Seismic Swarm S20161122.1 Near Namie, Japan: Geological Context and Event Analysis

The region 48 km east-southeast of Namie in Fukushima Prefecture, Japan, lies within the tectonically active forearc of the Tohoku subduction zone. Here the Pacific Plate subducts beneath the Okhotsk Plate at approximately 8–9 cm per year, producing frequent moderate earthquakes and occasional seismic swarms. Post-2011 Tohoku-oki megathrust rupture, the area has exhibited elevated rates of aftershock activity and swarm-like sequences driven by fluid migration along pre-existing faults and viscoelastic relaxation of the crust.

Swarm S20161122.1 began at 21:17 JST on 21 November 2016 and concluded at 21:59 JST on 23 November 2016, spanning 48 hours and 41 minutes. During this interval, 42 earthquakes were recorded, with magnitudes ranging from 4.0 to 4.9 and focal depths predominantly between 5 km and 34 km. The majority of events clustered at depths of 10 km, consistent with shallow crustal faulting above the subduction interface.

Temporal analysis shows the highest event density during the first 12 hours, when 11 earthquakes occurred, including the two largest events of magnitude 4.9. Subsequent activity declined gradually, with isolated events continuing into the third day. Depths remained shallow (≤10 km) for most shocks, though a subset on 21–22 November reached 21–34 km, possibly reflecting activation of deeper segments of the same fault system.

This sequence forms part of a longer-term pattern. Since 1 January 2000, eleven swarms have been documented in the same source region. Eight occurred in 2011 immediately following the Tohoku-oki mainshock, one each in 2008 and 2014, and the present swarm in 2016. The concentration in 2011 underscores the role of static and dynamic stress changes from the megathrust event in promoting swarm behavior.

Seismic swarms in this setting typically arise from pore-pressure diffusion rather than progressive mainshock-aftershock cascades. The 2016 swarm’s rapid onset, similar magnitudes, and lack of a single dominant event align with this mechanism. Depths near 10 km coincide with the brittle-ductile transition zone where fluids can most effectively reduce effective normal stress on faults.

Continued monitoring by the Japan Meteorological Agency and international networks remains essential. Although individual swarm events rarely exceed magnitude 5.0, their occurrence indicates ongoing stress redistribution that could influence the probability of future larger earthquakes along the Japan Trench.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Database
Geological Survey of Japan, AIST – Tohoku Tectonic Summary (2023 update)