Location:

Period:

13 Feb 2021 14:07:49 - 16 Feb 2021 23:31:30 (3 days 9 hours 23 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.3; 57 km ENE of Namie, Japan; (16 Mar 2022) (10km from the swarm center)

9 swarms found nearby.

2011

PS20110309.1(53.0km)

9 Mar

7 days 12 hours

159 earthquakes

PS20110311.3(42.5km)

11 Mar

1 day 17 hours

44 earthquakes

PS20110312.2(68.2km)

11 Mar

20 hours

6 earthquakes

PS20110314.1(90.3km)

13 Mar

13 hours

14 earthquakes

PS20110319.1(113.7km)

18 Mar

1 day 7 hours

6 earthquakes

PS20110322.3(99.2km)

22 Mar

12 hours

6 earthquakes

2014

PS20140711.1(109.5km)

11 Jul

13 minutes

5 earthquakes

2016

PS20161121.1(47.8km)

21 Nov

20 hours

11 earthquakes

2022

PS20220316.1(29.2km)

16 Mar

1 hours

7 earthquakes

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Seismic Swarm S20210214.1: Earthquake Activity Near the East Coast of Honshu, Japan

Seismic swarm S20210214.1 occurred near the east coast of Honshu, Japan, from 14:07 on 13 February 2021 to 23:31 on 16 February 2021. Over 81 hours and 23 minutes, 49 earthquakes were recorded in this tectonically active region. The swarm initiated with a magnitude 7.1 event at a depth of 43 km, followed by numerous aftershocks and smaller events, the largest of which reached magnitude 5.4.

Japan's eastern coastline lies along a complex subduction zone where the Pacific Plate descends beneath the Okhotsk Plate. This setting generates frequent seismic activity, including both isolated large earthquakes and clustered swarms. The Honshu margin has produced major historical events, such as the 2011 Tohoku earthquake, which released immense energy and triggered widespread aftershock sequences. Swarm activity in the area reflects episodic stress release along the plate interface and within the overriding crust, often at depths between 30 and 70 km.

Data from the swarm show a clear temporal pattern. The initial magnitude 7.1 shock was followed within minutes by events of magnitude 4.3 and 5.4, with subsequent activity gradually decreasing in both frequency and size. Depths remained predominantly in the 40–60 km range, consistent with the regional subduction geometry. In total, 49 events were logged, with 18 exceeding magnitude 4.0.

Historical records since 2000 indicate eight comparable swarms in the vicinity. Earlier episodes occurred in 2011 (six swarms), 2014 (one swarm), and 2016 (one swarm). These clusters suggest recurring stress accumulation and release along the same structural corridor. On 16 March 2022, a magnitude 7.3 earthquake struck 57 km east-northeast of Namie, approximately 10 km from the 2021 swarm center, underscoring the persistent seismic hazard in the immediate offshore region.

Seismic swarms of this type provide valuable insight into the mechanics of subduction-zone deformation. The rapid onset, moderate magnitudes, and spatial concentration observed in S20210214.1 are characteristic of fluid-assisted or stress-triggered sequences rather than classic mainshock-aftershock sequences. Continued monitoring of such events contributes to improved understanding of rupture processes and long-term seismic hazard assessment along the Japan Trench.

References

SeismoSight internal swarm classification database (S20210214.1 parameters and statistics).
United States Geological Survey earthquake catalog (regional tectonics and 2022 M7.3 event).