THE SOUTH ALBORAN BASIN SEISMIC SWARM IN 2020-2021 AND ITS POSSIBLE RELATION TO THE SOUTHERN DEVELOPMENT OF THE AL-IDRISSI FAULT SYSTEM

The aim of this study is to quantify and map a recent seismic swarm sequence that occurred in the Alboran Sea between 2020 and 2021, focusing specifically on the South Alboran Basin. To identify the seismic clusters, a criterion based on seismicity is employed, considering the distribution of two parameters: the kernel density of earthquakes and the kernel density of seismic moments. The first parameter reveals the presence of two seismic clusters: one located north of Al Hoceima, specifically within the southern segment of the Al-Idrissi Fault System (AIFS), and another in the Granada Basin. The second parameter indicates that both clusters released significant amounts of energy, particularly in the southern part of the AIFS, with values reaching up to 1.6·10 14 J·km –2 ·year –1 . The 3D seismic modeling indicates a clustering of seismic occurrences in northern Morocco, displaying an eastward pattern in terms of depth. This observation leads to speculation about the existence of nearby geological formations, possibly associated with the AIFS system. Analysis according to the Gutenberg-Richter law shows that the seismic swarm sequence in the southern Alboran Basin exhibits a b-value close to 1, indicating a slip regime. Temporal analysis of the b-value variation reveals two stress regimes: an initial decreasing regime with a b-value close to one, succeeded by a sharp increase indicative of an extensional regime, possibly due to fracture opening. The focal mechanisms show an alternation between strikeslip and strike-slip with normal components, indicative of a transtensional regime. This type of faulting suggests both lateral motion and some degree of extensional force, which aligns with certain rises in the b-value, as these could correspond to stress release events associated with opening fractures or fault segments, although the timing does not perfectly match b-value fluctuations. Together, these findings highlight a complex tectonic environment marked by strike-slip and transtensional forces, supporting the ongoing southern development of the AIFS.