# Stochastic Modeling of the Al Hoseima (Morocco) Aftershock Sequences of 1994, 2004 and 2016
## 4 Seismic Series Stress Regime
After the ultimate optimization, the omitted focal planes must be reconsidered without modifying the stress tensor. If this is the case, the data are re-entered into the database, the stress tensor is re-optimized, and the software runs another check for the rejected data. This method was used to analyze the focal mechanism data for each seismic sequence. Table 2 and Figure 7 show the results of the stress inversion. Several authors (e.g., [19; 44; 47; 48; 49]) have used a similar approach to investigate the stress regime in other regions. Figure 6: 2016 seismic sequence showing earthquakes with magnitude above 3.0 and computed focal mechanisms. The variability in the stresses is highlighted in Figure 7 and Table 2. The different solutions of the mainshock, when considered together, and those determined for the aftershocks were not very distinct in the 1994 seismic series. A prolate stress ellipsoid with a near NW-SE horizontal (compressive stress regime) and roughly similar \(\sigma_{2}\) and \(\sigma_{3}\) values appeared in the mainshock. The slope then turns southeast, highlighting the NE-SW expansion with the horizontal \(\sigma_{3}\) being more noticeable (extensive stress regime). The seismic series from 2004 appears to be more uniform, with oblate stress ellipsoids and a noticeable NE-SW extension trend. While the main cluster shows NW-SE sub-horizontal \(\sigma_{1}\) and NE-SW sub-horizontal \(\sigma_{3}\) values, clearly indicating a strike-slip stress regime, in the \"eastern cluster\" [34], a cluster with a few low-energy earthquakes and \(\sigma_{1}\) and \(\sigma_{2}\) values becoming closer, and a NE-SW horizontal extension is dominant (extensional stress regime). In the 2016 sequence, the computed stress regime for the main cluster agrees with that obtained in the 2004 main cluster (strike-slip stress regime); however, there is a noticeable increase in the axial ratio, where \(\sigma_{1}\) and \(\sigma_{2}\) magnitudes are closer, highlighting the well-defined NE-SW sub horizontal extensive stresses. In contrast, the secondary cluster is dominated by NW-SE compressive \(\sigma_{1}\) stresses and a subvertical \(\sigma_{3}\), suggesting thrusting (compressional stress regime). Furthermore, the secondary cluster stresses are like those obtained for the mainshock of 1994, which had a primary NNW-SSE \(\sigma_{1}\) odd axis. The maximum compressive horizontal stress (SHmax) in all solutions was between 142\({}^{\circ}\) N and 153\({}^{\circ}\) N, which is consistent with previous results for regional stresses (e.g., [18; 19; 20; 50]). However, the extension directions are compatible with the Internal Zone movement of the Betic-Rif toward the southwest [26; 51; 52; 53]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sequence** & \(\sigma_{1}\) & \(\sigma_{2}\) & \(\sigma_{3}\) & **R** & **F5** \\ \hline
1994 main event & 327\({}^{\circ}\) N/13\({}^{\circ}\) & 226\({}^{\circ}\) N/40\({}^{\circ}\) & 072\({}^{\circ}\) N/47\({}^{\circ}\) & 0.19 & 7.1 \\
1994 aftershocks & 132\({}^{\circ}\) N/44\({}^{\circ}\) & 349\({}^{\circ}\) N/39\({}^{\circ}\) & 242\({}^{\circ}\) N/20\({}^{\circ}\) & 0.38 & 3.5 \\
2004 all, without ‘eastern’ cluster & 330\({}^{\circ}\) N/25\({}^{\circ}\) & 149\({}^{\circ}\) N/65\({}^{\circ}\) & 239\({}^{\circ}\) N/00\({}^{\circ}\) & 0.59 & 2.6 \\
2004 ‘eastern’ cluster & 020\({}^{\circ}\) N/79\({}^{\circ}\) & 141\({}^{\circ}\) N/06\({}^{\circ}\) &