Seismic Hazard of L'Aquila downtown (central Italy): new insights for 3D geological model based on high-resolution seismic reflection profile and borehole stratigraphy

Abstract. On 6 April 2009, a Mw 6.1 earthquake struck the Plio-Quaternary intermontane L'Aquila Basin in central Italy, causing strong damages in L'Aquila historical downtown and surroundings, which were affected by notable site effects. Previous works have suggested that different site effects may be related to the complex subsurface geologic architecture, given by the variability of thickness and lithology of L'Aquila Basin clastic deposits, on which the city was built. To improve the 3D geological model of L'Aquila downtown for seismic site response evaluation and to estimate the Seismic Hazard of possible buried active normal faults, a multitask project has been carried out consisting mainly of the integration of subsurface dataset, including geological and geophysical surveys. Data have been interpreted with the aim to conceive and build a detailed model for the Plio-Quaternary cover of the continental basin and the buried morphology of the Meso-Cenozoic bedrock. We report the results concerning the interpretation of a 1 km-long high-resolution seismic reflection profile and refraction tomography integrated with the stratigraphy from deep and shallow boreholes. The results allowed us to reconstruct the Plio-Quaternary succession below L'Aquila downtown. The Plio-Quaternary depocentre corresponds to a minor NNW-SSE graben, which is developed within the main regional graben that borders L'Aquila Basin. Finally, data interpretation allowed to reconstruct the Plio-Quaternary tectono-stratigraphic evolution of the basin, to evidence the recent activity of several faults, and to define the subsoil geological model of the study area. All these data, which are functional to define the seismic site effects and to detect the activity of faults, are useful to mitigate the Seismic Hazard of cultural heritage cities of central Italy, such as the case study of L'Aquila downtown.