (2013-2015; pale blue circles) and from ISIDE catalogue (2004-2012; crosses) events. In the two cross sections the projection of the focal mechanisms (see Fig.2) is shown. this restrict area of the Southern b) Hypocentral distribution and projection of focal mechanisms along the A-A’ cross section (Appenine axial direction). This Apennines the DISS catalog section highlights the differences in the tectonic regime between the northern domain (Sannio region) with a predominantly strike-slip regime and the southern one (Irpinia region) where a normal faulting regime prevails. Furthermore the hypocentral proposes a quite complex depths gradually decreases southward. arrangement of possible seismogenic sources (green box in Fig.1).In particular, the study area represents the connection between the NW- SE striking peri-axial structures and the roughly E-W ones affecting the frontal-foreland sector of the Appenine (Fig.1). Notwithstanding the short time interval investigated, the hypocentral distribution of the relocated events shows a deepening of almost all the focal mechanisms of isolated events, as well as those of the 2014 swarm, are compatible with the NW-SE striking peri-axial structures. The hypocentral depths (about 20 km) and the kinematics of events belonging to 2015 swarm agree with the overall dynamics of the Appenine frontal-foreland sectors (Fig.3). This last swarm is located just South of the known north-Apulia seismogenic sources responsible Figure 4 – Spatial distribution of the T- (blue) and P- (red) axes of the focal for the 2002 San Giuliano Earthquake. The T- and P- axes of all the computed focal mechanisms agree with the mechanisms reported in figure 2 (length of the axes is proportional to the cosine of the corresponding plunge). Main seismogenic sourses from DISS catalogue are also large scale stress regime acting in this portion of the Appenine (Fig.4) 4. REFERENCES - DISS Working Group (2015). Database of Individual Seismogenic Sources (DISS), Version 3.2.0: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/, Istituto Nazionale di Geofisica e Vulcanologia; DOI:10.6092/INGV.IT-DISS3.2.0 - Di Filippo D. & Peronaci F. (1963). Indagine preliminare della natura fisica del fenomeno che ha originato il periodo sismico dell’agosto 1962. Annali di Geofisica, 16, 625-643..