Seismic Swarm PS20161121.1 Near Namie, Japan: Geological Context and Event Analysis

The seismic swarm designated PS20161121.1 occurred approximately 40 km east-southeast of Namie in Fukushima Prefecture, Japan. It began at 20:59 on 21 November 2016 and concluded at 17:41 on 22 November 2016, spanning 20 hours and 41 minutes. During this interval, 11 earthquakes were recorded, with the sequence initiated by a magnitude 6.9 event at a depth of 9 km.

Subsequent events included multiple magnitude 5.0–5.4 shocks clustered at depths of 10 km in the initial hours, followed by deeper activity reaching 24 km and 31 km. The full sequence comprised one magnitude 6.9, eight magnitude 5.0–5.4 events, and two magnitude 4.6–4.6 events, reflecting typical swarm behavior with rapid aftershock decay rather than a single mainshock-aftershock pattern.

Namie lies within the tectonically active northeastern Honshu arc, where the Pacific Plate subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This convergent margin produces frequent intermediate-depth and shallow crustal seismicity. The 2011 Tohoku-Oki earthquake (Mw 9.0) induced widespread stress perturbations across the region, elevating swarm frequency in the years that followed. Post-2011 afterslip and viscoelastic relaxation continue to influence local fault systems, contributing to episodic swarm activity.

Historical records since 2000 indicate 20 swarms in the broader area, with notable concentrations in 2011 (17 swarms) and isolated occurrences in 2008 (2 swarms) and 2014 (1 swarm). These clusters align with periods of heightened crustal stress following major regional events. The November 2016 swarm fits this pattern, occurring five years after the Tohoku mainshock.

Stronger earthquakes have also been documented since 2000 within 100 km of the swarm centroid. These include a magnitude 7.3 event on 16 March 2022 located 57 km east-northeast of Namie, a magnitude 7.1 event on 13 February 2021 at 73 km east-northeast, and a magnitude 7.0 event on 19 July 2008 at 107 km east. Such events underscore the persistent seismic hazard along the Japan Trench margin.

Swarm sequences like PS20161121.1 provide insight into fluid migration and aseismic slip processes that can trigger clustered seismicity without culminating in larger ruptures. Depths ranging from 9 km to 31 km suggest activation across multiple crustal levels within the overriding plate.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Database
SeismoSight Internal Swarm Classification Records