Seismic Swarm PS20150420.1: Analysis of Activity Southwest of Yonakuni, Japan

The seismic swarm designated PS20150420.1 occurred in a tectonically active region 72 km southwest of Yonakuni, Japan. This area lies along the convergent boundary where the Philippine Sea Plate subducts beneath the Eurasian Plate, forming part of the Ryukyu subduction zone. Frequent seismic events result from this plate interaction, which has shaped the regional geology over millions of years through volcanic arc formation and crustal deformation.

The swarm commenced at 01:42 on 20 April 2015 and concluded at 12:23 the same day, spanning 10 hours and 40 minutes. During this interval, five earthquakes were recorded with the following parameters: a magnitude 6.4 event at 01:42:58 and 29 km depth; a magnitude 5.2 event at 01:49:11 and 35 km depth; a magnitude 6.0 event at 11:45:13 and 29 km depth; a magnitude 6.1 event at 12:00:00 and 29 km depth; and a magnitude 4.2 event at 12:23:09 and 28 km depth. Depths clustered between 28 and 35 km, consistent with intermediate-depth seismicity typical of subduction zone environments.

Such swarms reflect episodic stress release along fault systems influenced by plate convergence rates of approximately 5–7 cm per year. The 2015 sequence aligns with patterns observed in prior swarms recorded since 2000, which occurred in 2002, 2004, and 2013. These events underscore the persistent seismic hazard in the region without indicating immediate precursors to larger mainshocks.

Notable regional earthquakes since 2000 include a magnitude 7.1 event on 31 March 2002 located 67 km ENE of Hualien City, Taiwan (42 km from the swarm center), and a magnitude 7.4 event on 2 April 2024 located 15 km south of Hualien City (93 km from the swarm center). These illustrate the broader seismic productivity extending from the Ryukyu arc into northeastern Taiwan.

Geological records show that the Ryukyu Trench and adjacent forearc basins have hosted repeated seismic clusters due to slab dehydration and fluid migration at depth. Monitoring by regional networks continues to track such activity to refine hazard assessments for nearby islands and coastal areas.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Central Weather Administration, Taiwan (cwa.gov.tw)
SeismoSight internal swarm classification data