Location:

Period:

22 Mar 2011 09:44:29 - 23 Mar 2011 09:35:56 (23 hours 51 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.4; 100 km ENE of Miyako, Japan; (20 Apr 2026) (44km from the swarm center)

16 swarms found nearby.

2008

PS20080614.1(183.2km)

13 Jun

3 hours

6 earthquakes

2011

PS20110309.1(195.2km)

9 Mar

7 days 12 hours

159 earthquakes

PS20110311.6(168.0km)

11 Mar

2 days 8 hours

50 earthquakes

PS20110311.1(176.8km)

11 Mar

1 day 14 hours

28 earthquakes

PS20110311.7(24.9km)

11 Mar

3 days 15 hours

38 earthquakes

PS20110311.8(127.7km)

11 Mar

9 hours

5 earthquakes

PS20110315.1(75.6km)

14 Mar

1 day 10 hours

6 earthquakes

PS20110319.2(70.3km)

18 Mar

1 day 17 hours

8 earthquakes

PS20110413.1(22.4km)

13 Apr

8 hours

6 earthquakes

PS20110916.1(54.3km)

16 Sep

1 day 11 hours

10 earthquakes

2012

PS20120520.1(47.6km)

19 May

1 day 3 hours

9 earthquakes

2015

PS20150611.1(46.3km)

10 Jun

20 hours

5 earthquakes

2016

PS20160820.1(125.6km)

20 Aug

1 day 7 hours

6 earthquakes

2025

PS20251108.1(42.9km)

8 Nov

1 day 13 hours

25 earthquakes

PS20251208.1(172.3km)

8 Dec

18 hours

9 earthquakes

2026

PS20260420.1(58.3km)

20 Apr

21 hours

9 earthquakes

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Seismic Swarm PS20110323.1 Near Miyako, Japan: Geological Context and Event Analysis

The seismic swarm designated PS20110323.1 occurred approximately 33 km east-northeast of Miyako in Iwate Prefecture, Japan. Registered between 09:44 on 22 March 2011 and 09:35 on 23 March 2011, the sequence lasted 23 hours and 51 minutes and comprised six earthquakes. This activity took place in the immediate aftermath of the Mw 9.0 Tohoku-Oki megathrust earthquake of 11 March 2011.

The six events are listed below with their respective times, magnitudes, and focal depths:
22 March 2011 09:44:29, magnitude 6.4, depth 13 km
22 March 2011 11:21:39, magnitude 5.6, depth 30 km
22 March 2011 15:49:27, magnitude 5.1, depth 28 km
23 March 2011 00:06:43, magnitude 5.0, depth 40 km
23 March 2011 04:13:13, magnitude 4.8, depth 65 km
23 March 2011 09:35:56, magnitude 5.2, depth 27 km

All events were located within the overriding plate and along the subduction interface of the Japan Trench. The region forms part of the convergent boundary where the Pacific Plate subducts beneath the Okhotsk Plate at approximately 8–9 cm per year. This tectonic setting produces frequent megathrust earthquakes and associated aftershock sequences. The shallow to intermediate depths recorded during the swarm are consistent with crustal and upper-plate seismicity triggered by stress redistribution following the mainshock.

Since 1 January 2000, eight swarms have been identified in the Miyako area according to SeismoSight classification criteria. Prior episodes occurred in 2008 (one swarm) and 2011 (seven swarms). The 2011 cluster, including PS20110323.1, reflects elevated aftershock productivity along the ruptured portion of the plate interface and in the overlying crust.

A magnitude 7.4 earthquake was recorded 100 km east-northeast of Miyako on 20 April 2026, approximately 44 km from the PS20110323.1 centroid. This event further illustrates the persistent seismic hazard along the Japan Trench segment offshore Iwate Prefecture.

The combination of subduction-driven tectonics, historical swarm recurrence, and large-magnitude aftershocks underscores the continued monitoring needs for this portion of the northeastern Japan margin.

References

SeismoSight internal swarm classification database (PS20110323.1 parameters and historical statistics since 2000).
Japan Meteorological Agency seismic catalog for regional event verification.
USGS Earthquake Hazards Program for tectonic framework of the Japan Trench.