Seismic Swarm PS20231003.1 in the Izu Islands Region: Geological Context and Event Analysis

The Izu Islands, located south of the Japanese mainland along the Izu-Bonin volcanic arc, form a chain of volcanic islands resulting from the subduction of the Pacific Plate beneath the Philippine Sea Plate. This tectonic setting drives frequent seismic and volcanic activity across the region, with the arc extending over 1,000 kilometers and featuring active stratovolcanoes such as Miyakejima and Izu-Oshima. The crust here is relatively thin and influenced by back-arc spreading, contributing to elevated seismicity rates compared to surrounding areas.

Seismic swarms are common in the Izu Islands due to the interplay of tectonic stress accumulation and magmatic intrusions. Historical records document multiple swarms, including those preceding the 2000 Miyakejima eruption and the 1986 Izu-Oshima event, often characterized by clusters of moderate-magnitude earthquakes at shallow depths without a dominant mainshock. These episodes reflect fluid migration or dike propagation rather than simple fault rupture.

Swarm PS20231003.1 initiated at 08:29 on 3 October 2023 and concluded at 13:15 on 6 October 2023, spanning 76 hours and 46 minutes. During this period, 27 earthquakes were recorded, with magnitudes ranging from 4.5 to 6.1 and focal depths predominantly near 10 kilometers, though one event reached 35 kilometers. The sequence included two magnitude 6.1 events on 5 and 6 October, alongside several magnitude 5.8 shocks clustered on 3 October.

Analysis of the temporal distribution reveals peak activity on 3 and 5 October, with events spaced closely in time during afternoon and early morning hours. Depths remained consistent at approximately 10 kilometers for most shocks, suggesting activity within the upper crustal layers influenced by volcanic processes. This pattern aligns with typical swarm behavior in subduction-related arcs, where stress transfer and hydrothermal activity can sustain prolonged sequences.

The Izu Islands region continues to exhibit elevated seismic potential due to ongoing plate convergence at rates of 4–5 centimeters per year. Monitoring by agencies such as the Japan Meteorological Agency supports real-time assessment of such swarms for volcanic hazard evaluation.

References

• Japan Meteorological Agency seismic catalogs
• USGS Earthquake Hazards Program regional tectonics summaries
• Geological Survey of Japan arc volcanism reports