Seismic Swarm Activity off Eastern Japan: March 2011 Analysis

Seismic swarms represent clusters of earthquakes occurring in a localized area over a short period, often without a single dominant mainshock. The swarm designated PS20110325.1 unfolded 283 km southeast of Ōfunato, Japan, from 15:58 on 24 March 2011 to 12:43 on 25 March 2011. Within 20 hours and 44 minutes, six events were recorded, providing insight into post-mainshock stress adjustments along the Japan Trench subduction zone.

The sequence began with a magnitude 5.0 event at 15:58:17 on 24 March at 10 km depth. Roughly three hours later, at 19:17:18, a magnitude 5.4 shock occurred at 24 km depth, marking the largest event in the swarm. A magnitude 5.0 followed at 19:22:02 at 20 km depth. On 25 March, activity resumed with a magnitude 4.1 at 09:10:26 (10 km), a magnitude 5.3 at 09:43:45 (10 km), and concluded with a magnitude 5.1 at 12:43:11 (25 km). Depths remained shallow to intermediate, consistent with crustal and upper-plate responses in the overriding plate.

This swarm occurred in the tectonically active Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. The region experienced intense seismic energy release following the magnitude 9.0 Tohoku earthquake of 11 March 2011. Historical records since 2000 indicate eight prior swarms in the vicinity, with one in 2008 and seven in 2011, underscoring elevated aftershock productivity in the months after major events.

Notable strong earthquakes within 100 km of the swarm center include the magnitude 7.7 event of 11 March 2011 (71 km distant), the magnitude 7.0 of 10 July 2011 (66 km), the magnitude 7.3 of 7 December 2012 (20 km), the magnitude 7.1 of 25 October 2013 (90 km), and the magnitude 7.0 of 14 November 2005 (97 km). These events highlight the persistent seismic hazard along the trench, where megathrust ruptures and associated aftershock sequences dominate the geological record.

The 2011 swarm likely reflects triggered slip on subsidiary faults due to static and dynamic stress changes from the Tohoku mainshock. Such sequences aid in mapping fault networks and assessing evolving seismic risk in subduction settings.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog (historical events)
Japan Meteorological Agency seismic reports