GNGTS 2013 - Atti del 32° Convegno Nazionale

belt, the formation of which is related to the tectonic interaction between the African and European plates. Compressional deformation results from the westward subduction of the Adriatic lithosphere, causing the formation of compressional fronts migrating towards E and NE, thus progressively affecting the Adriatic foreland (Patacca et al. , 1990; Chiarabba et al. 2005; Molli et al. 2010; Bignami et al. , 2012). The May-June 2012 seismic sequence was characterized by two strong events of M w 6.1 and 6.0 occurred on May 20 and 29, respectively. Both events were located close to the buried front of the Ferrara northward-verging active thrust belt. The two main earthquakes were then followed by six M w ≥ 5.0 events and by many weaker shocks. The aftershock distribution covers an area of 800 km 2 extending in the E-W direction for a total length of approximately 55km (Mirandola Earthquake Working Group, 2012). Compared to the May 20 event, the earthquake occurred on May 29 is located westward and the related aftershocks cover the western and central parts of the thrust front (Mirandola Earthquake Working Group, 2012). As argued by some authors, the 2012 Emilia seismic sequence activated a portion of the buried outer thrust fronts of the northern Apennines (Bignami et al. , 2012). More specifically, two N-NNE-verging segments of the blind thrust of the external Ferrara-Romagna Arc were activated (Serpelloni et al. , 2012). Focal mechanisms indicate a prevalent compression (Pondrelli et al. , 2012), which is in agreement with the regional compressional tectonics characterizing the Apennine structures buried below the Po Plain sediments (Boccaletti et al. , 2004, Boccaletti et al. , 2011, Pondrelli et al. , 2012). Only in a few rare cases, a strike-slip component was observed. Of note is the rotation of the axis of maximum compression (P-axis). The major events show pure, low angle, thrust mechanisms with P-axis pointing towards north. Some aftershocks clearly present a rotation with respect to this dominant northward direction. In particular, those pointing towards NW are located close to the NE-striking part of the buried thrust front (Pondrelli et al. , 2012). Concerning past earthquakes, the seismicity was mostly concentrated along northeastern sector of the Apennine chain, at the border with the Po Plain. However, strong earthquakes also occurred in the blind thrust zone in the outer sector of the Apennines, such as the 1346 Ferrara ( M w = 5.8), 1570 Ferrara ( M w = 5.5), and 1688 Romagna ( M w = 5.9) earthquakes. Methodology. The calculation method proposed by Barani et al. (2010) is based on the zone- less, smoothed seismicity approach used by Frankel (1995) for the seismic hazard assessment of the Central and Eastern United States. The method of Barani et al. (2010) consists of calculating moment rates, , for each cell of a homogeneous grid covering the entire study area. Then, an elliptical kernel (here applied with smoothing parameters τ 1 = 30km and τ 2 = 20 km) with the major axis ( τ 1 is the major semi-axis) oriented parallel to the prevalent direction of compression characterizing the outer Apennines is applied on them to determine smoothed values. Based on this approach, the elliptical smoothing function allows for both the epicentral location error (quantified by τ 1 and τ 2 ) and the prevalent orientation of active faults within a region. values are then converted into strain rates, , by applying the Anderson formula (Anderson, 1979): (1) where k (= 0.66) is an empirical constant that depends on the regional stress field, µ is the shear modulus (taken as ), and V = Ah is the seismogenic volume ( A indicates the area of a grid cell and h is the thickness of the seimogenic layer). In this application, values are calculated by summing the moments M o of all events with M w ≥ 4.0 (and then dividing the cumulative moment by the observation period to obtain the rate per year) included in the catalog used by Barani et al. (2010) updated to year 2013 (to this 211 GNGTS 2013 S essione 1.2