Seismic Swarm in the Strait of Gibraltar: Analysis of the November 2021 Event

The Strait of Gibraltar marks a critical tectonic boundary where the African and Eurasian plates converge obliquely, driving ongoing seismic activity across the Gibraltar Arc. This narrow waterway, approximately 14 kilometers wide at its narrowest point, separates the Iberian Peninsula from North Africa and connects the Atlantic Ocean to the Mediterranean Sea. The region's geology features complex fault systems, including strike-slip and thrust faults, resulting from the northwestward motion of the African plate at rates of 4–5 millimeters per year relative to Eurasia.

Seismic swarms represent clusters of earthquakes occurring in a localized area over a short period without a dominant mainshock. The S20211114.1 swarm, recorded between 15:34 on 13 November 2021 and 21:22 on 18 November 2021, exemplifies this phenomenon. Over 125 hours and 48 minutes, 78 events were detected, with magnitudes ranging from 1.7 to 4.2 and focal depths primarily between 0 and 33 kilometers. The sequence included multiple events above magnitude 3.0, such as a 4.2-magnitude quake at 10 kilometers depth on 14 November at 04:48, followed by 3.6- and 3.5-magnitude events minutes later. Subsequent activity featured peaks on 15–16 November, including a 3.6-magnitude shock at 20 kilometers depth and a 3.3-magnitude event on 17 November.

These events clustered temporally, with heightened frequency on 14 and 15 November, reflecting fluid migration or stress transfer along pre-existing faults in the region. Depths indicate activity within the upper crust, consistent with the shallow seismogenic zone of the Gibraltar Arc. No surface rupture was reported, typical for moderate-magnitude swarms in this setting.

Historically, the Strait of Gibraltar has experienced recurrent seismic swarms since 2000, with four documented episodes. Prior occurrences include one in 2016 and three in 2021, underscoring episodic unrest rather than isolated incidents. The area's broader seismic history encompasses larger events, such as the 1755 Lisbon earthquake, which generated a transatlantic tsunami affecting the strait, and more recent moderate quakes linked to plate-boundary deformation.

Such swarms provide valuable data for understanding stress accumulation in convergent margins. Monitoring networks, including those operated by national seismological agencies, enable real-time tracking, aiding hazard assessment for nearby populations in southern Spain and northern Morocco. Continued observation remains essential given the region's potential for larger earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
EMSC Earthquake Database (emsc-csem.org)
Geological Survey of Spain (IGME) reports on the Gibraltar Arc
Moroccan National Seismic Network data summaries