Seismic Swarm PS20110314.1: Analysis of Post-Tohoku Activity Offshore Iwaki, Japan

The seismic swarm designated PS20110314.1 occurred in the offshore region 104 km east-southeast of Iwaki, Japan, between 19:59 on 13 March 2011 and 09:07 on 14 March 2011. Over 13 hours and 7 minutes, the sequence produced 14 earthquakes with magnitudes ranging from 4.7 to 5.5. All events were recorded at depths between 2 km and 62 km, consistent with activity within the subducting Pacific Plate and overlying crust in the Japan Trench subduction zone.

The sequence began with a magnitude 5.2 event at 25 km depth. Subsequent shocks included two magnitude 5.2 events within minutes of each other on 14 March at depths of 31 km and 35 km, and a magnitude 5.5 event at 11 km depth that represented the largest in the swarm. Depths showed considerable variation, with shallower events near 2–16 km interspersed among deeper ones exceeding 30 km, indicating distributed failure across multiple structural levels.

This swarm unfolded two days after the magnitude 9.0 Tohoku earthquake of 11 March 2011, whose epicenter lay approximately 66 km from the swarm centroid. The timing aligns with elevated aftershock rates and triggered seismicity expected in the outer-rise and forearc regions following a great subduction-zone rupture. A later magnitude 7.3 event on 16 March 2022, located 98 km from the swarm center, further illustrates the persistent seismic productivity of the same tectonic corridor.

Since 2000, ten swarms have been documented in the immediate area, with eight occurring in 2011 alone and two in 2008. This clustering underscores the region’s tendency toward episodic swarm behavior superimposed on the background aftershock decay following the 2011 mainshock. The Japan Trench setting, where the Pacific Plate converges with the Okhotsk Plate at roughly 8–9 cm per year, generates frequent moderate earthquakes through both interplate thrusting and intraslab normal faulting.

The 2011 swarm events released modest energy compared with the mainshock yet contributed to the prolonged redistribution of stress along the plate interface and within the incoming oceanic lithosphere. Depths extending to 62 km are typical for intraslab activity in this margin, where the Pacific slab dips beneath Honshu and experiences internal deformation.

References
SeismoSight internal swarm catalog PS20110314.1
USGS Earthquake Catalog (events since 2000)
Japan Meteorological Agency seismic database