Seismic Swarm PS20110311.8: Activity in the Tohoku Subduction Zone

On 11 March 2011, a seismic swarm designated PS20110311.8 was recorded 73 km east-southeast of Ofunato, Japan. The sequence began at 11:28 and concluded at 20:42, encompassing five earthquakes within a span of nine hours and thirteen minutes. These events occurred in the immediate vicinity of the M9.1 Great Tohoku Earthquake, whose epicenter lay only 25 km from the swarm centroid. The individual events displayed a range of magnitudes and focal depths. The first shock reached magnitude 5.2 at 27 km depth. Approximately ninety minutes later, a magnitude 5.4 event occurred at 50 km depth. A magnitude 5.0 earthquake followed at 59 km depth, succeeded by a shallower magnitude 4.4 shock at 20 km depth. The sequence ended with a magnitude 5.0 event at 10 km depth. Depths varied from 10 km to 59 km, indicating activity across both crustal and upper-mantle levels within the subduction interface. This swarm forms part of a broader pattern of clustered seismicity in the Tohoku region. Since 1 January 2000, six swarms have been identified, with one recorded in 2008 and five in 2011. The elevated count in 2011 coincides with the great megathrust rupture, suggesting that swarm activity may intensify during periods of major stress release along the plate boundary. The geological setting is defined by the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at a convergence rate of approximately 8–9 cm per year. This tectonic regime produces frequent megathrust earthquakes, intraslab events, and upper-plate deformation. Historical records since 2000 document multiple strong earthquakes within 100 km of the swarm center, including the M7.3 foreshock of 9 March 2011, the M7.1 aftershock of 7 April 2011, and later events in 2021. Earlier notable shocks comprise the M7.2 event of August 2005 and the M7.0 shocks of 2003. These occurrences illustrate the persistent seismic hazard associated with the subduction zone. The March 2011 swarm, occurring on the same day as the M9.1 mainshock, likely reflects dynamic stress changes and afterslip processes following the primary rupture. Such swarms provide valuable data for understanding how strain is redistributed along the megathrust and within the overriding plate. References SeismoSight internal swarm classification PS20110311.8 USGS Earthquake Catalog Japan Meteorological Agency seismic records