Seismic Swarm S20220720.1 in the Strait of Gibraltar

The Strait of Gibraltar marks the westernmost segment of the convergent boundary between the African and Eurasian tectonic plates. This narrow marine passage experiences ongoing oblique convergence, with the African plate moving northwestward relative to Eurasia at rates of approximately 4–5 mm per year. The resulting transpressional regime produces a complex network of strike-slip and thrust faults that accommodate both lateral shear and shortening. Historical records document recurrent moderate seismicity, including events that have caused localized damage in southern Spain and northern Morocco.

SeismoSight registered swarm S20220720.1 beginning at 10:28 UTC on 19 July 2022 and concluding at 13:29 UTC on 10 August 2022. During the 531-hour interval, 330 earthquakes were recorded within the Strait. Analysis of the first 100 events reveals a typical swarm signature: the majority of magnitudes clustered between 1.7 and 2.6, with a single peak event of 4.1 recorded on 20 July at 14:11 UTC. Focal depths ranged from 1 km to 33 km, indicating activity distributed through the upper and mid-crust. Early events showed a rapid succession of low-magnitude shocks, followed by a gradual decline in rate while maintaining similar depth distribution.

The 4.1-magnitude event occurred at a depth of 10 km and was preceded by several foreshocks of 2.0–3.3. Subsequent activity included multiple events above magnitude 3.0, yet none exceeded the initial peak. Depth histograms indicate a concentration between 10 km and 25 km, consistent with brittle failure within the seismogenic zone of the Gibraltar Arc. Temporal clustering suggests fluid migration or aseismic slip as possible driving mechanisms, although precise source modeling requires additional geodetic constraints.

Since 1 January 2000, twelve seismic swarms have been documented in the same sector. Prior episodes occurred in 2016 (one swarm), 2021 (seven swarms), and 2022 (four swarms, including the present sequence). This elevated frequency underscores the region’s capacity for episodic, swarm-type releases rather than isolated mainshock–aftershock sequences.

The geological setting of the Strait integrates the Betic-Rif orogenic system and the Gibraltar Arc, where inherited Miocene structures continue to respond to contemporary plate motion. Ongoing convergence sustains low to moderate seismicity, with swarms providing valuable windows into fault-zone processes at the plate boundary.

References
SeismoSight internal catalog, swarm classification S20220720.1.
USGS Earthquake Catalog (regional tectonics summary).
IGN (Instituto Geográfico Nacional) seismic bulletins for the Gibraltar region.