Location:

Period:

11 Mar 2011 06:35:20 - 11 Mar 2011 14:00:38 (7 hours 25 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.9; 47 km E of ?arai, Japan; (11 Mar 2011) (33km from the swarm center)

18 swarms found nearby.

2005

PS20050119.1(183.9km)

19 Jan

10 hours

6 earthquakes

2008

PS20080507.1(62.2km)

7 May

17 hours

11 earthquakes

PS20081220.1(125.9km)

20 Dec

22 hours

6 earthquakes

2011

PS20110311.2(62.8km)

11 Mar

21 hours

21 earthquakes

PS20110311.4(51.0km)

11 Mar

8 hours

57 earthquakes

PS20110312.2(158.8km)

11 Mar

20 hours

6 earthquakes

PS20110313.1(49.3km)

13 Mar

3 days 13 hours

14 earthquakes

PS20110314.1(99.7km)

13 Mar

13 hours

14 earthquakes

PS20110317.1(21.2km)

16 Mar

1 day 16 hours

10 earthquakes

PS20110319.1(110.9km)

18 Mar

1 day 7 hours

6 earthquakes

PS20110320.1(95.9km)

20 Mar

1 day 4 hours

6 earthquakes

PS20110322.1(34.4km)

22 Mar

2 days 5 hours

14 earthquakes

PS20110322.3(99.1km)

22 Mar

12 hours

6 earthquakes

PS20110411.1(137.1km)

10 Apr

1 day 7 hours

7 earthquakes

2014

PS20140711.1(160.5km)

11 Jul

13 minutes

5 earthquakes

2016

PS20160923.1(151.0km)

22 Sep

18 hours

8 earthquakes

PS20161121.1(150.0km)

21 Nov

20 hours

11 earthquakes

2021

PS20210804.1(87.8km)

3 Aug

18 hours

5 earthquakes

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Seismic Swarm PS20110311.5: Activity Offshore Eastern Japan on 11 March 2011

Seismic swarm PS20110311.5 was recorded 88 km east-southeast of Iwaki, Japan, beginning at 06:35 and concluding at 14:00 JST on 11 March 2011. Within this 7-hour-25-minute window, ten earthquakes occurred, with magnitudes ranging from 4.7 to 6.0 and focal depths between 9 km and 51 km. The sequence represents a short-lived cluster typical of foreshock or swarm activity in the tectonically active margin east of Honshu.

The events unfolded in two main phases. The initial pair at 06:35 (M5.8, 10 km) and 06:57 (M6.0, 47 km) was followed by a lull and then renewed activity between 10:05 and 11:46, featuring six additional shocks between M4.7 and M5.8. The final event at 14:00 (M5.5, 51 km) marked the end of the swarm. Depths varied without a clear deepening trend, indicating slip on multiple fault segments within the overriding and subducting plates.

This swarm occurred within the Japan Trench subduction zone, where the Pacific Plate converges with the Okhotsk Plate at approximately 8–9 cm per year. The region experiences frequent megathrust earthquakes and associated aftershock sequences. On the same day, a magnitude 7.9 event struck 47 km east of Ibaraki, only 33 km from the swarm centroid, underscoring the intense seismic energy release along this margin on 11 March 2011.

Since 1 January 2000, five swarms have been documented in the immediate vicinity. Earlier episodes occurred in 2005 (one swarm) and 2008 (two swarms), with two additional swarms recorded in 2011. These clusters illustrate the episodic nature of seismicity in the area, where stress transfer from larger events can trigger brief bursts of moderate-magnitude activity.

Geologically, the swarm lies above the locked portion of the plate interface that ruptured during the 2011 Tohoku-Oki mainshock. The varied depths suggest involvement of both the megathrust and subsidiary normal faults in the overriding plate, consistent with the extensional regime that develops after large subduction earthquakes. Such swarms provide valuable data on stress redistribution and can precede or accompany major seismic cycles.

Further monitoring of similar clusters continues to refine understanding of fault interactions in this high-hazard zone. Continued deployment of ocean-bottom seismometers and GPS networks improves resolution of these transient sequences and their relation to long-term subduction processes.

References

SeismoSight internal swarm catalogue PS20110311.5
USGS Earthquake Catalog (historical events since 2000)
Japan Meteorological Agency seismic database