Seismic Swarm PS20070420.1: Analysis of the April 2007 Event Northwest of Miyakojima

The seismic swarm designated PS20070420.1 occurred approximately 103 km NNW of Hirara on Miyakojima in the Ryukyu Islands, Japan. Registered between 00:26 and 05:23 on 20 April 2007, the sequence comprised eight earthquakes over 4 hours and 56 minutes. All events were shallow, with focal depths between 9 and 11 km, consistent with activity in the upper crust of this tectonically active margin.

The sequence began with a magnitude 6.1 event at 00:26:40, followed rapidly by a 5.7 event at 00:30:59. The largest shock, magnitude 6.3, occurred at 01:45:56. Subsequent events included magnitudes 6.0 at 02:23:34, 4.5 at 02:29:30, 5.3 at 03:00:58, another 5.3 at 03:28:54, and a final 5.5 at 05:23:11. The tight temporal clustering and similar depths indicate a localized stress release along a fault segment within the overriding plate.

This swarm represents the second such episode recorded in the immediate area since 2000, with the prior event occurring in 2002. Swarm activity of this nature is uncommon at this precise location, distinguishing it from the more persistent background seismicity of the broader Ryukyu arc.

Geologically, the region forms part of the Ryukyu subduction zone, where the Philippine Sea Plate converges northwestward beneath the Eurasian Plate at rates of approximately 6–8 cm per year. The resulting compressional regime produces frequent shallow crustal earthquakes, often along normal or strike-slip faults in the forearc. Depths near 10 km align with the brittle-ductile transition zone typical of island-arc crust. Historical records document recurrent moderate-to-large events throughout the Ryukyu chain, reflecting ongoing plate-boundary deformation that has shaped the islands over millions of years.

The 2007 swarm provides insight into episodic strain accumulation and release in a segment of the arc that experiences relatively infrequent clustered activity. Such sequences can occur when fluid migration or aseismic slip perturbs nearby faults, though the precise trigger for this event remains internal to the SeismoSight classification framework.

References

• SeismoSight internal swarm classification database (PS20070420.1 parameters).
• USGS Earthquake Catalog (regional tectonic framework of the Ryukyu arc).