DETAILED STUDY OF THE POTENTINO SEISMIC ZONE IN THE SOUTHERN APENNINES, ITALY

Most relevant macroseismic and instrumental data concerning past and current seismic activity of the Potentino, southern Italy, have been collected in order to study the seismic behaviour of this area. Ten among the most important recent and historical earthquakes occurred in the area from 1826 to 1990 have been studied by means of macroseismic methodologies; macroseismic fields for all the events indicate a general isoseismal lengthening in the direction of the Apennines chain (northwest-southeast), a rapid attenuation of intensity westward of the studied area and a strong propagation eastwards; this configuration is strongly dependent on the source mechanisms of the seismic events and the geological dishomogeneities of the concerned areas. As regards earthquakes which have been instrumentally recorded in the Campania-Basilicata area from 1980 to 1991, more than 1300 accurate hypocentral locations have been computed for events with M(D) greater than or equal to 2.5, using data from Osservatorio Vesuviano (O.V.) and Istituto Nazionale di Geofisica (I.N.G,) seismic networks. Special emphasis is put on the Potenza May 5, 1990 earthquake (M(S) = 5.4 NEIC-National Earthquake Information Center; I = VII MSK) which was the strongest recent event in the area and was felt in a large sector of southern Italy. As regards the seismic sequence following this event, about 110 aftershocks with M(D) greater than or equal to 1.9 have been located very close to the town of Potenza, northeast of it, with a clustering-type space distribution. The occurrence of the events is strongly time concentrated, in so defining a main shock-aftershocks typical sequence. Moreover, the macroseismic investigation of the main event has allowed to map out isoseismal lines bounding VII, VI, V and IV MSK areas; the isoseismals pattern shows that attenuation and amplification of intensity are observed in the same areas as for the previous events. Beginning from May 26, 1991 an increase of seismicity, initiated by an earthquake of M(L) = 4.7 NEIC and I = VI-VII MSK, was observed nearly in the same area as the 1990 sequence. Accurate locations of 84 events with M(D) greater than or equal to 1.9 have been computed. However, this sequence seems to have a slightly different character from the 1990 one, in that events are scattered in time and seismic energy release is comparable to a swarm-type seismic sequence. As regards the previous sequences, fault plane solutions for the highest-magnitude events and for appropriate groups of events have been worked out; the mechanisms indicate normal faulting motion with T axes oriented orthogonal to the Apennimic chain. In conclusion, detailed analyses on the historical and recent seismicity of the Potentino area and adjacent ones have allowed to identify at least four seismogenic zones which show activity with different features. Such zoning represents a contribution to the seismic hazard assessment in the Southern Apennines on a regional scale.