Seismic Swarm Activity in the Strait of Gibraltar: November 2021

The Strait of Gibraltar marks the tectonic boundary between the African and Eurasian plates, where oblique convergence occurs at rates of approximately 4–5 mm per year. This region features a complex mix of thrust and strike-slip faulting, contributing to moderate seismicity along the Gibraltar Arc and adjacent Rif-Betic cordillera. Historical records document destructive events such as the 1755 Lisbon earthquake, which generated widespread shaking across the strait, and more recent activity including the 2016 Alboran Sea sequence. SeismoSight internal classification recorded swarm S20211121.1 from 11:28 on 20 November 2021 to 21:19 on 25 November 2021. Over 129 hours and 50 minutes, 57 earthquakes were detected, with magnitudes ranging from 1.7 to 3.5 and focal depths predominantly between 1 km and 44 km. The sequence began with two events of magnitudes 2.5 and 2.6 at 10 km depth within the first four minutes. Subsequent activity included peaks at magnitude 3.5 on 20 November at 15:41 and again on 23 November at 04:57, both at 10 km depth. The majority of events clustered in the first three days, with later shocks showing slightly greater depth variability. This swarm aligns with the area’s established pattern of episodic seismic clusters. Since 1 January 2000, five such swarms have occurred in the Strait of Gibraltar. Earlier episodes were limited to a single swarm in 2016 and four additional swarms throughout 2021. These sequences typically consist of low-to-moderate magnitude events and rarely culminate in larger mainshocks, reflecting fluid migration or minor stress adjustments along local fault segments. The November 2021 activity remained below thresholds associated with significant ground shaking or tsunami risk. Depths shallower than 10 km accounted for roughly one-third of events, while deeper events (20–35 km) indicated activity within the subducting slab or lower crust. No surface rupture or notable aftershock migration beyond the initial cluster was observed. Ongoing monitoring by regional networks continues to track microseismicity in this convergent setting, providing data for refined hazard assessments along the plate boundary. References:
SeismoSight internal swarm classification S20211121.1
USGS Earthquake Hazards Program – Gibraltar Arc seismicity summaries
IGN (Instituto Geográfico Nacional) seismic bulletins for the Iberian–Maghreb region