Earthquake swarms in the Southern Apennines chain (Italy): the 1997 seismic sequence in the Sannio-Matese mountains

In this paper we analyze a seismic sequence that occurred in the Sannio-Mat ese mountains (Southern Apennines, Italy), one of the most dangerous seismi c areas of the Italian peninsula. On 19 March 1997 at 23:10 (UTC) a seismic event of M-D = 4.1 hit a zone near the border between the Campania and the Molise regions. Swarm-type earthquake activity followed the main event and lasted for more than six months, with alternation of intense activity and relative quiescence periods. Evolution of the seismic sequence both in time and space has allowed us to divide it into three stages with rather simila r characteristics. The seismic energy release process of this sequence exhi bits the typical swarm pattern of the minor seismic sequences occurring in the Southern Apennines and involving relatively minor faults located in the marginal border of the chain. Such pattern, unlike the main shock-aftersho ck model, consists of low energy events closely clustered in time and space without a single main shock. About 2800 micro-earthquakes were recorded fr om March to September 1997 by the seismic networks of the Istituto Nazional e di Geofisica and of the Osservatorio Vesuviano; among them, only 20 event s had M-D greater than or equal to 3.0. A selection of 370 earthquakes reco rded at a minimum of five stations with at least three S phase readings hav e been located between latitudes 41 degrees 20' and 41 degrees 30' and long itudes 14 degrees 36' and 14 degrees 42'. The events are clustered along a nearly vertical plane striking NNE-SSW and dipping towards the east; their hypocentral depths range from 5 to 15 km. The focal mechanisms of 67 events show a variety of different solutions, even if a prevalence of normal dip- slip solutions with a strike component is observed. The Taxes distribution of all mechanisms show two trends: the prevalent one is aligned along a NNW -SSE direction, whereas the other one is along a NNE-SSW direction. The hyp ocentral distribution, the prevalence of focal mechanisms with fault planes striking about NE-SW as well as the Taxes distribution, suggest that earth quakes relative to this sequence took place along NNE-SSW-oriented faults a nd that the local stress field acting in this sector of the Southern Apenni nes is prevalently transtensional and is strongly controlled by local fault s heterogeneities. (C) 1999 Elsevier Science B.V. All rights reserved.