# New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore
## 7 Discussion and Conclusions  
In the present paper, the seismicity of southeastern Murge was reconsidered and analyzed. Thanks to the registration of the OSUB and OTRIONS seismic networks, new recordings from stations close to the epicenters were retrieved, improving the quality of the relocations in terms of minimum epicentral distance and location uncertainty. The seismic activity is sporadic, as evidenced by the very low number of significant events recorded (only 32 earthquakes with M \(\geq\) 2.0 in 25 years). The collected earthquakes, distributed over a horizontal distance of \(\sim\)50 km, with a range of focal depths from 4.5 km to 27 km, indicate that the whole Earth crust is involved in this sporadic seismic activity. The seven focal mechanisms, although obtained using a limited number of P-wave polarities, appear quite well constrained and homogeneous, as can be observed in Figure 8. All the focal solutions have a common nodal plane in the approximately E-W direction, as shown by the \(\varphi_{1}\) values, which range between 80\({}^{\circ}\) N and 127\({}^{\circ}\) N (see Table 4). This is the same direction along which earthquake epicenters tend to align (Figure 6), neglecting the earthquakes in the neighboring of Taranto City, so, between the two nodal planes of the  
Figure 9: Results of the FMSI inversion for the considered event group. From the left: stereonet plot with 95% confidence limits for the principal stress axes \(\sigma\)1 (in fuchsia), \(\sigma\)2 (in green) and \(\sigma\)3 (in light blue); plunge/trend angles of \(\sigma\)1, \(\sigma\)2, \(\sigma\)3, R value and misfit angle of solution; double-couple focal mechanism corresponding to the mean stress tensor solution (black triangle for P-axis and white triangle for T-axis). mechanism solution, the plane 1 should correspond to the actual fault plane. It should be noted that this E-W fault striking fits the seismogenic zonation of Italy [57] very well. The stress regime inferred in this study indicates that seismicity, even if it is of low frequency and magnitude, occurs according to right-lateral transtensional kinematics of the seismogenic faults. The homogeneity of the focal plane solutions provided a high-quality stress-inversion solution. The orientation of Shmin, according to the direction of the trend of the \(\sigma\)3 axis (36\({}^{\circ}\) N), agrees with the results of the Shmin in adjacent areas (as shown in Figure 10, modified from Mariucci and Montone [12]), indicating that this region is subject to a regional stress regime that controls the tectonics of the Adria plate. In an attempt to correlate earthquakes and their seismogenic structures, we selected the seismic events with the best constrained location (20120505, 20180323, 20190213 and 20190520), recorded by a more recent and denser network of seismographs, which provided locations with a total quality factor A or B. In agreement with [PERSON] et al. [11], the location of the seismic event 20180323 is consistent with its association to the western branch of the fault bounding the Monte Giove structural high and the adjacent Rosaria Mare basin (Figure 11a). Moreover, the eastern branch of this fault seems to have been the seismogenic source for the seismic event 20190213 (Figure 11a). A good consistency is also observed between the eastern branch of the fault belonging to the Central Deformation Zone (Figure 4) and the location of the seismic event 20190520. Finally, the epicenter of the seismic event 20120505 is close to the fault striking from WNW (near the town of Matera) to ESE (south of the town of Brindisi), bounding the southern Murge (Figure 11a). In the southeastern Murge area, the ca. NE-SW regional elongation has occurred since Middle Pleistocene [58]. Accordingly, such a feature of the regional strain field is coherent with the obtained stress field (Figure 9), which could determine, in the southeastern Murge, the reactivation of ca. E-W striking ancient faults (at least of the Late Cretaceous) with a right horizontal component of simple shear (Figure 11a).