Seismic Swarm PS20220316.1: Analysis of Activity Near Namie, Japan

A seismic swarm designated PS20220316.1 was recorded 62 km east-northeast of Namie, Japan, on 16 March 2022. The sequence began at 14:34 JST and concluded at 15:52 JST, encompassing seven earthquakes within 1 hour and 18 minutes. The largest event reached magnitude 7.3 at a depth of 41 km, followed by events of magnitudes 6.0, 5.5, 5.3, 5.2, and 5.0 at depths ranging from 41 km to 66 km. This swarm occurred within the tectonically active zone of the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year.

The Tohoku region exhibits persistent seismicity driven by this subduction dynamics. Historical records since 2000 document 25 swarms in the vicinity, with notable clusters in 2011 (16 events) and smaller occurrences in 2008, 2012, 2014, 2016, and 2021. These swarms often reflect stress adjustments along the plate interface and within the overriding continental crust. Depths consistently between 40 km and 70 km align with the megathrust environment and associated intraslab faulting.

Strong earthquakes since 2000 further illustrate the area's hazard profile. The 11 March 2011 Great Tohoku Earthquake (magnitude 9.1) struck 70 km from the swarm center, triggering widespread aftershock sequences that continue to influence regional stress fields. Subsequent large events include the magnitude 7.3 on 16 March 2022 itself, the magnitude 7.1 of 13 February 2021 located only 11 km away, and earlier shocks such as the magnitude 7.0 of 20 March 2021 and magnitude 7.1 of 7 April 2011. These events cluster along the same rupture segments, demonstrating how postseismic viscoelastic relaxation and afterslip sustain elevated seismicity for years to decades.

Swarm characteristics in this setting typically arise from fluid migration along faults or aseismic slip transients rather than a single dominant mainshock-aftershock pattern. The 2022 sequence's rapid onset and decay, combined with its proximity to prior rupture zones, suggests localized stress triggering within the 2011 aftershock volume. Depths and magnitudes indicate a mix of interface and intraslab sources, consistent with the complex fault architecture of the subduction margin.

Ongoing monitoring by networks such as those operated by the Japan Meteorological Agency remains essential for distinguishing swarm behavior from foreshock sequences that could precede larger events. The geological framework of the Japan Trench ensures that such activity will persist, underscoring the importance of preparedness in coastal Fukushima and Miyagi prefectures.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Data
Geological Survey of Japan, AIST – Subduction Zone Reports