Active compressional tectonics, Quaternary capable faults, and the seismic landscape of the Po Plain (N Italy)

Abstract

It is commonly believed that the Po Plain is an area of low seismic haz- ard. This conclusion is essentially a combination of two factors: (1) the historical record of earthquakes, which shows a relatively small number of events of moderate magnitude, and only two significant earthquakes, which occurred in the Middle Ages; and (2) the lack of ad-hoc research on the geology of earthquakes in this area, as although many studies have highlighted the local Quaternary tectonics, only a very few of them have discussed the observed evidence in terms of seismic hazard. In contrast, the data presented in the present study strongly suggest that the level of earthquake hazard in the Po Plain is comparable to that of the well- known seismic areas of the Apennine range, at least in terms of maxi- mum magnitudes. Indeed, the high population density and the concentration of industrial facilities make the Po Plain today one of the more high-risk areas of the Italian territory. The Po Plain represents the foredeep of two growing mountain belts, the southern Alps and the north- ern Apennines. Recently, modern active tectonics studies have been con- ducted along its margins to the south, along the northern Apennine Piedmont belt, and to the northeast, along the eastern southern Alpine Piedmont belt. However, in the central and western sectors of the Po Plain, where the south-verging western southern Alpine front links up with the north-verging Monferrato, Emilia and Ferrara arcs, the Qua- ternary history of tectonic deformation and faulting are still relatively poorly understood. These lie beneath the relatively flat alluvial surface of the Po River, and provide the evidence for paleoseismicity and the result- ing seismic hazard. In this review, we compile the data from the literature to reassess the style and magnitude of the ongoing crustal deformation and the associated earthquake faulting. This includes detailed informa- tion on historical and instrumental seismicity, extensive subsurface in- formation from the ENI industrial exploration, structural interpretation of three regional seismic reflection profiles, analysis of novel global posi- tioning system data, field mapping at selected key areas, and new paleo- seismological investigations. We show that along the western southern Alpine belt between Lake Garda and Lake Maggiore, the active tectonic setting is characterized by a segmented belt of fault-propagation folds. These are 50 km wide, and are controlled by the growth of out-of-se- quence, 10-to-20-km-long, north and south verging thrusts. Regional global positioning system data show ongoing shortening rates of the order of 1 mm/yr. Quaternary fault slip rates typically range between 0.2 mm/yr and 0.4 mm/yr. Pleistocene shortening is obvious not only along the western southern Alpine outer fronts that are buried beneath the Po Plain, but also along the south Alpine foothills between Brescia and Varese. Similar styles and rates of active folding and thrusting have also been documented along the frontal sector of the northern Apennine arcs, from Torino to Ferrara, and along the base of the Apennine mountain front between Piacenza and Bologna. We selected the Brescia and Como sectors in the western southern Alps and the Monferrato and Mirandola structures in the northern Apennines as examples to illustrate the seismic landscape of the study area, in terms of typical active structural, geo- morphic and paleoseismic features. We argue that the level of earthquake hazard in the Po Plain is comparable to that of the Apennine range. On May 20, 2012, a few days after this review was formally accepted for pub- lication, a M W 5.9 earthquake ruptured the Mirandola structure. The seismic sequence following this mainshock is ongoing, and we have added further information about this event (updated on June 3rd, 2012), which substantially confirms the conclusions arrived at here.