Tectonic geomorphology of the Paganica Fault Zone (2009 L’Aquila Earthquake, Italy) for the understanding of its seismic behavior

The Mw 6.3, April 6, 2009 earthquake occurred on the Paganica fault (PF hereinafter) and it produced a 3 km-long co-seismic surface rupture along its central section, with few centimeters of vertical displacement. The PF, NW-SE striking and SW dipping, is a normal active structure running for a total length of 20 km and, along with antithetic faults on its hanging-wall, it forms the graben of the Middle Aterno River Valley. Extensive 1:10,000-scale geological and geomorphological mapping has been carried out, focusing on the reconstruction of the long-term expression of the PF. Particular attention has been devoted to the study of the deposits infilling this graben with the aim to reconstruct the Quaternary deformational history of the fault. Field mapping was integrated by observations, made on 1:33,000 scale aerial photographs (GAI), 10-m-resolution Digital Elevation Model and standard morphometric derivatives (hill-shaded and slope angle maps, Spatial Analyst™). The whole fault system was studied and variable characteristics of the tectonic deformation affecting the continental deposits at the surface were identified. The PF long-term morphologic signature is represented by a set of prominent scarps formed by the tectonic juxtaposition of Pliocene-middle Pleistocene and late Pleistocene alluvial deposits, and by smaller scarps in late Pleistocene-Holocene deposits. The study of the long-term fault expression results helpful to recognize deformed geomorphologic markers, essential for long-term slip-rate calculations, and to locate and interpret paleoseismological trench sites. In order to calculate the long-term slip rate of the PF, we sampled depositional and erosional surfaces for 14C and OSL (Optically Stimulated Luminescence) dating. The preliminary results are valuable for a comparison with the estimates carried out by paleoseismological investigation of the PF (see abstract Cinti et al., same session). Furthermore, we correlated the 2009 co-seismic features with the long-term morphologies and tectonic structures. The co-seismic ruptures showed a general coherence with the long-term PF trace, both in terms of location and style of the deformation pattern. Despite this agreement, the 2009 ruptures are confined only to the central portion of the PF trace where the long-term deformation results to be well localized. Conversely, the co-seismic deformation resulted in a distributed and not evident extensional strain where the PF system branches out, with many splays capable to accommodate it. The preserved fault-related geomorphology is witness of the persistence, for several Late Quaternary seismic cycles, of the rupture complexities. Although the long-term expression of the PF system suggests that this structure is capable of larger Magnitude earthquakes than the April 6, 2009 event, further studied are needed in order to characterize the style of fault activity and the model describing the patterns in which displacement may reoccur on faults.