Seismic Swarm PS20040905.1: Analysis of the September 5, 2004 Event Sequence off Shingu, Japan

The seismic swarm designated PS20040905.1 occurred on September 5, 2004, approximately 102 km southeast of Shingu, Japan. The sequence began at 10:07 and concluded at 20:30, encompassing seven earthquakes over a span of 10 hours and 23 minutes. This cluster of events took place within a tectonically active zone characterized by ongoing subduction processes.

The sequence opened with a magnitude 7.2 earthquake at 10:07:07 local time, at a focal depth of 14 km. Subsequent events included two magnitude 5.1 shocks at depths of 10 km each, followed by a magnitude 4.0 event. The swarm reached its peak intensity with a magnitude 7.4 earthquake at 14:57:18, occurring at 10 km depth, accompanied immediately by a magnitude 6.5 event at 15 km depth. Activity tapered with a final magnitude 5.7 shock at 20:30:59, again at 10 km depth. These events illustrate the rapid succession and variable energy release typical of swarm behavior in subduction settings.

The broader region lies along the Nankai Trough, where the Philippine Sea Plate converges with the Eurasian Plate at rates of approximately 4–5 cm per year. This convergence produces frequent moderate to large earthquakes, as well as occasional great megathrust events with magnitudes exceeding 8. Historical records document major ruptures in 1944 (Tonankai) and 1946 (Nankai), both of which generated significant ground shaking and tsunamis along the Kii Peninsula and surrounding coasts. Paleoseismic studies indicate recurrence intervals for such great earthquakes on the order of 100–200 years.

Since 2000, several strong earthquakes have been recorded near the swarm center. These include a magnitude 7.4 event on September 5, 2004, located 118 km east-southeast of Shingu and 17 km from the swarm centroid, as well as a magnitude 7.2 shock on the same day, 94 km southeast of Shingu and 29 km from the center. A magnitude 7.1 earthquake occurred on August 9, 2009, 161 km south of Oyama and 98 km from the swarm location. Such occurrences underscore the persistent seismic hazard in the area.

Swarm activity like PS20040905.1 often reflects fluid migration or stress transfer along fault networks within the overriding or subducting plate. Depths clustered near 10–15 km suggest involvement of the plate interface or upper crustal structures. Continued monitoring by regional networks helps refine models of strain accumulation and release along the trough.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Database
Geological Survey of Japan, AIST reports on Nankai Trough tectonics