Seismic Swarm in the Izu Islands, Japan: October 8, 2023

On October 8, 2023, a seismic swarm designated PS20231008.1 occurred in the Izu Islands region of Japan. The sequence began at 05:36 UTC and concluded at 21:17 UTC, spanning 15 hours and 40 minutes. During this period, five earthquakes were recorded, with magnitudes ranging from 5.0 to 5.4 and focal depths consistently at 10 km.

The events unfolded as follows: a magnitude 5.2 quake at 05:36, followed by four additional shocks in the evening hours, including a peak magnitude 5.4 event at 21:05. All events clustered tightly in time and space, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

The Izu Islands form part of the Izu-Bonin volcanic arc, resulting from the subduction of the Pacific Plate beneath the Philippine Sea Plate. This tectonic setting produces frequent seismic and volcanic activity along the island chain, which extends southward from the Izu Peninsula. The region has a well-documented history of both tectonic earthquakes and volcanic unrest, with shallow seismicity often linked to magma movement or fluid migration within the crust.

Historical records since 2000 indicate that this swarm represents the sole swarm event identified in the area under current classification criteria. Such episodes highlight the dynamic nature of the arc, where clusters of moderate-magnitude earthquakes can occur without a single dominant shock.

Seismic swarms in subduction-related arcs like the Izu-Bonin system provide valuable data for understanding stress transfer and potential precursors to larger events or volcanic activity. Continued monitoring by regional networks remains essential for assessing ongoing hazards in this tectonically active zone.

References

USGS Earthquake Catalog
Japan Meteorological Agency seismic reports
Geological Survey of Japan, AIST – Tectonic overview of the Izu-Bonin arc