GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 - POSTER GNGTS 2023 mean value of V P /V S equal to 1.76 is obtained for the area (Fig. 1b). The focal mechanism has been computed for the 7 strongest earthquakes (2.6 <= M <= 3.6), obtaining in all cases normal kinematics with P axis approximately vertical and T axis oriented NW-SE (Fig. 3a). These mechanisms, as a whole, can be described by a single solution defined by strike = 239°, dip = 56°, rake = -93° (or strike = 64°, dip = 34°, rake = -85° considering the other nodal plane), computed according to the method of Michael (1984). Relative location has been computed on 54 events using the arrival times at the stations shown in Figure 2a, using the event occurred on July 2, 2020 at 10:15 UTC (lat 38.494°, lon 16.147°, depth 18.6 km, M=2.2) as a reference. The distribution of the hypocenters shows a strong orientation in space, and allowed us to identify a plane with strike equal to 58° and dip equal to 59° toward southeast (Fig. 2b), which is compatible with one of the nodal planes provided by the focal mechanism. The seismogenetic volume has been computed from the polyhedron enveloping the hypocenters, then its size has been increased by an amount computed by assuming a thickness Δ l equal to 0.1 km surrounding the polyhedron to account for the finite extent of the faults associated with single events. The result of this procedure gives a volume of 0.6 km 3 for the Mesima swarm (Fig. 2c). Assimilating the distribution of hypocenters to a flat surface, the size of the corresponding portion of the fault is 1.0 x 1.8 km 2 . These values are much larger than the extent of the source of the mainshock, which has a radius of about 0.4 km if we assume a circular geometry with uniform dislocation and stress drop of 3 Mpa. The Bagnara Calabra swarm occurred from September to November 2018, with a main event of magnitude M=4.0 which happened on September 28 at 05:24 UTC (lat 38.375°, lon 15.731°, depth 18.5 km). The swarm includes about a hundred earthquakes, and most of them, about 80, had magnitudes lower than 2. The absolute location results of 98 events show a distribution elongated in the EW direction, a depth range of 15 to 20 km, and an average V P /V S ratio of 1.8, this latter estimated from the modified Wadati diagram. Since this is an offshore sequence, for most of these earthquakes the azimuthal gap is wider than that of the previous study case, and it is generally comprised between 120° and 150°, with values greater than 180° for the smaller earthquakes. The focal mechanism has been computed for the 4 strongest earthquakes, with magnitude ranging from 3 to 4. All the solutions show a normal kinematics, with P axis approximately vertical and T axis varying from N-S to NW-SE (Fig. 3b). The Kagan angle (Kagan, 2007), computed among the mechanisms of the 3 smaller magnitude earthquakes and the mainshock, gives values between 24° and 33°, indicating rather similar solutions. This analysis strengthened the reliability of the results obtained, despite the difficulties in calculating the focal mechanism mainly related to the high azimuthal gap. Relative location has been performed on the 37 events for which it was possible to pick the P-wave arrival time to at least 5 of the stations in Figure 2a, with an uncertainty lower than one hundredth of a second. The distribution of the hypocenters with respect to the mainshock (chosen as the reference event) does not show any planar distribution so it wasn’t possible to fit them in order to constrain one of the solutions obtained from the focal mechanism. Anyway, this analysis confirms the EW trend of the epicenters already observed, and allows us to estimate a seismogenetic volume of 1.1 km 3 , computed as the previous case. The distribution of hypocenters indicates an extent of 2 km which is, again, much greater than the size of the fault that generated the strongest event of the swarm. For all swarms analyzed in this paper, the result