Location:

Period:

11 Mar 2011 05:54:31 - 12 Mar 2011 03:34:43 (21 hours 40 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.9; 47 km E of ?arai, Japan; (11 Mar 2011) (95km from the swarm center)

18 swarms found nearby.

2000

PS20000629.1(193.0km)

28 Jun

1 day 0 hours

10 earthquakes

2005

PS20050119.1(121.9km)

19 Jan

10 hours

6 earthquakes

PS20050121.1(141.2km)

20 Jan

1 day 3 hours

8 earthquakes

2008

PS20080507.1(88.7km)

7 May

17 hours

11 earthquakes

PS20081220.1(156.3km)

20 Dec

22 hours

6 earthquakes

2011

PS20110311.4(72.8km)

11 Mar

8 hours

57 earthquakes

PS20110311.5(62.8km)

11 Mar

7 hours

10 earthquakes

PS20110313.1(64.9km)

13 Mar

3 days 13 hours

14 earthquakes

PS20110314.1(158.1km)

13 Mar

13 hours

14 earthquakes

PS20110317.1(60.4km)

16 Mar

1 day 16 hours

10 earthquakes

PS20110319.1(173.7km)

18 Mar

1 day 7 hours

6 earthquakes

PS20110320.1(131.5km)

20 Mar

1 day 4 hours

6 earthquakes

PS20110322.1(62.4km)

22 Mar

2 days 5 hours

14 earthquakes

PS20110322.3(160.8km)

22 Mar

12 hours

6 earthquakes

PS20110411.1(199.2km)

10 Apr

1 day 7 hours

7 earthquakes

2014

PS20140711.1(194.3km)

11 Jul

13 minutes

5 earthquakes

2016

PS20160923.1(88.7km)

22 Sep

18 hours

8 earthquakes

2021

PS20210804.1(105.9km)

3 Aug

18 hours

5 earthquakes

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Seismic Swarm Activity Near Namie, Japan, March 2011

A seismic swarm occurred 58 km east-northeast of Namie, Japan, from 05:54 on 11 March 2011 to 03:34 on 12 March 2011. In 21 hours and 40 minutes, 21 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.3 and focal depths between 1 km and 66 km.

The sequence began with a magnitude 6.3 event at 32 km depth, followed within minutes by a magnitude 5.9 shock at 8 km. Subsequent events included multiple magnitude 6.0–6.3 quakes at depths of 21–52 km, interspersed with smaller shocks. Activity tapered with a final magnitude 5.3 event at 13 km depth. Depths varied significantly, indicating activity across both shallow crustal levels and the deeper plate interface.

This region lies along the Japan Trench, where the Pacific Plate subducts westward beneath the Okhotsk Plate at approximately 8–9 cm per year. The tectonic setting produces frequent megathrust earthquakes and associated aftershock sequences. Historical records show elevated swarm activity in the area since 2000, with prior episodes in 2000 (one swarm), 2005 (two swarms), and 2008 (two swarms). These swarms typically reflect stress adjustments along the subduction interface or within the overriding plate.

On the same day, a magnitude 9.1 megathrust earthquake struck approximately 95 km from the swarm center, with its epicenter 70 km east of the Oshika Peninsula. The event ruptured a 500 km segment of the plate boundary and generated a major tsunami. The swarm began shortly after this mainshock, consistent with patterns of triggered seismicity in subduction zones.

Such swarms provide insight into post-seismic stress redistribution. Depths spanning 1–66 km suggest involvement of both the shallow accretionary prism and the deeper seismogenic zone. Continued monitoring of similar sequences aids in understanding afterslip and viscoelastic relaxation following great earthquakes.

References

USGS Earthquake Catalog
Japan Meteorological Agency seismic reports
Global CMT catalog for subduction zone parameters