Location:

Period:

20 Apr 2026 07:53:00 - 21 Apr 2026 05:29:09 (21 hours 36 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.4; 100 km ENE of Miyako, Japan; (20 Apr 2026) (45km from the swarm center)

15 swarms found nearby.

2011

PS20110311.6(181.5km)

11 Mar

2 days 8 hours

50 earthquakes

PS20110311.1(144.0km)

11 Mar

1 day 14 hours

28 earthquakes

PS20110311.7(33.7km)

11 Mar

3 days 15 hours

38 earthquakes

PS20110311.8(170.0km)

11 Mar

9 hours

5 earthquakes

PS20110315.1(38.2km)

14 Mar

1 day 10 hours

6 earthquakes

PS20110319.2(127.6km)

18 Mar

1 day 17 hours

8 earthquakes

PS20110323.1(58.3km)

22 Mar

23 hours

6 earthquakes

PS20110413.1(46.7km)

13 Apr

8 hours

6 earthquakes

PS20110916.1(52.8km)

16 Sep

1 day 11 hours

10 earthquakes

2012

PS20120314.1(169.2km)

14 Mar

2 hours

6 earthquakes

PS20120520.1(20.0km)

19 May

1 day 3 hours

9 earthquakes

2015

PS20150611.1(19.6km)

10 Jun

20 hours

5 earthquakes

2016

PS20160820.1(86.8km)

20 Aug

1 day 7 hours

6 earthquakes

2025

PS20251108.1(24.9km)

8 Nov

1 day 13 hours

25 earthquakes

PS20251208.1(172.6km)

8 Dec

18 hours

9 earthquakes

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

Seismic Swarm PS20260420.1: Analysis of Activity Off the East Coast of Honshu, Japan

A seismic swarm designated PS20260420.1 was recorded off the east coast of Honshu, Japan, beginning at 07:53 on 20 April 2026 and concluding at 05:29 on 21 April 2026. Over 21 hours and 36 minutes, nine earthquakes were detected in the region, with magnitudes ranging from 4.4 to 7.4 and focal depths between 10 and 47 km.

The sequence initiated with a magnitude 7.4 event at 35 km depth, followed by eight additional shocks. Notable events included a magnitude 5.6 quake at 24 km depth and several magnitude 5+ aftershocks clustered at shallower depths around 10 km. This pattern reflects typical swarm dynamics in subduction-related settings, where stress redistribution along the plate interface triggers clustered seismicity without a single dominant mainshock-aftershock sequence.

The location lies within the Japan Trench subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic interaction generates frequent earthquakes and has shaped the region’s geological history, including the devastating 2011 Tohoku earthquake and associated tsunami. Historical records since 2000 indicate 15 prior swarms in the vicinity, with notable clusters in 2011 (9 events) and smaller occurrences in 2012, 2015, 2016, and 2025. These episodes underscore the persistent seismic hazard along Honshu’s eastern margin.

The swarm’s leading magnitude 7.4 shock occurred roughly 45 km from the reported swarm center and 100 km east-northeast of Miyako, consistent with activity along the megathrust or adjacent crustal faults. Depths in the 10–47 km range align with both interplate and intraslab seismicity characteristic of the area.

Such swarms provide valuable data for understanding stress transfer and precursory signals in subduction environments. Continued monitoring by regional networks remains essential for assessing evolving hazard levels in this tectonically active zone.

References

SeismoSight internal swarm classification PS20260420.1
USGS Earthquake Catalog (regional tectonics and historical context)
Japan Meteorological Agency seismic reports (subduction zone parameters)