Seismic Swarm S20220206.1 in the Strait of Gibraltar: Analysis and Regional Context

The Strait of Gibraltar represents a key tectonic boundary where the Nubian Plate converges with the Eurasian Plate at rates of approximately 4–5 mm per year. This oblique convergence drives distributed deformation across the region, producing both shallow crustal earthquakes and occasional deeper events linked to slab dynamics beneath the Alboran Sea. The area forms part of the Gibraltar Arc, a complex orogenic system shaped by Miocene subduction and subsequent slab rollback, resulting in elevated seismicity compared to surrounding stable continental domains.

Seismic swarms are recurrent in this setting due to fluid migration along faults and stress transfer within the plate-boundary zone. Since 2000, eight such swarms have been documented in the Strait of Gibraltar. Earlier episodes occurred in 2016 (one swarm) and 2021 (seven swarms), indicating a pattern of episodic clustered activity rather than isolated mainshock-aftershock sequences.

Swarm S20220206.1 began at 11:50 on 5 February 2022 and concluded at 03:05 on 14 February 2022, spanning 207 hours and 14 minutes. During this interval, 147 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-to-moderate magnitudes, with values ranging from 1.8 to 4.2. Depths clustered between 0 and 40 km, the majority occurring at shallow levels (under 20 km), consistent with brittle failure in the upper crust. Notable events include a magnitude 3.8 quake at 10 km depth on 6 February, a magnitude 4.2 event at 11 km depth on 8 February (the largest in the sequence), and several magnitude 3.2 shocks at depths of 5–10 km. Temporal distribution shows peak activity on 6–8 February, with events spaced minutes to hours apart during intense phases, followed by a gradual decline.

This swarm exhibited typical characteristics of tectonic swarms in convergent margins: absence of a dominant mainshock, migration of hypocenters along fault segments, and a high proportion of events below magnitude 3.0. Depths remained largely crustal, supporting interpretations of activity along the Trans-Alboran Shear Zone and associated strike-slip and thrust structures. No surface rupture or significant damage was associated with these events, aligning with the moderate energy release.

The 2022 swarm reinforces the ongoing seismic hazard in the Gibraltar region, where historical records document destructive earthquakes linked to the same plate-boundary processes. Continued monitoring is essential given the proximity to densely populated coastal areas in southern Spain and northern Morocco.

References:
SeismoSight internal swarm classification records (S20220206.1 dataset).
Tectonic framework derived from studies on Nubian-Eurasian convergence and Gibraltar Arc evolution (e.g., geological surveys of the western Mediterranean).