Seismic Swarm PS20110317.1: Post-Tohoku Activity Near Iwaki

Seismic swarm PS20110317.1 was recorded 79 km east-southeast of Iwaki, Japan, beginning at 15:27 on 16 March 2011 and concluding at 08:01 on 18 March 2011. Over 40 hours and 34 minutes, the sequence produced ten earthquakes with magnitudes ranging from 5.0 to 5.8. The events occurred at focal depths between 12 km and 52 km, consistent with crustal and upper-mantle seismicity in the subduction zone offshore eastern Honshu.

This swarm unfolded five days after the magnitude 9.0 Tohoku earthquake of 11 March 2011. A magnitude 7.9 aftershock located 47 km east of Ibaraki on the same date lay only 38 km from the swarm centroid, indicating that the March 2011 sequence formed part of the extensive aftershock series triggered by the mainshock. The regional tectonic setting involves rapid subduction of the Pacific Plate beneath the Okhotsk Plate at approximately 8–9 cm per year, generating frequent megathrust and intraslab earthquakes along the Japan Trench.

The ten events of swarm PS20110317.1 are listed chronologically below in narrative form. The initial magnitude 5.0 shock occurred at 36 km depth on 16 March at 15:27. Roughly three hours later a magnitude 5.1 event was recorded at 21 km depth. On 17 March, four additional shocks took place between 03:50 and 18:55, including two magnitude 5.2 events at 12 km and 41 km and a magnitude 5.8 event at 29 km depth that represented the largest in the swarm. Two further magnitude 5.1 events followed at 26 km depth within ten minutes of each other on the evening of 17 March. The final two shocks, both magnitude 5.1 and 5.3, occurred at depths of 22 km and 24 km early on 18 March.

Since 1 January 2000, nine seismic swarms have been identified in the same offshore sector. One occurred in 2005 and two in 2008; the remaining six, including PS20110317.1, took place in 2011. This temporal clustering reflects elevated crustal stress following the Tohoku mainshock and its immediate aftershocks. Depths recorded during the swarm span the typical range for both interplate and intraslab events in this segment of the subduction zone, where the plate interface lies between roughly 10 km and 60 km depth.

The Iwaki region lies within the Fukushima tectonic domain, characterized by active normal and strike-slip faults that accommodate extension behind the volcanic arc. Historical records show that similar moderate-magnitude sequences have repeatedly followed great subduction earthquakes, serving as indicators of stress redistribution along the plate boundary and in the overlying crust. Continued monitoring of such swarms provides valuable data for refining aftershock forecasts and understanding post-seismic relaxation processes in this high-hazard margin.

References

USGS Earthquake Catalog (event data 2000–2023)
Japan Meteorological Agency Seismological Bulletin
Geological Survey of Japan, Tectonic Map of Eastern Honshu