Seismic swarm PS20110322.1 occurred approximately 91 km east of Iwaki, Japan, in a tectonically active zone where the Pacific Plate subducts beneath the Eurasian Plate along the Japan Trench. This region forms part of the Pacific Ring of Fire and experiences frequent seismicity due to ongoing plate convergence at rates of 7–9 cm per year. The swarm was recorded between 01:13 UTC on 22 March 2011 and 07:03 UTC on 24 March 2011, lasting 53 hours and 50 minutes and comprising 14 events.
The sequence began eleven days after the magnitude 9.0 Tohoku earthquake of 11 March 2011. A magnitude 7.9 event located 47 km east of Ibaraki (46 km from the swarm centroid) on that date formed part of the early aftershock series and likely contributed to elevated stress in the surrounding crust. The subsequent swarm reflected continued readjustment along pre-existing faults in the overriding plate and within the subducting slab.
Event parameters are listed chronologically below:
Depths ranged from 19 km to 75 km, indicating activity both in the upper crust and within the subducting slab. The largest event reached magnitude 6.1 at 31 km depth on 22 March.
Since 1 January 2000, fourteen swarms have been identified in the same source region. Earlier episodes occurred in 2005 (two swarms) and 2008 (two swarms). Ten swarms, including the present sequence, took place in 2011, consistent with heightened aftershock productivity following the Tohoku mainshock.
The geological setting features a well-developed accretionary prism and forearc basin overlying the subduction interface. Historical records show that similar clusters of moderate-magnitude events commonly follow great earthquakes in this segment of the Japan Trench, reflecting viscoelastic relaxation and fluid migration along faults.
References
SeismoSight internal swarm catalogue PS20110322.1
USGS Earthquake Catalog (Tohoku aftershock sequence)
Japan Meteorological Agency regional seismicity reports