Seismic Swarm PS20110309.1: Foreshock Activity Preceding the 2011 Tohoku Earthquake

The seismic swarm designated PS20110309.1 was recorded in the offshore region of eastern Japan, centered 146 km east-southeast of Ofunato. Activity commenced at 02:45 on 9 March 2011 and concluded at 14:46 on 16 March 2011, encompassing 159 earthquakes over approximately 180 hours. This sequence unfolded within the seismically active forearc of the Japan Trench subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates exceeding 8 cm per year. The tectonic setting produces frequent megathrust events and associated foreshock-aftershock sequences along the plate interface and within the overriding plate. The swarm initiated with a magnitude 7.3 event at 32 km depth, followed by a rapid succession of aftershocks ranging from magnitude 4.7 to 6.5. Depths varied between 0 and 55 km, reflecting both shallow crustal faulting and deeper intraslab activity. Notable early events included multiple magnitude 5.0–6.0 shocks on 9 March, with peaks at magnitude 6.0 (22 km depth) and 6.5 (15 km depth) later that evening. Activity intensified further on 11 March, culminating in the magnitude 9.1 mainshock at 05:46:24, located at 29 km depth. Subsequent events in the immediate hours reached magnitudes up to 6.3, with depths predominantly between 15 and 52 km. Analysis of the first 100 events reveals a classic foreshock pattern: an initial large trigger followed by clustered moderate-magnitude releases that migrated slightly in space and time. Magnitudes clustered between 5.0 and 6.2 after the opening M7.3, with occasional deeper events exceeding 40 km. This progression aligns with stress transfer along the subduction interface, a common precursor mechanism in the Tohoku segment. Since 2000, only two such swarms have occurred in the immediate vicinity, the earlier episode taking place in 2008. The broader region has hosted multiple strong earthquakes during the same period, including the 2011 Great Tohoku mainshock itself (68 km from swarm center), the M7.1 event of 7 April 2011 (19 km distant), and earlier M7+ shocks in 2003, 2005, 2008, 2021, and 2022. These events underscore the persistent seismic hazard along this portion of the Japan Trench. Geologically, the swarm area lies within a zone of heterogeneous coupling on the megathrust, where patches of high friction alternate with conditionally stable regions that can generate foreshock sequences. Historical records and modern instrumentation confirm that similar precursory swarms have preceded several large Tohoku earthquakes, highlighting the value of real-time monitoring for understanding rupture nucleation. References
SeismoSight internal swarm catalog PS20110309.1
USGS Earthquake Catalog (events 2000–2022)
Japan Meteorological Agency seismic bulletins