Seismic Swarm S20040224.1 in the Strait of Gibraltar: Analysis of the 2004 Event Sequence

The Strait of Gibraltar represents a key tectonic boundary where the African and Eurasian plates converge obliquely. This region forms part of the Gibraltar Arc, characterized by thrust faults, strike-slip systems, and moderate to high seismic activity driven by northwest-southeast compression. The crust here exhibits variable thickness, with shallow seismicity often linked to active faulting in the Alboran Sea basin and surrounding continental margins. Historical records document recurrent earthquakes, including notable events in the 19th century that caused damage along the coasts of southern Spain and northern Morocco.

Seismic monitoring in the area has improved significantly since the late 20th century, enabling detailed recording of swarm activity. Swarm S20040224.1 began at 02:27 UTC on 24 February 2004 and continued until 01:50 UTC on 11 March 2004, spanning 383 hours and 22 minutes. During this period, 396 earthquakes were registered, with the sequence dominated by shallow-focus events.

Analysis of the first 100 events reveals a classic swarm pattern initiated by a mainshock of magnitude 6.4 at 02:27:46 UTC on 24 February, located at 0 km depth. This event was followed within minutes by aftershocks reaching magnitudes 4.5 and 4.7. Subsequent events showed a rapid decline in maximum magnitude, interspersed with occasional spikes above 4.0. Depths remained predominantly shallow, clustering between 0 and 15 km, though isolated events reached 40 km, suggesting involvement of both upper crustal and deeper lithospheric structures.

The temporal distribution indicates intense activity in the initial hours, with multiple events per hour on 24 February. Magnitudes transitioned from frequent values above 4.0 early on to a predominance of 3.0–3.7 events by late 24 February and into 25 February. Later events in the subset maintained a steady rate of smaller quakes, with the final listed event at magnitude 4.1. This progression reflects stress redistribution along local fault networks without a single dominant aftershock sequence.

Geological context supports that such swarms arise from fluid migration or aseismic slip triggering brittle failure in the complex plate-boundary zone. The 2004 activity aligns with the region's long-term pattern of episodic clusters rather than isolated large events.

References

SeismoSight internal swarm classification database.
USGS Earthquake Hazards Program regional tectonic summaries.
Instituto Geográfico Nacional (Spain) seismic bulletins.