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

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In this paper we analyze a seismic sequence that occurred in the Sannio–Matese mountains (Southern Apennines, Italy), one of the most dangerous seismic 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 similar characteristics. The seismic energy release process of this sequence exhibits 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–aftershock model, consists of low energy events closely clustered in time and space without a single main shock. About 2800 micro-earthquakes were recorded from March to September 1997 by the seismic networks of the Istituto Nazionale di Geofisica and of the Osservatorio Vesuviano; among them, only 20 events had M D≥3.0. A selection of 370 earthquakes recorded at a minimum of five stations with at least three S phase readings have been located between latitudes 41°20′ and 41°30′ and longitudes 14°36′ and 14°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 T-axes 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 hypocentral distribution, the prevalence of focal mechanisms with fault planes striking about NE–SW, as well as the T-axes distribution, suggest that earthquakes relative to this sequence took place along NNE–SSW-oriented faults and that the local stress field acting in this sector of the Southern Apennines is prevalently transtensional and is strongly controlled by local faults heterogeneities.