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Note:This page contains AI-generated content for informational and entertainment purposes only. It may contain inaccuracies. Raw event data is from USGS and EMSC. All statistics, lists, and derived information are generated by this site. Full disclaimerFound an error?
Location:
STRAIT OF GIBRALTAR
Period:
5 Dec 2021 08:13:11 - 11 Dec 2021 02:29:32 (5 days 18 hours 16 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
57
16 swarms found nearby.
2016
PS20160125.1(16.1km)
25 Jan
1 hours
6 earthquakes
2021
15 May
3 days 23 hours
51 earthquakes
6 Jun
11 days 1 hours
144 earthquakes
S20210730.3(19.4km)
29 Jul
57 days 9 hours
1361 earthquakes
13 Nov
5 days 5 hours
78 earthquakes
20 Nov
5 days 9 hours
57 earthquakes
20 Dec
10 days 0 hours
135 earthquakes
2022
5 Feb
8 days 15 hours
147 earthquakes
7 Apr
28 days 16 hours
392 earthquakes
19 May
21 days 19 hours
412 earthquakes
16 Jun
6 days 9 hours
74 earthquakes
19 Jul
22 days 3 hours
330 earthquakes
15 Sep
4 days 1 hours
55 earthquakes
3 Oct
5 days 4 hours
91 earthquakes
S20221016.1(13.2km)
15 Oct
5 days 23 hours
62 earthquakes
2024
15 Mar
2 days 2 hours
32 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm in the Strait of Gibraltar: December 2021 Event Analysis

The Strait of Gibraltar marks a critical tectonic boundary between the African and Eurasian plates, where oblique convergence drives ongoing deformation across the Gibraltar Arc. This complex zone encompasses the Betic Cordillera in southern Spain and the Rif Mountains in northern Morocco, featuring a combination of thrust faults, strike-slip structures, and remnants of ancient subduction. Seismicity in the region typically occurs at shallow to intermediate depths, reflecting both crustal faulting and deeper slab-related processes.

Seismic swarms represent clusters of earthquakes closely spaced in time and location without a single dominant mainshock. Swarm S20211206.1 began at 08:13 on 5 December 2021 and concluded at 02:29 on 11 December 2021, spanning 138 hours and 16 minutes. During this interval, 57 earthquakes were recorded within the Strait of Gibraltar. Magnitudes ranged from 2.0 to 3.7, with the largest event measuring 3.7 at a depth of 10 km on 5 December. Depths varied between 0 km and 30 km, indicating activity across multiple crustal levels.

The temporal distribution showed the highest frequency on the first day, followed by a gradual decline. Early events included several above magnitude 3.0, such as a 3.4 at 10 km depth and a 3.2 at 20 km depth. Later stages featured predominantly smaller events below magnitude 2.5, with occasional spikes such as a 3.0 on 10 December. Depths remained variable throughout, with clusters at 7–10 km and 20–30 km suggesting involvement of distinct fault segments or rheological boundaries.

Historical records since 2000 document six swarms in the same area. One occurred in 2016, while five took place in 2021, underscoring elevated swarm activity during that year. These episodes typically involve low-to-moderate magnitudes and do not culminate in damaging events, consistent with the background tectonic strain accumulation along the plate boundary.

Such swarms contribute to understanding stress transfer and fluid migration within the Gibraltar Arc. Continued monitoring supports regional hazard assessment, particularly given the proximity to densely populated coastal zones in Spain and Morocco. The December 2021 sequence aligns with the established pattern of diffuse, moderate seismicity that characterizes this convergent margin.

References

  • United States Geological Survey Earthquake Catalog
  • European-Mediterranean Seismological Centre Data Archive
  • Instituto Geográfico Nacional (Spain) Seismic Reports