Seismic Swarm PS20120520.1: Earthquake Activity East of Yamada, Japan

Seismic swarm PS20120520.1 was recorded 144 km east of Yamada, Japan, in the Pacific Ocean. The sequence began at 19:05 on 19 May 2012 and concluded at 22:21 on 20 May 2012, spanning 27 hours and 15 minutes. During this period, nine earthquakes were detected, with magnitudes ranging from 4.4 to 6.3.

The swarm location lies along the Japan Trench, a convergent plate boundary where the Pacific Plate subducts westward beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic setting generates intense seismicity, including both isolated events and episodic swarms. The region forms part of the broader Ring of Fire and has produced several large earthquakes throughout recorded history, most notably the magnitude 9.1 Tohoku earthquake of 11 March 2011, whose epicenter was situated roughly 200 km south of the swarm area.

Within the swarm, the largest events reached magnitude 6.3 and occurred within one minute of each other on 20 May at 07:20. One of these events was located at a focal depth of 11 km, while the second occurred at 40 km. Additional events included a magnitude 5.9 shock at the onset on 19 May at 10 km depth, followed by magnitude 5.6, 5.3, 5.1, and 5.0 earthquakes clustered between 10 km and 28 km depth. The final event registered magnitude 5.1 at 28 km depth. Depths remained predominantly shallow to intermediate, consistent with activity near the plate interface and within the overriding plate.

Earthquake swarms in this subduction zone often reflect fluid migration or aseismic slip along the megathrust rather than a classic mainshock-aftershock sequence. The nine events occurred without a single dominant mainshock, and their temporal clustering over less than 30 hours aligns with documented swarm behavior in the Tohoku forearc. Since 1 January 2000, ten such swarms have been identified in the broader region, with nine occurring in 2011 and one in 2012.

Seismic monitoring in the area relies on networks operated by the Japan Meteorological Agency and international catalogs, which provide precise hypocenter locations and moment tensor solutions confirming predominantly thrust mechanisms along the subduction interface.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Database
Global CMT Project
Scientific literature on Japan Trench tectonics (e.g., reviews in Tectonophysics)