Location:

Period:

21 Mar 2011 19:01:19 - 22 Mar 2011 13:05:12 (18 hours 3 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

16 swarms found nearby.

2008

PS20080507.1(170.0km)

7 May

17 hours

11 earthquakes

PS20081220.1(109.8km)

20 Dec

22 hours

6 earthquakes

2011

PS20110311.6(147.6km)

11 Mar

2 days 8 hours

50 earthquakes

PS20110311.4(183.8km)

11 Mar

8 hours

57 earthquakes

PS20110311.3(184.5km)

11 Mar

1 day 17 hours

44 earthquakes

PS20110312.1(26.1km)

11 Mar

1 day 3 hours

8 earthquakes

PS20110314.1(193.8km)

13 Mar

13 hours

14 earthquakes

PS20110315.2(71.4km)

15 Mar

1 day 17 hours

6 earthquakes

PS20110320.1(139.9km)

20 Mar

1 day 4 hours

6 earthquakes

PS20110325.1(109.1km)

24 Mar

20 hours

6 earthquakes

2012

PS20121207.1(108.9km)

7 Dec

14 hours

10 earthquakes

S20121207.1(110.7km)

7 Dec

1 day 20 hours

39 earthquakes

2013

PS20131025.1(119.4km)

25 Oct

4 hours

6 earthquakes

2014

PS20140711.1(90.2km)

11 Jul

13 minutes

5 earthquakes

2021

PS20210804.1(145.6km)

3 Aug

18 hours

5 earthquakes

2022

PS20220316.1(181.4km)

16 Mar

1 hours

7 earthquakes

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Seismic Swarm PS20110322.2: Insights from the Japan Trench Offshore Iwaki

The seismic swarm designated PS20110322.2 occurred in the Japan Trench subduction zone, approximately 216 km east-southeast of Iwaki in Fukushima Prefecture. This region lies at the convergent boundary where the Pacific Plate subducts beneath the Okhotsk Plate at rates of about 8–9 cm per year, generating frequent megathrust earthquakes and associated aftershock sequences. The swarm initiated at 19:01 on 21 March 2011 and concluded at 13:05 on 22 March 2011, spanning 18 hours and 3 minutes during which five earthquakes were recorded.

Event parameters from the sequence include a magnitude 5.0 event at 36 km depth on 21 March at 19:01, followed by a magnitude 6.4 shock at 11 km depth on 22 March at 07:18. Subsequent activity comprised a magnitude 5.6 event at 10 km depth (12:01), a magnitude 5.1 event at 22 km depth (12:56), and a magnitude 4.5 event at 35 km depth (13:05). These focal depths indicate activity spanning both the shallow megathrust interface and slightly deeper crustal levels within the overriding plate.

Geologically, the Japan Trench hosts one of the most active subduction systems on Earth. The 11 March 2011 Tohoku earthquake (Mw 9.0) ruptured a 500 km segment of the plate boundary immediately west of this swarm location, producing widespread stress redistribution that triggered numerous aftershocks and secondary swarms. The March 22 sequence aligns with this post-mainshock phase, reflecting adjustment along the fault system. Historical records since 2000 document nine comparable swarms in the vicinity, with two occurring in 2008 and seven in 2011, underscoring episodic clustering tied to the great earthquake cycle.

Such swarms typically arise from fluid migration, aseismic slip, or stress triggering within the forearc and outer-rise domains. Depths ranging from 10 km to 36 km are consistent with the geometry of the subducting slab and the seismogenic zone in this segment of the trench. The occurrence of a magnitude 6.4 event within the swarm highlights the potential for moderate-to-strong shaking even in secondary sequences following a megathrust rupture.

Long-term monitoring of the Japan Trench reveals that seismic swarms often precede or accompany major strain release, although individual swarms rarely exceed the energy of the primary event. Continued observation in this tectonically active corridor remains essential for refining hazard models.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Japan Meteorological Agency seismic database
SeismoSight internal swarm classification records