Seismic swarm S20111224.1 occurred in Spain between 22:07 on 23 December 2011 and 22:41 on 27 December 2011. Over 96 hours and 33 minutes, the sequence produced 94 earthquakes. Magnitudes ranged from 0.3 to 2.3, with the largest event recorded at 2.3 on 25 December at a depth of 5 km. Depths throughout the swarm remained shallow, predominantly between 0 and 10 km, consistent with upper-crustal activity.
The temporal distribution showed the highest event rate on 24 and 25 December, followed by a gradual decline. Early events clustered tightly in time on 24 December, with multiple occurrences within minutes. Later stages featured more dispersed timing yet maintained similar magnitude and depth ranges. Such patterns typify earthquake swarms, in which energy releases without a dominant mainshock and aftershock sequence.
Spain lies at the convergent boundary between the Eurasian and African plates. This tectonic setting drives moderate seismicity, especially along the southern margin in the Betic Cordillera and the Gulf of Cádiz. Shallow crustal faults accommodate the oblique convergence, producing frequent low-to-moderate magnitude events. Historical records document destructive earthquakes in 1884 (Andalusia) and 1954 (Granada), both linked to the same plate-boundary dynamics. Instrumental monitoring by the Instituto Geográfico Nacional confirms that swarms of small events occur regularly in these zones.
The 2011 swarm’s maximum magnitude of 2.3 and shallow focal depths align with background activity observed in Spain’s intraplate and plate-boundary regions. Depths clustered near 0–10 km suggest activation of brittle upper-crustal structures rather than deeper mantle processes. The absence of a single large event and the steady production of similar-sized shocks further support classification as a swarm driven by fluid migration or aseismic slip along pre-existing faults.
No damage or felt reports were associated with this low-magnitude sequence. Its documentation contributes to the long-term seismic catalogue used for hazard assessment in Spain. Continued monitoring refines understanding of swarm recurrence and possible precursors to larger events along the Iberian margin.
Instituto Geográfico Nacional (IGN), Spanish Seismic Catalogue. USGS Earthquake Hazards Program, Tectonic Summary of the Iberian Peninsula.