Seismic Swarm S20210730.3 in the Strait of Gibraltar

The seismic swarm designated S20210730.3 occurred in the Strait of Gibraltar, beginning at 17:56 on 29 July 2021 and concluding at 03:47 on 25 September 2021. Over 1377 hours and 51 minutes, the sequence produced 1361 earthquakes. This event represents one of four documented swarms in the region since 2000, with prior episodes recorded in 2007 and 2016, followed by a second swarm in 2021.

The Strait of Gibraltar occupies a tectonically complex zone at the western terminus of the Mediterranean, where the African and Eurasian plates converge at rates of approximately 4–5 mm per year. The area forms part of the Gibraltar Arc, characterized by the Betic-Rif orogenic system and associated crustal deformation. Moderate seismicity arises from strike-slip and reverse faulting along inherited structures, with hypocenters commonly located between 1 km and 30 km depth. The region has experienced significant historical earthquakes, underscoring its long-term seismic hazard.

Analysis of the first 100 events reveals a typical swarm pattern of clustered, low-to-moderate magnitude activity without a single dominant mainshock. Magnitudes ranged from 1.6 to 4.3, with the largest event (magnitude 4.3) occurring at 09:13 on 30 July 2021 at 10 km depth. Depths varied between 1 km and 30 km, averaging around 16–20 km for many events. Early activity on 29–30 July included several events above magnitude 3.0, such as magnitudes 4.0 and 3.7, followed by a sustained sequence of magnitude 2.0–2.6 shocks through early August. Temporal distribution showed peak rates within the first 48 hours, with events distributed across both shallow and mid-crustal levels.

This distribution aligns with fluid-driven or stress-transfer mechanisms often observed in swarm sequences within convergent margins. Depths consistently below 30 km indicate activity within the brittle upper crust, consistent with the regional tectonic framework of the Gibraltar Arc.

Historical records confirm that such swarms remain infrequent yet recurrent features of the Strait of Gibraltar. The 2021 episodes, including S20210730.3, contribute to ongoing monitoring efforts that track cumulative strain release and inform seismic hazard assessments for surrounding coastal populations in southern Spain and northern Morocco.

Continued observation of similar sequences supports improved understanding of fault interactions and potential precursors to larger events in this key tectonic gateway between the Atlantic and Mediterranean.