GNGTS 2017 - 36° Convegno Nazionale

50 GNGTS 2017 S essione 1.1 gathered data until the end of 2015 The map shows a concentration of recent events in the areas of the eastern Emilia Folds and the whole Ferrara Folds, along the Apennine-Po Plain margin and Apennine hills between the Nure and Reno valleys, along the Apennine ridge and in the Romagna Apennines. The frequency of events with M>3 is lower in the Piacenza plain and in the hills between Trebbia and Nure valleys, between the Apennine-Po Plain margin and the Ferrara Folds, in the Po delta, in the middle Taro valley, in the high Enza and Secchia valleys, in the middle Reno valley, in the upper Casentino and in the low Conca valley. The analysis of focal depth is of particular interest to identify the seismogenic structures and their tectonic style. Specific studies are necessary for the correct identification of focal depth but, in general, we can observe that in the study area earthquakes are superficial, with a focal depth between 5 and 15 km; deeper earthquakes, 15÷35 km, are widespread in the Emilia- Romagna side and in the Po Plain-Adriatic area. Deepest earthquakes are located only below the chain. For the characterization of seismogenic sources, focal mechanisms are a key information, being the representation of the movement that occurred along the fault which generated the seismic event. In the map we represented the focal mechanisms available for earthquakes with a Mw≥4 derived from different catalogues. In addition to the solutions provided by the RCMT (Pondrelli et al. , 2006, 2015), to have a longer time interval represented in our focal mechanism dataset, we added data from EMMA (Vannucci and Gasperini, 2004). In summary, we gathered from these catalogues data for 150 focal mechanisms for earthquakes occurred from 1939 until present. The focal mechanisms of the events listed in the ISIDe catalogue were located in the map on the basis of coordinates listed in this catalog; the focal mechanisms of events in 1951, near Lodi, were located using the relocation by Caciagli et al. (2015). The dataset collected for this map shows that in the Apennine area, where the strongest earthquakes occurred, faulting is predominantly extensional, with mostly shallow focal depth (< 15 km). On the Po side of the chain extensional and compressive earthquakes coexist, usually with extensional events at a depth < 15 km and compressional earthquakes at a greater depth. Along the Apennines-Po Plain margin the focal mechanisms are predominantly compressive in Romagna, while they are compressive and strike-slip in Emilia. Dominant compressive focal mechanisms occur in correspondence of the Emilia and Ferrara Folds. The 2012 Emilia sequence clearly shows these features. The western part of the region is characterized by a higher frequency of strike-slip events. In the map we also report the main individual seismogenic sources derived from DISS 3.2 (DISS Working Group, 2015). Field surveys. There are different surveys which, associated with the geological survey, allow the identification of active faults (e.g., seismic, geomorphological, geophysical, geodetic, historical or archaeological surveys). In particular, it is essential to identify, through morphotectonic investigations, particular associations of the relief forms which may be indicative of recent or active tectonics. The distinction between active faults, potentially active faults and non-active faults in this new edition was based on the analysis and comparison of geologic, morphotectonic and seismologic data. The different geomorphological environments present in the study area (axial zone of the chain, the Apennine-Po Plain margin, plain and coast) required different criteria of analysis and investigation, resulting in different degrees of knowledge. The recognition of recent and active structures in the axial zone was based on the morphotectonic analysis, i.e. on the identification of associations of geomorphological features and characteristic forms of relief such as triangular facets, fault scarps, tectonic terraces, control of the drainage network, dislocation and/or deformation of recent deposits and surfaces. A detailed structural analysis involved the