Seismic Swarm S20130417.2 Near Yonakuni, Japan: Characteristics and Regional Context

Seismic swarm S20130417.2 occurred 61 km NNE of Yonakuni in the Ryukyu Islands, Japan. The sequence began at 04:58 on 17 April 2013 and concluded at 08:37 on 18 April 2013, spanning 27 hours and 38 minutes. During this period, 41 earthquakes were recorded.

The events exhibited magnitudes between 4.0 and 5.3, with the majority clustered at depths of 9–12 km. Two events reached magnitude 5.3 at depths of 2 km and 9 km, respectively. Activity peaked in the morning hours of 17 April, followed by a gradual decline through the evening and into the next day. Depths remained predominantly shallow, consistent with crustal processes in the overriding plate.

This swarm represents the third such episode in the region since 2000. Prior events occurred in 2002 (one swarm) and 2004 (one swarm), indicating infrequent swarm-type activity relative to the broader tectonic setting.

The Yonakuni area lies within the Ryukyu arc, formed by subduction of the Philippine Sea Plate beneath the Eurasian Plate. Convergence rates average 5–7 cm per year, driving persistent seismicity along the plate interface and in the overlying crust. Shallow crustal earthquakes, such as those in this swarm, often arise from stress accumulation and release along local faults rather than direct megathrust slip. Historical records show the region experiences both isolated events and occasional clustered sequences, though swarm frequency remains low compared with other segments of the arc.

The 2013 swarm did not produce reported damage or tsunami, aligning with its moderate magnitudes and limited spatial extent. Such sequences provide insight into short-term stress redistribution without escalation to larger mainshock-aftershock patterns.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonics summary)
Japan Meteorological Agency seismic monitoring reports