Seismic Swarm S20221016.1 in the Strait of Gibraltar: Analysis and Regional Context

The seismic swarm designated S20221016.1 occurred in the Strait of Gibraltar, a tectonically active zone marking the boundary between the African and Eurasian plates. The event sequence began at 20:03 on 15 October 2022 and concluded at 19:19 on 21 October 2022, spanning 143 hours and 16 minutes. During this period, 62 earthquakes were recorded, with magnitudes ranging from 1.5 to 4.5 and focal depths between 1 and 33 km.

The largest event reached magnitude 4.5 at a depth of 15 km on 16 October. Most events clustered at shallow to intermediate depths, with notable activity on 16 October including multiple shocks above magnitude 2.0. Subsequent days showed a gradual decline in both frequency and intensity, with the final recorded event at magnitude 2.0 on 21 October. Depths varied considerably, indicating activity across different crustal levels within the Gibraltar Arc system.

This swarm fits into a pattern of seismic unrest in the region. Since 1 January 2000, 14 such swarms have been documented. Earlier episodes include one in 2016, six in 2021, and seven in 2022 prior to this sequence. These recurrent swarms reflect ongoing stress accumulation along the plate margin, where oblique convergence drives both strike-slip and thrust faulting.

Geologically, the Strait of Gibraltar forms part of the western Mediterranean's complex boundary zone. The area lies within the Gibraltar Arc, an orogenic belt shaped by the collision and subduction dynamics between Africa and Eurasia. The region experiences moderate seismicity due to the interaction of major fault systems, including the Trans-Alboran Shear Zone. Historical records document significant events, such as the 1755 Lisbon earthquake, which produced widespread effects across the strait and adjacent coasts.

The October 2022 swarm underscores the persistent seismic hazard in this narrow waterway separating Europe and Africa. Depths predominantly under 30 km suggest involvement of the upper crust, consistent with the tectonic regime of the arc. Monitoring such sequences provides critical data for understanding strain release patterns and improving hazard assessment for nearby populations in southern Spain and northern Morocco.

Continued observation remains essential given the area's history of clustered seismicity. The S20221016.1 event contributes to the growing dataset of recent activity, highlighting the dynamic nature of the Gibraltar region.

References:
SeismoSight internal swarm classification records.
USGS Earthquake Catalog for regional tectonic framework.
Scientific literature on the Gibraltar Arc and Alboran Sea tectonics (updated through 2023).