Seismic Swarm Activity in the Izu Islands, Japan: The October 2023 Event

The Izu Islands region of Japan experienced a notable seismic swarm on October 4, 2023, classified internally as PS20231004.1. The sequence began at 04:55 and concluded at 23:47, encompassing six earthquakes over 18 hours and 51 minutes. All events occurred at shallow depths between 8 and 10 km, with magnitudes ranging from 4.9 to 5.6.

The largest event registered 5.6 at 04:55:47, followed closely by a 5.5 at 05:57:17. Subsequent activity included a 4.9 at 11:00:12 and two 5.0 events at 11:13:20 and 11:13:32. The swarm ended with a 5.1 at 23:47:20. These parameters indicate clustered shallow seismicity typical of the region's tectonic setting.

The Izu Islands form part of the Izu-Bonin volcanic arc, resulting from subduction of the Pacific Plate beneath the Philippine Sea Plate. This convergent margin drives frequent volcanic and seismic episodes, with the arc extending southward from the Izu Peninsula. Historical records document persistent activity linked to both plate boundary deformation and magmatic processes beneath the islands.

Since 2000, three swarms have been recorded in the area under consistent classification criteria: one each in 2015, 2022, and 2023. The 2023 sequence aligns with this pattern of episodic clustering rather than isolated mainshock-aftershock sequences.

Such swarms provide insight into stress accumulation along the subduction interface and potential fluid migration in the crust. Depths consistently near 10 km suggest involvement of the upper plate or shallow volcanic plumbing systems common throughout the Izu chain.

References

USGS Earthquake Hazards Program regional tectonic summaries
Japan Meteorological Agency seismic monitoring archives
Smithsonian Institution Global Volcanism Program Izu-Bonin arc overview