Seismic Activity: Earthquake Swarm PS20160820.1 Near Miyako, Japan

An earthquake swarm designated PS20160820.1 occurred approximately 168 km east-northeast of Miyako, Japan, in the Pacific Ocean. The sequence began at 05:14 UTC on 20 August 2016 and concluded at 12:49 UTC on 21 August 2016, spanning 31 hours and 34 minutes. During this period, six earthquakes with magnitudes of 5.2 or greater were recorded. The events originated at depths between 10 km and 23 km, characteristic of shallow crustal activity within the subduction zone.

The individual events unfolded as follows. The initial shock measured magnitude 5.3 at a depth of 23 km. Roughly four hours later, the largest event reached magnitude 6.0 at 10 km depth. Subsequent shocks included a magnitude 5.8 at 11 km depth, followed by two magnitude 5.2 events at 10 km depth each. The sequence ended with a final magnitude 5.2 event at 10 km depth. All but the first event clustered at similar shallow depths, suggesting a localized stress release along a common fault segment.

This swarm took place in a tectonically active region along the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at a rate of approximately 8–9 cm per year. The area experiences frequent seismic activity due to plate convergence, which generates both interplate and intraplate earthquakes. Historical records indicate that 12 swarms have occurred in the same vicinity since 2000, with notable clusters in 2003 (two swarms), 2004 (one swarm), 2011 (six swarms), 2012 (two swarms), and 2015 (one swarm). These recurrent swarms reflect episodic stress accumulation and release along the subduction interface and overlying crust.

Geological studies of the Japan Trench highlight its potential for both moderate swarms and great earthquakes. The 2011 Tohoku-Oki event, centered nearby, demonstrated how accumulated strain in this setting can produce megathrust ruptures. The 2016 swarm, while modest in scale, aligns with patterns of foreshock-like activity sometimes observed prior to larger events, though no immediate large mainshock followed.

No earthquakes exceeding magnitude 7.0 have been reliably associated with this specific swarm location in verified records since 2000 apart from the well-documented regional history of the trench. Ongoing monitoring by seismic networks continues to track activity in this high-hazard zone to improve understanding of subduction dynamics.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Japan Meteorological Agency seismic reports
SeismoSight internal swarm classification data