Seismic Swarm PS20110315.2: Analysis of Activity Offshore Eastern Japan

Seismic swarm PS20110315.2 was recorded 242 km east of Namie, Japan, from 03:21 on 15 March 2011 to 20:44 on 16 March 2011. In 41 hours and 23 minutes, the sequence produced six earthquakes with magnitudes between 4.7 and 5.2. The events occurred at depths ranging from 10 km to 35 km, consistent with crustal and upper-plate seismicity in the region.

The sequence began with a magnitude 5.0 event at 26 km depth on 15 March at 03:21:35 UTC. Subsequent shocks included a magnitude 5.1 at 34 km depth later that morning, followed by two magnitude 5.0 events in the evening. A magnitude 4.7 and a magnitude 5.2 at only 10 km depth rounded out activity on 15 March. The final event, magnitude 5.1 at 10 km depth, occurred on 16 March at 20:44:41 UTC. These closely spaced events illustrate typical swarm behavior, in which energy release occurs through multiple moderate shocks rather than a single dominant mainshock.

The swarm location lies seaward of the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at a rate of approximately 8–9 cm per year. This tectonic setting produces frequent seismicity, including both interplate thrust events and intraplate normal-faulting earthquakes within the bending Pacific slab. The March 2011 swarm took place four days after the magnitude 9.0 Tohoku earthquake of 11 March 2011, suggesting the activity formed part of the extensive aftershock sequence that redistributed stress across the megathrust and surrounding crust.

Since 1 January 2000, six swarms have been identified in the same offshore sector. One swarm occurred in 2008 and five took place in 2011, underscoring elevated seismic clustering following the Tohoku mainshock. Nearby strong earthquakes recorded since 2000 include a magnitude 7.1 event on 25 October 2013 located 82 km from the swarm center, a magnitude 7.3 shock on 7 December 2012 situated 58 km away, and a magnitude 7.0 event on 10 July 2011 positioned 84 km distant. These larger earthquakes demonstrate the region’s capacity for generating damaging events along both the plate interface and within the subducting slab.

Seismic swarms in subduction zones often reflect fluid migration, afterslip, or stress triggering on secondary faults. Depths between 10 km and 35 km place the PS20110315.2 events within the seismogenic zone where brittle failure predominates. Continued monitoring of such sequences contributes to refined models of post-seismic deformation and long-term hazard assessment along the Japan Trench.

References
United States Geological Survey Earthquake Catalog
Japan Meteorological Agency Seismic Database
SeismoSight internal swarm classification records