Seismic Swarm PS20110311.3: Analysis of Post-Tohoku Activity off Yamada, Japan

Seismic swarm PS20110311.3 occurred in the offshore region 94 km east-southeast of Yamada, Japan, within the tectonically active Japan Trench subduction zone. This area marks the boundary where the Pacific Plate converges with and subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. The subduction process generates intense compressional stresses, producing frequent earthquakes at varying depths along the plate interface and within the overriding plate.

The swarm initiated at 06:06 on 11 March 2011 and concluded at 23:24 on 12 March 2011, spanning 41 hours and 18 minutes. During this interval, 44 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.7 and focal depths between 2 km and 100 km. Events clustered in the hours immediately following the great 11 March 2011 Tohoku mainshock, reflecting aftershock processes and stress redistribution along the megathrust fault. Notable events included a magnitude 6.7 earthquake at 2 km depth early in the sequence and a magnitude 6.6 event at 25 km depth later on 11 March. Later activity on 12 March featured a magnitude 6.1 shock at 38 km depth and another magnitude 6.1 at 15 km depth near the swarm’s close.

The Tohoku region has a well-documented history of large earthquakes driven by subduction. Since 2000, five seismic swarms have been identified in the vicinity, with prior episodes occurring in 2008 (three swarms) and additional activity in 2011 (two swarms including the present event). Strong earthquakes since 2000 have repeatedly affected areas near the swarm center, underscoring persistent seismic hazard. The 11 March 2011 Great Tohoku Earthquake (magnitude 9.1) occurred 57 km from the swarm center and remains the dominant event in the modern record. Other significant shocks include the magnitude 7.3 event of 16 March 2022 located 38 km from the center, the magnitude 7.1 of 13 February 2021 at 25 km distance, and the magnitude 7.1 of 7 April 2011 at 49 km distance. Earlier events encompass the magnitude 7.3 of 9 March 2011 (98 km distant), the magnitude 7.0 of 19 July 2008 (47 km), and the magnitude 7.2 of 16 August 2005 (40 km).

These patterns illustrate the characteristic behavior of the Japan Trench, where megathrust ruptures periodically release accumulated strain while triggering extensive aftershock sequences and secondary swarms. Depths recorded during swarm PS20110311.3 span both shallow crustal levels and intermediate depths consistent with the subducting slab, consistent with the geometry of plate convergence in this sector.

References

USGS Earthquake Catalog
Japan Meteorological Agency seismic records
Global CMT Project focal mechanism data