Spain experiences moderate seismic activity primarily due to its position at the convergent boundary between the Eurasian and African tectonic plates. This interaction produces distributed deformation across the Iberian Peninsula, the Alboran Sea, and the Canary Islands, where both tectonic and volcanic processes can generate earthquake swarms. Historical records indicate that such swarms remain relatively infrequent, with only one documented swarm occurring in 2011 since systematic monitoring began in 2000.
SeismoSight internal classification identifies Swarm S20210124.1 as a distinct sequence that began at 10:14 on 23 January 2021 and concluded at 20:24 on 10 February 2021. Over 442 hours and 10 minutes, the network recorded 556 earthquakes across Spain. Analysis of the first 100 events reveals a typical swarm pattern dominated by low-to-moderate magnitudes and shallow focal depths.
The initial event registered magnitude 1.5 at 6 km depth. Within the first hour, a magnitude 4.2 earthquake occurred at 10 km, followed shortly by a magnitude 2.7 event at the same depth. Subsequent activity remained largely concentrated between 1 km and 12 km depth, with the majority of hypocenters clustered near 10 km. Magnitudes during this period ranged from 1.5 to a peak of 4.3, recorded on 26 January at 10 km depth. Several additional events exceeded magnitude 4.0, including magnitudes 4.1, 4.0, and 3.5, all within a narrow time window on 26 January.
Depth distribution shows strong clustering: approximately 40 % of the first 100 events occurred at or very near 10 km, while shallower events (0–5 km) accounted for roughly 25 %. A smaller subset reached depths of 11–12 km. This vertical distribution is consistent with brittle failure within the upper crust under regional tectonic stress.
Temporal evolution indicates an initial burst of activity on 23 January, followed by a steady rate punctuated by short bursts of higher-magnitude events on 24 and 26 January. The largest events did not appear to trigger prolonged aftershock sequences; instead, the swarm maintained a diffuse, swarm-like character without a clear mainshock–aftershock decay pattern.
Such sequences in Spain are often linked to fluid migration or slow slip along pre-existing faults within the plate-boundary zone. Continued monitoring by national and international networks remains essential for assessing whether similar swarms may recur in the coming years.
References:
SeismoSight internal swarm catalogue S20210124.1
Instituto Geográfico Nacional (IGN) seismic database
European-Mediterranean Seismological Centre (EMSC) event bulletins