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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:
20 Dec 2021 04:50:38 - 30 Dec 2021 05:06:02 (10 days 15 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
135
16 swarms found nearby.
2016
PS20160125.1(13.2km)
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(22.5km)
29 Jul
57 days 9 hours
1361 earthquakes
S20211114.1(11.8km)
13 Nov
5 days 5 hours
78 earthquakes
20 Nov
5 days 9 hours
57 earthquakes
5 Dec
5 days 18 hours
57 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(12.8km)
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 Analysis: Strait of Gibraltar, December 2021

The Strait of Gibraltar marks the tectonic boundary between the African and Eurasian plates, where oblique convergence drives moderate seismicity. This narrow marine corridor, approximately 14 km wide at its narrowest point, experiences ongoing deformation linked to the broader western Mediterranean tectonic regime. Geological records indicate that the region has hosted multiple seismic swarms since 2000, with seven documented episodes prior to and including late 2021.

SeismoSight internal data classify Swarm S20211221.1 as having begun at 04:50 on 20 December 2021 and concluded at 05:06 on 30 December 2021. Within this 240-hour window, 135 earthquakes were recorded. Analysis of the first 100 events reveals a compact temporal distribution dominated by low-to-moderate magnitudes. The largest events reached 3.7 on 20 and 21 December, while the majority fell between 1.7 and 2.8. Focal depths ranged from 2 km to 33 km, with many events clustered between 10 km and 30 km, consistent with brittle failure within the upper crust.

Early activity on 20 December showed rapid succession of events between 16:00 and 16:27, followed by the first magnitude-3.7 shock at 21:39. Subsequent peaks occurred near midnight on 21 December, including a second 3.7 event and a 3.6 shock within an hour. Magnitudes then declined, with only isolated events exceeding 3.0 later in the sequence. Depth variations suggest both shallow crustal and slightly deeper sources, possibly reflecting heterogeneous stress release along local fault segments.

Swarm recurrence since 2000 follows a pattern of infrequent but clustered episodes. Prior swarms occurred in 2016 (one event) and six times in 2021, indicating elevated activity during the latter year. The 2021 swarm sequence aligns with this trend of short-duration, high-event-count episodes rather than isolated mainshock-aftershock sequences.

Such swarms in the Strait of Gibraltar are interpreted as expressions of distributed strain accommodation across a complex plate-boundary zone. Continued monitoring remains essential for distinguishing background seismicity from potential precursors to larger events.

References

SeismoSight internal classification records for Swarm S20211221.1.
Geological Survey of Spain (IGME) tectonic framework reports on the Gibraltar Arc.
USGS Earthquake Catalog regional summaries for the western Mediterranean.