Seismic Swarm PS20110311.6: Post-Tohoku Aftershock Cluster off Eastern Japan

Seismic swarm PS20110311.6 occurred 204 km east of Iwaki, Japan, within the Japan Trench subduction zone. The sequence began at 05:55 UTC on 11 March 2011 and concluded at 14:41 UTC on 13 March 2011, encompassing 50 events over 56 hours and 45 minutes. This activity took place in the immediate aftermath of the Mw 9.1 Great Tohoku Earthquake, whose epicenter lay approximately 79 km from the swarm centroid.

The swarm featured repeated moderate-to-strong aftershocks. The largest event reached Mw 6.5 at 01:47 UTC on 12 March at 20 km depth. Other notable shocks included Mw 6.4 at the swarm onset (35 km depth) and multiple Mw 5.9 events distributed across the two-day period. Focal depths ranged from 2 km to 69 km, with the majority clustered between 10 km and 35 km. This depth distribution aligns with the geometry of the plate interface and overlying crustal faults activated by static and dynamic stress changes following the mainshock.

Geologically, the swarm region lies where the Pacific Plate subducts westward beneath the Okhotsk Plate at rates of 8–9 cm per year. The March 2011 mainshock ruptured a roughly 500 km segment of the megathrust, producing widespread afterslip and viscoelastic relaxation that triggered secondary sequences such as PS20110311.6. Historical records since 2000 document only one prior swarm in the immediate vicinity (2008), underscoring the exceptional nature of the 2011 cluster. Strong regional events recorded in the same interval include the Mw 7.3 foreshock of 9 March 2011 (51 km from the swarm center) and subsequent aftershocks reaching Mw 7.0 in July and December 2011.

Analysis of event timing reveals two peaks of activity: an initial burst on 11 March followed by a more intense phase on 12 March that included the Mw 6.5 mainshock of the swarm. Depths shallowed notably during the later hours of 11 March (down to 6–10 km), suggesting involvement of shallower crustal structures before activity returned to the deeper interface. Such spatiotemporal patterns are consistent with fluid migration and stress redistribution along the subduction interface in the days after a great megathrust rupture.

The swarm underscores the prolonged aftershock hazard in the Tohoku region. Although no events exceeded Mw 6.5, the cumulative energy release and proximity to the mainshock rupture zone illustrate how post-seismic processes can sustain elevated seismicity for days to weeks. Continued monitoring of similar clusters remains essential for understanding stress evolution along the Japan Trench.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Japan Meteorological Agency aftershock database
• Tectonic framework from the Geological Survey of Japan