Background seismicity in the Central Apennines of Italy: The Abruzzo region case study

We investigate background seismic activity of the Abruzzo region, a 5000 km2 area located within the Central Apennines of Italy, where in the past 600 years at least 5 large earthquakes (I=XI–X) have occurred. Between April 2003 and September 2004, a dense temporary seismic network composed of 30 digital three-component seismic stations recorded 850 earthquakes with 0.9bMLb3.7. We present earthquake locations and focal mechanisms obtained by standard procedures and an optimized velocity model computed with a search technique based on genetic algorithms. The seismicity occurs at a low and constant rate of ∼2.6 e−04 events/day⁎km2 and is sparsely distributed within the first 15 km of the crust. Minor increases in the seismicity rate are related to the occurrence of small and localised seismic sequences that occur at the tip of major active normal faults along secondary structures. We observe that during the 16 months of study period, the Fucino fault system responsible for the 1915 Fucino earthquake (MS=7.0), and the major normal faults of the area, did not produce significant seismic activity. Fault plane solutions evaluated using P-wave polarity data show the predominance of normal faulting mechanisms (∼55%) with NE-trending direction of extension coherent with the regional stress field active in this sector of the Apennines. Around 27% of the focal solutions have pure strike–slip mechanisms and the rest shows transtensional faulting mechanisms that mainly characterise the kinematics of the secondary structures activated by the small sequences. We hypothesize that the largest known NW-trending normal faults are presently locked and we propose that in the case of activation, the secondary structures located at their tips may act as transfer faults accommodating a minor part of the extensional deformation with strike–slip motion.