Seismic Swarm PS20110413.1: Analysis of Earthquake Activity East of Yamada, Japan

Seismic swarm PS20110413.1 was recorded east of Yamada in Iwate Prefecture, Japan, beginning at 19:57 on 13 April 2011 and concluding at 04:46 on 14 April 2011. Over 8 hours and 49 minutes, six earthquakes occurred at a location approximately 127 km east of Yamada. This sequence took place within the Japan Trench subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates of 8–9 cm per year. The tectonic setting produces frequent seismic activity, including aftershock sequences and episodic swarms driven by stress transfer and fluid migration along the plate interface.

The swarm events unfolded as follows. The initial shock reached magnitude 6.0 at a depth of 22 km. Within minutes, two magnitude-5.0 events occurred at depths of 14 km and 6 km. Subsequent shocks included a magnitude-4.7 event at 35 km depth, a magnitude-5.5 event at 18 km depth, and a final magnitude-5.0 event at 30 km depth. Depths ranged from shallow crustal levels to intermediate values typical of the subducting slab, reflecting activity both near the plate interface and within the overriding plate.

Such swarms represent clustered seismicity without a dominant mainshock-aftershock pattern. In subduction margins like the Japan Trench, they often arise from localized stress perturbations following major events, such as the 2011 Tohoku earthquake. Historical records since 1 January 2000 indicate eight swarms in the broader region, with one occurring in 2008 and seven in 2011. This clustering aligns with elevated post-seismic activity in the years immediately following the magnitude-9.0 Tohoku mainshock.

A notable strong earthquake in the vicinity was the magnitude-7.4 event located 100 km east-northeast of Miyako, Japan, on 20 April 2026, only 22 km from the swarm center. This proximity underscores the persistent seismic hazard along this segment of the trench, where megathrust ruptures and associated swarms remain characteristic features of regional tectonics.

Geological studies of the Japan Trench highlight its role in generating both great earthquakes and smaller clustered sequences. Bathymetric and seismic reflection data reveal complex fault structures that facilitate fluid flow and slow slip events, mechanisms commonly linked to swarm occurrence. The April 2011 swarm fits this pattern, occurring amid ongoing viscoelastic relaxation and afterslip following the 2011 mainshock.

In summary, swarm PS20110413.1 illustrates typical behavior in a highly active subduction environment. Continued monitoring of similar sequences contributes to refined understanding of stress evolution and seismic hazard assessment along the Japan Trench.

References
SeismoSight internal swarm classification database
USGS Earthquake Catalog (historical event parameters)
Japan Meteorological Agency seismic reports
Tectonic summaries from the Geological Survey of Japan