# New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore
## 5 Focal Mechanisms  
Information on the orientation of the seismogenic structures can be derived from the determination of focal mechanisms. These are also necessary to calculate the stress field to which the area is subjected, since it is well inside the Adria microplate, which is highly deformed at the edges, not too far from the area under study (as discussed in Section 1 and shown in Figure 1). We computed focal mechanisms by using P-wave polarities and the FPFIT code [49]. Since the magnitude of the earthquakes selected for this study is generally low, it is not easy to correctly distinguish polarities from the signal noise in a sufficient number to determine the focal mechanisms. P-wave polarities were determined manually and picked on the seismograms of the OSUB and OT network for the entire period examined and on those of the INGV network only for the events from 2008 onward. The velocity model used for the computation of take-off angles is the same as that used for location [45]. Fault plane solutions were considered well constrained only if derived using a minimum of 10 clearly readable polarities homogeneously distributed on the focal sphere. This quality criterion has led us to accept the focal mechanism solution for only 7 earthquakes (ID 10, 16, 17, 23, 24, 25 and 29) out of the initial 32. Three of these earthquakes (ID 10, 16 and 17) were already analyzed by [PERSON] et al. [48]. For each of these events, we have chosen, from the multiple solutions proposed by FPFIT, the one that minimized the distance of the nodal planes from the mismatching polarity data points. To better constrain the FPFIT inversion, we assigned a weight to the polarity from 0 to 2 (following the picking weighting used by the SAC v. 101.6a software). Assigning weights to the polarity data used in the FPFIT inversion resulted in a reduction in the misfit, F. The best fit solution of each event was determined by minimizing the residuals between the observed and theoretical amplitudes, exploring a search grid of values of strike, \(\varphi\); dip, \(\delta\); and rake, \(\lambda\), spaced at 5\({}^{\circ}\). The results are shown in Table 4, where we labelled with the subscripts 1 and 2 the two nodal planes of the double-couple solution in terms of \(\varphi\), \(\delta\) and \(\lambda\); we also reported the P and T axes orientation as the plunge (inclination measured downward relative to the horizontal plane) and trend (azimuth measured in the direction of the plunge) angles. The P and T axes represent the directions of the maximum, \(\sigma\)1, and the minimum, \(\sigma\)3, principal stress axes, respectively. The fault type (FT in Table 4) can be identified, as proposed by [PERSON] et al. [50], by plotting the combination of T and P plunges: based on the position of the point representing the fault plane solution on this graph, the fault type can be defined according to the diagram legend, as shown in Figure 7.  
In Table 4, N\({}_{\text{pol}}\) is the number of polarities matching with the focal solution compared to the total number of polarities, Ntot, available for each event. The quality of the solution is expressed by the quality factors, Qf and Qp. Qf gives information about the solution misfit, F, of the polarity data and assumes values that depend on F; Qp reflects the solution uniqueness in terms of the 90% confidence region for the three angular parameter uncer \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Layer** & **Vp (km/s)** & **Vs (km/s)** & **D (km)** \\ \hline
1 & 4.00 & 2.25 & 2.0 \\
2 & 6.10 & 3.43 & 19.0 \\
3 & 6.80 & 3.83 & 33.0 \\
4 & 8.10 & 4.55 & 90.0 \\
5 & 8.20 & 4.61 & \(\infty\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Velocity model [45] used in seismic event relocation by the Hypoellipse code [44]: Vp and Vs are P-wave and S-wave velocities (Vp/Vs = 1.78); D is the depth of the bottom of each layer. tainties, \(\Delta\varphi\), \(\Delta\delta\) and \(\Delta\lambda\).