Seismic Swarm PS20050119.1 Offshore Southeast of Katsuura, Japan

On 19 January 2005, a seismic swarm designated PS20050119.1 was recorded approximately 192 km southeast of Katsuura in Chiba Prefecture. The sequence began at 06:11 JST and concluded at 16:38 JST, encompassing six events within a span of roughly ten hours. This activity occurred in a region characterized by ongoing subduction along the boundary between the Philippine Sea Plate and the Eurasian Plate, where convergence rates average 4–5 cm per year and produce frequent moderate-to-large earthquakes.

The individual events unfolded as follows. The initial shock at 06:11:36 registered magnitude 6.6 at a depth of 27 km. Six minutes later, a magnitude 5.3 event occurred at 10 km depth. Following a period of quiescence, a magnitude 0.0 signal was noted at 13:27:02 at 10 km depth. Two closely spaced events followed at 13:51:51 (magnitude 5.3, 2 km depth) and 13:51:53 (magnitude 5.0, 16 km depth). The final event at 16:38:10 reached magnitude 5.5 at 2 km depth. Depths clustered between 2 km and 27 km, consistent with shallow crustal and upper-plate seismicity typical of the offshore Boso region.

Geologically, the swarm location lies east of the Sagami Trough and near the western margin of the Pacific Plate influence. The area experiences both interplate thrust earthquakes and intraplate normal-faulting events driven by slab bending. Historical records indicate elevated swarm activity in this sector, often linked to episodic fluid migration or stress transfer along pre-existing faults. Since 1 January 2000, four such swarms have been documented in the immediate vicinity, with the earliest occurring in 2000. These clusters typically last from several hours to a few days and rarely culminate in larger mainshocks, though they contribute to the cumulative seismic hazard assessment for eastern Honshu.

The 2005 swarm did not produce reported damage onshore, reflecting both its offshore position and the moderate magnitudes involved. Continued monitoring by the Japan Meteorological Agency and regional networks remains essential for distinguishing swarm behavior from foreshock sequences in this tectonically complex setting.

References
SeismoSight internal swarm catalogue PS20050119.1
Japan Meteorological Agency earthquake database (historical swarm statistics 2000–present)
Geological Survey of Japan, tectonic framework of the Sagami Trough region (updated regional synthesis)