GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 15 ADRIATIC EARTHQUAKES FROM THE MID-LOWER CRUST: POSSIBLE HINTS FOR THE PALEOGEOGRAPHY OF ADRIA A. Argnani CNR, ISMAR-Bologna, Italy The Emilia 2012 seismic sequence, that occurred from the end of May to early June, is characterizedby a large numer of earthquakes that range from16 to2kmindepth (e.g., Carannante et al. , 2015), with the epicentres that are located west of the city of Ferrara. However, on the 6th June, towards the end the seismic sequence, an earthquake occurred way to the east, near the city of Ravenna. This event worried the citizenship as it was perceived as a potential eastward propagation of the Emilia fault rupture. The hypocentral depth of this Ravenna earthquake, however, was ca. 30 km, significantly deeper than the earthquakes of the Emilia sequence, and it remained an isolated event. On the 15th January 2019 an earthquake of Mw 4.3 occurred near Ravenna at ca. 21 km depth, suggesting that tectonic stress in the area can be released by earthquake ruptures in the middle-lower crust. Focal mechanisms indicate strike-slip or reverse fault solutions, and the nodal planes suggest the occurrence of ruptures along relatively high angle faults. It is likely that these eathquakes nucleate on pre-existing faults characterized by some inherited weakness (e.g., Middleton and Copley, 2014). The aim of this contribution is to attempt relating the mid-lower crustal earthquakes in the Adria domain to the inherited Mesozoic fault system and to infer a palaeogeography of Adria that still has some control over the current seismicity of the Adriatic foreland. The distribution of instrumental seismicity in Italy shows that several mid-lower crustal earthquakes (deeper than 15 km) occurred in the Adriatic foreland (Chiarabba et al. , 2005: 2015).Where focal mechanisms are available, the solutions point to reverse or strike.slip faults. In some instances the relationships of these earthquakes with pre- existing extensional faults seem to be convincingly proven (Herak et al. , 2005). Moreover, several lines of geological evidence indicate that tectonic inversion is rather widespread in the Adriatic foreland (Argnani et al. , 1993, 2002; Argnani and Frugoni, 1997; Scisciani, 2009; Stanculete et al. , 2014; Pace, 2018), extending also into the Po Plain foreland (Fantoni et al. , 2004, 2017). The large-scale control exerted by the Mesozoic paleogeography of Adria on the evolution of the fold-and-thrust belts that currently Fig. 1 - Map showing the epicenters of the 6th June 2012 (blue square) and 15th January 2019 (red square) Ravenna earthquakes and the simplified tectonic structures. The small square and circles indicate earthquakes of Ml 3 and around 2, respectively. Dashed lines are thrusts affecting mainly the Pliocene-Quaternary sediments. Continuous lines with open and filled triangles are thrusts involving the Mesozoic succession and the basement, respectively (from Carannante et al. , 2015). The inset shows the nodal planes of the two larger earthquakes, with the same colour code.