Location:

Period:

3 Aug 2021 10:29:17 - 4 Aug 2021 04:40:25 (18 hours 11 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.9; 47 km E of ?arai, Japan; (11 Mar 2011) (83km from the swarm center)

21 swarms found nearby.

2008

PS20080507.1(25.8km)

7 May

17 hours

11 earthquakes

PS20081220.1(51.8km)

20 Dec

22 hours

6 earthquakes

2011

PS20110311.2(105.9km)

11 Mar

21 hours

21 earthquakes

PS20110311.4(38.2km)

11 Mar

8 hours

57 earthquakes

PS20110311.3(194.5km)

11 Mar

1 day 17 hours

44 earthquakes

PS20110311.5(87.8km)

11 Mar

7 hours

10 earthquakes

PS20110312.2(169.5km)

11 Mar

20 hours

6 earthquakes

PS20110312.1(135.5km)

11 Mar

1 day 3 hours

8 earthquakes

PS20110313.1(134.6km)

13 Mar

3 days 13 hours

14 earthquakes

PS20110314.1(101.2km)

13 Mar

13 hours

14 earthquakes

PS20110317.1(67.5km)

16 Mar

1 day 16 hours

10 earthquakes

PS20110319.1(146.2km)

18 Mar

1 day 7 hours

6 earthquakes

PS20110320.1(36.6km)

20 Mar

1 day 4 hours

6 earthquakes

PS20110322.2(145.6km)

21 Mar

18 hours

5 earthquakes

PS20110322.1(55.1km)

22 Mar

2 days 5 hours

14 earthquakes

PS20110322.3(118.4km)

22 Mar

12 hours

6 earthquakes

PS20110411.1(177.9km)

10 Apr

1 day 7 hours

7 earthquakes

2014

PS20140711.1(90.0km)

11 Jul

13 minutes

5 earthquakes

2016

PS20160923.1(178.2km)

22 Sep

18 hours

8 earthquakes

PS20161121.1(145.9km)

21 Nov

20 hours

11 earthquakes

2022

PS20220316.1(182.7km)

16 Mar

1 hours

7 earthquakes

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

Seismic Swarm PS20210804.1: Analysis of Activity East of Hasaki, Japan

A seismic swarm designated PS20210804.1 occurred 144 km ENE of Hasaki, Japan, from 10:29 on 3 August 2021 to 04:40 on 4 August 2021. The event sequence lasted 18 hours and 11 minutes and included five earthquakes. This swarm unfolded within the tectonically active offshore zone east of Honshu, where the Pacific Plate subducts beneath the Okhotsk Plate along the Japan Trench. Such settings produce frequent seismic episodes driven by plate convergence, fluid migration, and stress transfer along the subduction interface.

The earthquakes registered as follows. The initial event at 10:29:17 on 3 August reached magnitude 5.0 at a depth of 16 km. A larger shock of magnitude 5.8 occurred at 20:33:34 the same day at 27 km depth. Ten minutes later, at 20:43:10, a magnitude 5.2 event struck at 10 km depth. On 4 August, a magnitude 4.6 quake was recorded at 04:11:51 at 10 km depth, followed by a magnitude 5.4 event at 04:40:25 at 14 km depth. These events clustered at shallow to intermediate depths typical of the outer forearc region.

Seismic swarms in this area reflect episodic strain release rather than mainshock-aftershock sequences. The 2021 swarm aligns with patterns observed since 2000, during which 20 swarms have occurred. Prior episodes took place in 2008 (two swarms), 2011 (15 swarms), 2014 (one swarm), and 2016 (two swarms). Elevated swarm frequency often correlates with post-seismic relaxation following major events.

The strongest regional earthquake since 2000 was the magnitude 7.9 event on 11 March 2011, located 47 km east of Ibaraki and 83 km from the 2021 swarm center. That quake formed part of the broader 2011 Tohoku sequence and produced lasting changes in crustal stress that continue to influence local seismicity.

Ongoing monitoring of such swarms provides insight into subduction dynamics and helps refine hazard assessments for eastern Japan. Continued observation remains essential given the region's persistent tectonic loading.

References:
SeismoSight internal swarm classification records.
USGS earthquake catalog for regional tectonic context.
Japan Meteorological Agency historical seismicity summaries.