The Mechanisms Behind Triggering the 2021–2022 South Alboran Seismic Swarm: Constrains from Combined Catalogues, Relocation and Spatiotemporal Analysis

Abstract. The Alboran system reflects the interplay of slow Nubia–Iberia convergence, inherited structures, and ongoing lithospheric attenuation. Earthquake occurrence mainly tracks active crustal fault networks that partition the oblique plate motion into strike-slip and extension. The 2021–2022 seismic swarm in the western Alboran Sea represents an exceptional episode characterized by complex temporal-spatial evolution, primarily influenced by fluid-driven processes interacting with inherited fault systems. We analyzed arrivals of approximately 7,000 seismic events recorded by Spanish (IGN) and Moroccan (CNRST) seismic networks. Both bulletins have been individually and jointly processed using the double-difference algorithm (HypoDD) and a regionally optimized velocity model. This approach significantly improved hypocentral precision, reducing event scatter and delineating a clearly defined, near-vertical seismic conduit-oriented NW–SE. Spatiotemporal analyses revealed distinct episodes of seismic migration, consistent with episodic fluid overpressure pulses, confirmed by diffusivity values (1.2–13.9 m²/s) characteristic of fluid-controlled swarms. Focal mechanisms predominantly indicated strike-slip motion, aligning with the regional transtensional tectonics and pinpointing the unrecognized Ras Tarf Fault as the primary seismogenic structure likely linked to the Al-Idrissi Fault System (AIFS). Integration with vertical and horizontal shear-wave velocity models (VSH and VSV) highlighted velocity anomalies at depths of 30–70 km, suggesting the presence of partially serpentinized mantle wedges above a delaminating slab segment, further supporting fluid involvement. Our results emphasize the critical interplay between deep lithospheric fluids, inherited fault structures, and regional tectonic stress, providing a comprehensive framework for understanding the 2021–2022 swarm dynamics which could improve the seismic hazard assessment in the region.