Historical earthquakes and variable kinematic behaviour of the 2009 L'Aquila seismic event (central Italy) causative fault, revealed by paleoseismological investigations

Paleoseismological investigations has been carried out along the causative fault of the Mw 6.1 2009 L'Aquila earthquake. We dug three trenches in two different sites on the Paganica fault to understand its Holocene kinematic behavior. Paleoseismological results show a minimum of three faulting events preceding the 2009 earthquake. The most recent recognized event occurred after 1420–1610 A.D., tentatively attributed to the 1703 (February 2) seismic shock; a previous event happened between 1st–2nd century A.D. and 7th–9th century A.D., probably in the interval 4th–6th century A.D., while another identified previous event occurred prior to 1st–2nd century A.D. The observed minimum displacement per event varies from 0.25 m to 0.50 m consistent with a fault length of about 22–27 km, from which we infer a maximum expected Mw 6.6–6.7. This implies that the Paganica fault may occasionally rupture during 2009-like seismic events as well as during larger earthquakes, activating contemporaneously to nearby active faults kinematically and structurally linked to the Paganica fault. More specifically, on the basis of our data and of recent paleoseismologic studies, we hypothesize a linkage of the investigated fault with the Upper Aterno Valley Fault system. In this perspective, the Paganica Fault can rupture as an independent fault segment with recurrence interval of about 3 hundreds of years and together with the Upper Aterno Valley Fault system with recurrence interval that varies between 1000 and 1500 years.