A detailed analysis of two seismic sequences in Abruzzo, Central Apennines, Iitaly

A three-component digital seismic network has been installed along central Apennines since the end of 1991. Two seismic sequences having main shocks o f magnitudes 3.9 and 3.7 were recorded in August 1992 and June 1994, respec tively. A detailed analysis of these sequences, including multiplet relocat ion, fault-plane solutions and source parameter estimation, is performed in the present paper. A correlation analysis allowed us to recognize a number of correlated events in the two sequences which were used for relative loc ations using a master event technique. This analysis allowed to obtain a be tter alignment of epicentral data along two almost orthogonal directions, f ollowing an Apenninic and an anti-Apenninic trend. For the two sequences, f ault-plane solutions were evaluated by using a first arrival technique, res ulting in mechanisms with predominant normal faulting for the 1992 and 1994 swarms. S-wave polarization analysis allowed to check the stability of the previous solutions and to reduce their range of uncertainty. The same tech nique was also applied to derive the composite fault-plane solutions from t he aftershocks, resulting in solutions which are in good agreement with tho se derived from the main shocks of both sequences. Source parameters were t hen derived from the three-component records of 28 well-recorded events wit h seismic moment in the range 8.5 x 10(10)-1.0 x 10(14) Nm. Stress drops ra nged in the interval 0.3-52.3 bar and source radii were of the order of 100 m. Their scaling relations are in good agreement with other results derive d from the analysis of other Italian earthquakes that occurred in regions o f predominantly normal faulting tectonics (Apennines and Calabrian arc).