Seismic Swarm S20220617.1 in the Strait of Gibraltar

The Strait of Gibraltar marks the tectonic boundary between the Eurasian and African plates, where oblique convergence drives ongoing deformation and moderate seismicity. This narrow marine corridor, approximately 14 km wide at its narrowest point, overlies a complex transition zone involving the Gibraltar Arc and the Rif-Betic cordillera. Plate motion rates average 4–5 mm per year, producing both strike-slip and thrust faulting that can generate earthquake swarms when fluid migration or stress transfer activates clusters of small faults.

Between 14:19 UTC on 16 June 2022 and 23:22 UTC on 22 June 2022, SeismoSight recorded swarm S20220617.1 comprising 74 earthquakes. The sequence lasted 153 hours and three minutes. Magnitudes ranged from 1.6 to 3.3, with the largest events occurring on 17 June (M 3.3 at 15 km depth) and 20 June (M 3.3 at 20 km depth). Focal depths were predominantly shallow to intermediate, spanning 0–30 km, consistent with brittle failure within the upper crust and the seismogenic portion of the plate interface.

The temporal distribution showed two main peaks: an initial burst on 16–17 June that included twelve events above magnitude 2.0, followed by renewed activity on 19–20 June featuring the second M 3.3 shock and several M 2.5–2.8 events. Depths shallowed noticeably during the second pulse, with multiple events located at 4–10 km, suggesting possible upward migration of seismicity. All but one event remained below magnitude 3.0, indicating a typical swarm pattern driven by aseismic slip or pore-pressure changes rather than a single mainshock-aftershock sequence.

Since 1 January 2000 the region has hosted eleven documented swarms. Earlier episodes occurred in 2016 (one swarm) and 2021 (seven swarms). The 2022 sequence represents the third swarm of the year, underscoring a recent uptick in clustered activity along the Gibraltar Strait segment of the plate boundary.

Such swarms contribute to the long-term seismic hazard assessment of adjacent coastal areas in southern Spain and northern Morocco. Although individual events rarely exceed moderate magnitudes, their cumulative energy release and proximity to critical infrastructure warrant continued monitoring. Future swarms may exhibit similar magnitude–depth distributions, providing valuable data for refining local velocity models and fault segmentation.

References
SeismoSight internal swarm catalogue S20220617.1
USGS Earthquake Catalog (regional plate-boundary events)
IGN Spain seismic bulletins (Gibraltar Strait region)