Location:

Period:

14 Mar 2012 09:08:35 - 14 Mar 2012 11:40:17 (2 hours 31 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

12 swarms found nearby.

2003

PS20030925.1(132.7km)

25 Sep

1 day 0 hours

14 earthquakes

PS20030928.1(107.8km)

27 Sep

2 days 15 hours

13 earthquakes

2004

PS20041114.1(132.2km)

14 Nov

1 hours

5 earthquakes

2011

PS20110311.7(197.0km)

11 Mar

3 days 15 hours

38 earthquakes

PS20110315.1(145.7km)

14 Mar

1 day 10 hours

6 earthquakes

PS20110916.1(176.8km)

16 Sep

1 day 11 hours

10 earthquakes

2012

PS20120520.1(188.4km)

19 May

1 day 3 hours

9 earthquakes

2015

PS20150611.1(188.5km)

10 Jun

20 hours

5 earthquakes

2016

PS20160820.1(102.6km)

20 Aug

1 day 7 hours

6 earthquakes

2025

PS20251108.1(193.9km)

8 Nov

1 day 13 hours

25 earthquakes

PS20251208.1(190.6km)

8 Dec

18 hours

9 earthquakes

2026

PS20260420.1(169.2km)

20 Apr

21 hours

9 earthquakes

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Seismic Swarm PS20120314.1: Offshore Activity South of Kushiro

Seismic swarm PS20120314.1 was recorded 232 km south of Kushiro, Japan, beginning at 09:08 on 14 March 2012 and concluding at 11:40 the same day. Within the 2-hour 31-minute window, six earthquakes occurred in rapid succession, characteristic of swarm behavior where events cluster without a dominant mainshock.

The sequence opened with a magnitude 6.9 earthquake at 09:08:35, located at 12 km depth. Roughly 54 minutes later, at 10:02:11, a magnitude 5.1 event struck at only 2 km depth, followed immediately by a magnitude 4.0 shock at 10 km depth. Subsequent activity included a magnitude 6.1 event at 10:49:24 (10 km depth), a magnitude 5.6 shock at 10:57:40 (12 km depth), and a final magnitude 5.4 earthquake at 11:40:17 (10 km depth). All events remained at shallow crustal depths between 2 and 12 km, consistent with tectonic release along the plate interface.

This region lies within the subduction zone where the Pacific Plate descends beneath the Okhotsk Plate, part of the broader Pacific Ring of Fire. The offshore area south of Kushiro experiences frequent seismicity driven by plate convergence rates of approximately 8–9 cm per year. Shallow depths amplify ground-motion potential, though the swarm’s modest magnitudes limited widespread impact.

Since 1 January 2000, six swarms have been documented in the same locale. Earlier episodes occurred in 2003 (two swarms), 2004 (one swarm), and 2011 (three swarms). These recurrent clusters highlight persistent stress accumulation along the megathrust and associated crustal faults.

Swarm PS20120314.1 exemplifies how short-term seismic sequences can illuminate active deformation without escalating to larger ruptures. Continued monitoring of such patterns supports improved understanding of subduction dynamics in this tectonically active margin.

References

SeismoSight internal swarm classification records
Japan Meteorological Agency regional seismicity summaries
USGS Earthquake Hazards Program tectonic framework reports