Seismic Swarm PS20230501.1: Analysis of Earthquake Activity Southeast of Katsuren-haebaru, Japan

On May 1, 2023, seismic monitoring systems recorded swarm PS20230501.1 approximately 79 km southeast of Katsuren-haebaru, Japan. The sequence began at 02:21 and concluded at 07:59, encompassing six earthquakes over a period of five hours and thirty-eight minutes. This event represents the second swarm documented in the area since 2000, following a prior occurrence in 2018. The earthquakes exhibited magnitudes ranging from 4.5 to 5.9, with focal depths between 9 km and 20 km. The sequence initiated with a magnitude 5.1 event at 02:21:02 (16 km depth), followed closely by a magnitude 5.3 quake at 02:22:29 (20 km depth). A peak magnitude 5.9 event occurred at 03:22:10 (9 km depth), succeeded by a magnitude 4.5 shock at 04:01:22 (13 km depth) and a magnitude 5.0 event at 04:02:32 (14 km depth). The swarm concluded with a magnitude 5.3 earthquake at 07:59:37 (10 km depth). Such clustered activity is characteristic of swarm sequences, where multiple events occur without a dominant mainshock-aftershock pattern. The region lies within the Ryukyu Islands arc, formed by the subduction of the Philippine Sea Plate beneath the Eurasian Plate along the Ryukyu Trench. This tectonic setting generates frequent seismic activity, including both shallow crustal events and deeper Wadati-Benioff zone earthquakes. Historical records indicate a magnitude 7.0 earthquake on February 26, 2010, located 70 km southeast of Katsuren-haebaru and approximately 11 km from the 2023 swarm center, underscoring the area's potential for significant seismic release. Geological studies of the Ryukyu subduction zone highlight ongoing plate convergence rates of approximately 6–8 cm per year, contributing to the observed seismicity. The swarm's shallow depths align with typical upper-crustal faulting in this convergent margin. No surface rupture or significant secondary effects were associated with the 2023 sequence.

References

SeismoSight internal classification data for swarm PS20230501.1.
USGS Earthquake Catalog for regional historical events.
Japan Meteorological Agency seismic reports for Ryukyu Islands tectonics.