Seismic Swarm S20220520.1 in the Strait of Gibraltar

The Strait of Gibraltar forms a narrow marine passage between the Iberian Peninsula and northwest Africa, marking a key segment of the diffuse plate boundary between the African and Eurasian plates. This region belongs to the Gibraltar Arc, where ongoing convergence at rates of approximately 4–5 mm per year produces complex deformation involving strike-slip, thrust, and normal faulting. Seismicity is frequent and often occurs in swarms rather than isolated mainshock-aftershock sequences, reflecting fluid migration and stress redistribution within the crust. Swarm S20220520.1 began at 20:01 UTC on 19 May 2022 and concluded at 15:52 UTC on 10 June 2022, spanning 523 hours and 50 minutes. During this interval, 412 earthquakes were recorded. The first 100 events, which occurred primarily between 19 and 21 May, displayed magnitudes ranging from 1.9 to 4.1 and focal depths between 0 and 40 km. The largest event reached magnitude 4.1 at 20:11 UTC on 20 May at a depth of 10 km. Most events clustered at depths of 5–20 km, consistent with activity in the upper to mid-crust. Shallow events (≤5 km) and a few deeper events (30–40 km) were also observed, indicating a vertically distributed source volume. Temporal analysis of the initial sequence shows a rapid onset with multiple events exceeding magnitude 3.0 within the first 24 hours, followed by a gradual decline in both rate and maximum magnitude. Depths remained predominantly shallow throughout, supporting the interpretation of a brittle crustal response rather than deeper mantle involvement. This pattern aligns with prior swarms in the same tectonic corridor, where episodic fluid-driven triggering has been inferred from similar depth distributions and swarm durations. Historical records indicate that ten seismic swarms have occurred in the Strait of Gibraltar since 1 January 2000. These include one swarm in 2016, seven in 2021, and two in 2022. The recurrence of swarm activity underscores the region’s persistent susceptibility to clustered seismicity driven by the broader plate-boundary dynamics. The geological setting features Mesozoic and Cenozoic sedimentary sequences overlying a heterogeneous basement affected by the Alpine orogeny. Active faults such as the Yusuf Fault and segments of the Trans-Alboran Shear Zone contribute to the observed seismicity. Although individual swarm events rarely exceed magnitude 5, the cumulative effect of hundreds of small earthquakes can produce perceptible ground motion and minor structural stress in nearby coastal communities in southern Spain and northern Morocco. Continued monitoring by regional seismic networks remains essential for refining source models and assessing any evolution toward larger-magnitude events. The characteristics of Swarm S20220520.1 reinforce the value of high-resolution catalogs in understanding transient deformation along this tectonically active gateway between the Atlantic and Mediterranean.

References

• Instituto Geográfico Nacional (IGN) seismic catalog
• United States Geological Survey (USGS) Earthquake Hazards Program
• European-Mediterranean Seismological Centre (EMSC) bulletins
• SeismoSight internal swarm classification records