Contributing to seismic hazard assesment in the French-Italian Alps-Mediterranean region : an onshore-offshore, multi-disciplinary and multi-scale approach

The seismic hazard in Italy is important because of high seismic activity through all the Apennines. Only some areas are considered less active, with a seismic hazard thus lower. This is the case in the area of Pisa, where no major active fault is recorded on the most current maps. However large normal faults exist, most structuring sedimentary basins like that of Viareggio. The objective of my work was therefore to revisit the issue of hazard in northern Italy. Based on literature data, I first studied the geological and tectonic setting of this region, including seismic activity and crustal structures. By combining digital topographic data and aerial and satellite images, I then analyzed the major extensive basins in the region by focusing on the morphology, geometry and distribution of their bounding faults and the links between these faults and instrumental seismicity recorded. Finally, on the basis of observations and field measurements, coupled with analysis of satellite images and seismic data, I analyzed the Viareggio basin, and its normal faults. The main fault system that controls the basin is oriented NW-SE and is about 70 km long. The morphology of the high escarpment, steep, linear and locally associated with triangular facets, attests to its recent activity. Other smaller structures show evidence of activity in other parts of the basin, or nearby (eg Camaiore Basin), usually emphasized by significant seismic activity. A dense instrumental seismicity is also recorded offshore near Viareggio Basin, suggesting the existence of active faults. On land, strong historical earthquakes are also known, such as in Garfagnana in 1920 (Mw = 6.5). Finally, the analysis of micro-fracturing associated with major faults (orientations, striations and stress tensors inferred) confirms that most of these secondary structures were formed recently in a direction NE-SW extension. All this work shows that most normal faults in northern Italy are active, and among them, the large fault systems that border the Viareggio basin, near the city of Pisa. The rupture of these faults, approximately 70 km long, could produce earthquakes with magnitudes of at least 6.5. Thus, the seismic hazard in the populated region of Pisa is high, contrary to the current suggestions.

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