Seismic Swarm in the Strait of Gibraltar: Analysis of Event S20240316.1

The Strait of Gibraltar marks the narrow marine passage separating the Iberian Peninsula from northwest Africa and serves as the sole surface connection between the Atlantic Ocean and the Mediterranean Sea. Geologically, the region occupies the diffuse plate boundary where the African and Eurasian plates converge at rates of approximately 4–5 mm per year. This convergence produces a complex tectonic regime involving strike-slip faulting, thrust structures, and localized extension within the Alboran Sea domain. The resulting crustal deformation accounts for the moderate but recurrent seismicity observed throughout the area.

Between 18:32 UTC on 15 March 2024 and 21:00 UTC on 17 March 2024, a seismic swarm comprising 32 events was recorded in the Strait of Gibraltar. The sequence lasted 50 hours and 27 minutes. Magnitudes ranged from 1.6 to 4.5, with the largest shock (magnitude 4.5) occurring at a shallow focal depth of 8 km on 15 March at 20:17 UTC. Most events clustered between 10 km and 30 km depth, although several very shallow foci (0–3 km) were also detected. The temporal distribution showed peak activity during the first 12 hours, followed by a gradual decline, consistent with swarm behavior driven by fluid migration or slow-slip processes rather than a single mainshock-aftershock sequence.

The 4.5-magnitude event and subsequent smaller shocks were felt locally along the coasts of southern Spain and northern Morocco, yet no significant damage or tsunami generation was reported. Depths remained predominantly crustal, indicating that brittle failure occurred within the upper 35 km of the lithosphere. Such shallow-to-intermediate depths align with the known seismogenic thickness of the Gibraltar Arc, where inherited structures from the Miocene collision continue to accommodate present-day strain.

Historical records maintained since 1 January 2000 document 16 prior swarms in the same sector. These episodes occurred in 2016 (one swarm), 2021 (seven swarms), and 2022 (eight swarms). The 2024 swarm therefore represents the first such cluster identified in the current year and continues a pattern of episodic, low-to-moderate energy release that has characterized the region over the past two decades.

From a hazard perspective, the Strait of Gibraltar experiences background seismicity at a rate sufficient to produce occasional felt events, although great earthquakes remain rare. The ongoing convergence implies that accumulated strain will continue to be released through both isolated shocks and swarm-type sequences. Monitoring networks operated by Spanish and Moroccan agencies, supplemented by regional catalogues, provide the data necessary to track these patterns and refine probabilistic forecasts.

Continued observation of swarm statistics, focal mechanisms, and any associated geodetic signals will improve understanding of the interplay between tectonic loading and transient processes in this key gateway between two major ocean basins.

References

• European-Mediterranean Seismological Centre (EMSC) earthquake catalogue
• Instituto Geográfico Nacional (IGN), Spain, seismic bulletins
• U.S. Geological Survey (USGS) Earthquake Hazards Program
• Geological Survey of Morocco (SGM) regional reports