Location:

Period:

20 Mar 2011 01:24:42 - 21 Mar 2011 05:54:12 (1 day 4 hours 29 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.9; 47 km E of ?arai, Japan; (11 Mar 2011) (77km from the swarm center)

23 swarms found nearby.

2008

PS20080507.1(42.9km)

7 May

17 hours

11 earthquakes

PS20081220.1(30.7km)

20 Dec

22 hours

6 earthquakes

2011

PS20110309.1(181.2km)

9 Mar

7 days 12 hours

159 earthquakes

PS20110311.2(131.5km)

11 Mar

21 hours

21 earthquakes

PS20110311.4(58.7km)

11 Mar

8 hours

57 earthquakes

PS20110311.3(158.6km)

11 Mar

1 day 17 hours

44 earthquakes

PS20110311.5(95.9km)

11 Mar

7 hours

10 earthquakes

PS20110312.2(136.7km)

11 Mar

20 hours

6 earthquakes

PS20110312.1(123.9km)

11 Mar

1 day 3 hours

8 earthquakes

PS20110313.1(145.2km)

13 Mar

3 days 13 hours

14 earthquakes

PS20110314.1(71.1km)

13 Mar

13 hours

14 earthquakes

PS20110315.2(190.5km)

15 Mar

1 day 17 hours

6 earthquakes

PS20110317.1(79.8km)

16 Mar

1 day 16 hours

10 earthquakes

PS20110319.1(121.6km)

18 Mar

1 day 7 hours

6 earthquakes

PS20110322.2(139.9km)

21 Mar

18 hours

5 earthquakes

PS20110322.1(71.3km)

22 Mar

2 days 5 hours

14 earthquakes

PS20110322.3(91.4km)

22 Mar

12 hours

6 earthquakes

PS20110411.1(152.9km)

10 Apr

1 day 7 hours

7 earthquakes

2014

PS20140711.1(65.0km)

11 Jul

13 minutes

5 earthquakes

2016

PS20161121.1(111.2km)

21 Nov

20 hours

11 earthquakes

S20161122.1(97.2km)

21 Nov

2 days 0 hours

42 earthquakes

2021

PS20210804.1(36.6km)

3 Aug

18 hours

5 earthquakes

2022

PS20220316.1(146.5km)

16 Mar

1 hours

7 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm PS20110320.1: Post-Tohoku Activity Offshore Iwaki

Seismic swarm PS20110320.1 occurred 122 km ESE of Iwaki, Japan, in the Japan Trench subduction zone. It began at 01:24 on 20 March 2011 and concluded at 05:54 on 21 March 2011, spanning 28 hours and 29 minutes with six recorded events. This sequence followed the magnitude 9.0 Tohoku earthquake of 11 March 2011 by nine days and was located approximately 77 km from the epicenter of a magnitude 7.9 aftershock that struck 47 km east of Oarai on the same day.

The swarm events exhibited the following parameters:

20 March 2011 01:24:42, magnitude 5.0, depth 27 km
20 March 2011 01:30:45, magnitude 5.4, depth 21 km
20 March 2011 05:55:30, magnitude 5.6, depth 57 km
20 March 2011 07:56:30, magnitude 5.2, depth 31 km
20 March 2011 13:55:35, magnitude 4.2, depth 35 km
21 March 2011 05:54:12, magnitude 5.0, depth 35 km

These earthquakes reflect typical aftershock behavior in a region where the Pacific Plate subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. The varying focal depths indicate activity across the plate interface and within the overriding crust, consistent with stress redistribution following the mainshock rupture.

Since 1 January 2000, fourteen swarms have been documented in this sector of the subduction zone. Earlier swarms occurred in 2008 (two events) and 2011 (twelve events), underscoring elevated seismic clustering during periods of heightened regional strain. The 2011 Tohoku mainshock and its immediate aftershocks released substantial accumulated energy, triggering secondary sequences such as PS20110320.1 through both static and dynamic stress changes.

Geologically, the offshore area east of Iwaki lies within the Fukushima-Ibaraki segment of the Japan Trench, characterized by thick sedimentary cover and active megathrust faults. Historical records show recurrent large-magnitude events, with the 2011 sequence representing one of the most energetic episodes in the instrumental era. Depths between 21 km and 57 km align with the expected range for interplate and intraslab seismicity in this tectonic setting.

Ongoing monitoring by regional networks continues to track residual activity, contributing to refined models of subduction-zone dynamics and long-term hazard assessment.

References

SeismoSight internal swarm catalog PS20110320.1
USGS Earthquake Catalog (Tohoku sequence parameters)
Japan Meteorological Agency regional seismicity reports