Seismic Swarm PS20110312.1: Analysis of Post-Tohoku Activity Offshore Japan

The seismic swarm designated PS20110312.1 occurred 162 km east-southeast of Namie, Japan, in the Pacific Ocean. Registered by SeismoSight from 22:36 on 11 March 2011 to 01:42 on 13 March 2011, the sequence lasted 27 hours and 5 minutes and included eight earthquakes with magnitudes between 4.4 and 5.2.

This swarm unfolded within the Japan Trench subduction zone, where the Pacific Plate converges with and subducts beneath the Okhotsk Plate at approximately 8–9 cm per year. The tectonic setting produces frequent megathrust earthquakes and associated aftershock sequences. The March 2011 timing places the swarm in the immediate aftermath of the magnitude-9.0 Tohoku earthquake, whose rupture extended along the trench and triggered widespread aftershock activity across the outer-rise and forearc regions.

Earthquake depths in the swarm ranged from 7 km to 35 km, consistent with the seismogenic zone above the plate interface and within the overriding plate. Shallower events near 7–22 km likely reflect brittle failure in the upper crust, while deeper events at 32–35 km align with the transition toward the slab interface. Magnitudes clustered tightly around 5.0, indicating a swarm-like pattern of similar-sized events rather than a classic mainshock-aftershock decay.

Historical records maintained by SeismoSight show seven swarms in the region since 1 January 2000. Earlier episodes occurred in 2008 (two swarms) and 2011 (five swarms, of which PS20110312.1 is one). These recurrent swarms highlight persistent stress heterogeneity along the trench, possibly influenced by slab bending, fluid migration, or post-seismic viscoelastic relaxation following major events.

The 2011 Tohoku earthquake fundamentally altered regional stress fields, increasing the rate of moderate-magnitude seismicity for months afterward. Swarms such as PS20110312.1 exemplify how aftershock productivity can manifest in episodic clusters rather than steady decay. Depths and magnitudes recorded during the swarm remain within the expected range for this segment of the subduction zone, reinforcing its classification as part of the broader post-Tohoku aftershock sequence.

Continued monitoring of the Japan Trench remains essential given the potential for both intraslab and interface events. The geological framework of rapid plate convergence and complex fault architecture ensures that seismic swarms will remain a characteristic feature of the region’s seismicity.

References

USGS Earthquake Hazards Program – Tectonic summary of the Japan Trench
Japan Meteorological Agency – Seismicity catalog for the Tohoku region
Geological Survey of Japan – Subduction zone characteristics of northeast Japan