Location:

Period:

7 Dec 2012 08:54:02 - 9 Dec 2012 05:24:39 (1 day 20 hours 30 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

7 swarms found nearby.

2011

PS20110311.6(58.5km)

11 Mar

2 days 8 hours

50 earthquakes

PS20110312.1(104.1km)

11 Mar

1 day 3 hours

8 earthquakes

PS20110315.2(41.7km)

15 Mar

1 day 17 hours

6 earthquakes

PS20110322.2(110.7km)

21 Mar

18 hours

5 earthquakes

PS20110325.1(14.0km)

24 Mar

20 hours

6 earthquakes

2012

PS20121207.1(1.8km)

7 Dec

14 hours

10 earthquakes

2013

PS20131025.1(111.4km)

25 Oct

4 hours

6 earthquakes

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Seismic Swarm S20121207.1: Earthquake Activity Offshore Ishinomaki

Seismic swarm S20121207.1 occurred in the Pacific Ocean approximately 228 km east-southeast of Ishinomaki, Japan. The sequence began at 08:54 on 7 December 2012 and concluded at 05:24 on 9 December 2012, spanning 44 hours and 30 minutes. During this period, 39 earthquakes were recorded, with magnitudes ranging from 4.1 to 5.1 and focal depths primarily between 10 km and 35 km.

The swarm exhibited a clustered temporal pattern, with the majority of events concentrated in the first 12 hours. Initial activity included multiple events of magnitude 4.7 at depths around 35 km, followed by a gradual decline in frequency. Later phases featured slightly shallower events, including the largest shock of magnitude 5.1 at 19 km depth on 9 December. Depths showed modest variation, with several events occurring at 10–19 km, potentially indicating activity along different segments of the fault system.

This region lies along the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at a convergence rate of approximately 8–9 cm per year. The subduction interface generates frequent seismicity, including large megathrust earthquakes and associated aftershock sequences. The Tohoku area experienced the magnitude 9.0 Great East Japan Earthquake on 11 March 2011, whose rupture zone extended across much of the same offshore segment. Post-2011 stress redistribution has influenced subsequent seismic patterns, contributing to episodic swarm activity.

Historical records since 2000 indicate six documented swarms in the vicinity, with five occurring in 2011 and one additional swarm in 2012. These episodes reflect the dynamic stress environment typical of subduction zones, where fluid migration, aseismic slip, or localized stress perturbations can trigger clustered events without a single dominant mainshock.

Analysis of the magnitude distribution shows a relatively high proportion of events between 4.4 and 4.9, consistent with moderate-energy release distributed across multiple small ruptures. The absence of events exceeding magnitude 5.1 suggests limited rupture dimensions during this swarm. Depth clustering near 35 km aligns with the expected position of the plate interface in this segment of the trench.

Such swarms provide valuable data for understanding subduction-zone mechanics. Continued monitoring by regional networks helps refine models of interplate coupling and afterslip processes that persist years after major events like the 2011 Tohoku earthquake.

References

Japan Meteorological Agency seismic catalogs
United States Geological Survey earthquake database
SeismoSight internal swarm classification records