Earthquake Swarm S20200615.2 Near Yonakuni, Japan

An earthquake swarm designated S20200615.2 occurred approximately 51 km north-northeast of Yonakuni in the Ryukyu Islands, Japan. The sequence began at 17:28 on 14 June 2020 and concluded at 13:35 on 16 June 2020, spanning 44 hours and 7 minutes. During this period, 30 earthquakes were recorded, with magnitudes ranging from 4.1 to 5.2 and focal depths predominantly at 10 km.

The swarm exhibited a clustered temporal pattern, with the majority of events occurring within the first 24 hours. Notable shocks included a magnitude 5.1 event at 19:26 on 14 June and a magnitude 5.2 event at 13:47 on 15 June, both at 10 km depth. Such sequences are characteristic of swarm activity, where numerous events of similar magnitude occur without a single dominant mainshock, often linked to fluid migration or stress redistribution along fault networks.

Yonakuni lies within the Ryukyu arc, a tectonically active region formed by the northwestward subduction of the Philippine Sea Plate beneath the Eurasian Plate at rates of approximately 6–8 cm per year. This convergent margin produces frequent seismicity, including both interplate thrust events and shallower crustal earthquakes. The arc's geology features volcanic islands, accretionary complexes, and active faults that accommodate oblique convergence and back-arc extension in the Okinawa Trough to the north.

Historical records indicate that swarm activity in this sector has been infrequent since 2000, with only three prior episodes documented: one in 2002, one in 2004, and one in 2013. These events underscore the episodic nature of seismic clustering in the region, potentially influenced by regional stress fields or hydrothermal processes associated with nearby volcanic systems.

The 2020 swarm did not produce reported damage or tsunami, consistent with its moderate magnitudes and offshore location. Monitoring by regional seismic networks continues to track aftershock decay and any potential for renewed activity, contributing to improved understanding of subduction-zone dynamics in the western Pacific.

References

SeismoSight internal swarm classification S20200615.2 dataset.
USGS Earthquake Catalog (historical regional seismicity).
Global CMT Project (tectonic setting of Ryukyu arc).