# New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore
## 5 Focal Mechanisms
Qf and Qp range from class A to class C for decreasing quality, according to Table 5.  
We can observe that four of the selected focal mechanisms (ID = 10, 17, 24 and 29) represent normal/strike-slip faults, whereas only one (ID = 16) is a pure strike-slip mechanism and one (ID = 25) is a pure normal fault. The focal mechanism of the strongest event (ID = 23) has an uncertain character, as it is of type U; the mechanism found is quite similar to that obtained by INGV using the Time-Domain Moment Tensor (TDMT) technique [51] on eight stations (\"[[https://terremoti.ingvit/event/18504011](https://terremoti.ingvit/event/18504011)]([https://terremoti.ingvit/event/18504011](https://terremoti.ingvit/event/18504011)) (accessed on 15 April 2024)\"), also classified as type U. For most events (see Figure 8), the best solution has a pressure axis (P) with a trend of about 300\({}^{\circ}\) and a plunge of about 40\({}^{\circ}\), whereas the tension (T) axis has a trend of about 60\({}^{\circ}\) (NE-SW direction) and a plunge of 20\({}^{\circ}\). These events reveal normal/strike-slip faulting mechanisms along approximately E-W striking planes, and, in particular, the best constrained mechanism of the 5 May 2012 earthquake (ID = 17) is the great representative of this kind of solution. A composite fault plane solution was also obtained by combining the 115 P-onset polarities of these seven events; in Figure 8, the beach-ball is shown with black and white quadrants; the joint solution of the normal/strike-slip type shows, on average, a trend and plunge of T and P axes similar to those obtained for the individual focal mechanisms. Analysis of the T-axis orientations of normal and normal/strike-slip solutions suggests a widespread NE-SW extensional regime. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline
**ID** & \(\varphi_{1}\) & \(\delta_{1}\) & \(\lambda_{1}\) & \(\varphi_{2}\) & \(\delta_{2}\) & \(\lambda_{2}\) & **Trend P** & **Plunge P** & **Trend T** & **Plunge T** & \(\mathbf{N_{pol}/N_{tot}}\) & **Qf** & **Qp** & **FT** \\ \hline
10 & 100 & 35 & \(-\)160 & 353 & 79 & \(-\)57 & 297 & 46 & 58 & 26 & 10/11 & C & C & NS \\
16 & 95 & 55 & \(-\)180 & 5 & 90 & \(-\)35 & 314 & 24 & 56 & 24 & 11/13 & C & B & SS \\
17 & 80 & 65 & \(-\)150 & 336 & 63 & \(-\)28 & 299 & 38 & 208 & 1 & 10/10 & A & A & NS \\
23 & 117 & 84 & 125 & 215 & 35 & 10 & 179 & 31 & 59 & 41 & 44/48 & B & B & U \\
24 & 80 & 80 & \(-\)140 & 342 & 51 & \(-\)13 & 309 & 35 & 205 & 19 & 10/10 & A & B & NS \\
25 & 281 & 31 & \(-\)107 & 120 & 60 & \(-\)80 & 55 & 73 & 203 & 14 & 10/10 & A & A & NO \\
29 & 127 & 31 & \(-\)161 & 20 & 80 & \(-\)60 & 321 & 47 & 86 & 29 & 11/13 & C & A & NS \\ joint & 70 & 30 & \(-\)160 & 323 & 80 & \(-\)62 & 262 & 47 & 30 & 29 & 84/115 & C & B & NS \\ \hline \end{tabular}
\end{table} 
Table 4: List of fault plane solutions of the events. For each event, the table reports the identification number (ID); the angles of strike (\(\varphi\)), dip (\(\delta\)) and rake (\(\lambda\)) of the 2 nodal planes; the trend and plunge angles of the P and T axes; the number of polarities, \(\mathbf{N_{pol}}\), that match compared to the total; the quality factors (Qf and Qp); and the fault type (FT: NS, normal/strike–slip; SS, strike–slip; U, unknown; and NO, normal). The fault plane solution obtained for the composite mechanism is also reported. \begin{table}
\begin{tabular}{c c c} \hline
**F** & **Qf** & \(\Delta_{\text{STR}}\), \(\Delta_{\text{DIP}}\), \(\Delta_{\text{RAK}}\) & **Qp** \\ \hline F \textless{} 0.025 & A & \textless{}20\({}^{\circ}\) & A \\
0.025 \textless{} F \textless{} 0.1 & B & \(20^{\circ}\) to 40\({}^{\circ}\) & B \\ F \textgreater{} 0.1 & C & \textgreater{}40\({}^{\circ}\) & C \\ \hline \end{tabular}
\end{table}