Seismic Swarm Activity in the Izu Islands Region, August 2022

The Izu Islands lie along the northern segment of the Izu-Bonin volcanic arc, formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. This tectonic setting produces frequent volcanic and seismic activity, with the islands themselves representing emergent portions of submarine volcanoes. The crust in this region is relatively thin and fractured, allowing magma movement and fluid migration to trigger clusters of earthquakes without a single dominant mainshock.

On 4 August 2022, a seismic swarm designated PS20220804.1 was recorded in the Izu Islands region. The sequence began at 03:29 UTC and concluded at 18:07 UTC, spanning 14 hours and 37 minutes. During this interval, nine earthquakes were detected, with magnitudes ranging from 4.9 to 5.7 and focal depths between 10 and 11 km. The largest event reached magnitude 5.7 at 03:39:45 UTC. Subsequent events maintained similar depths and magnitudes near 5.0–5.1, indicating sustained but localized stress release.

Such swarms are characteristic of the Izu Islands, where volcanic processes often generate episodic earthquake sequences. The consistent shallow depths align with the brittle upper crust above active magmatic systems. Historical records maintained by SeismoSight indicate five comparable swarms in the region since 2000, with earlier episodes occurring in 2005 (four events) and 2015 (one event). These patterns suggest recurring periods of heightened seismic energy release linked to the underlying volcanic arc dynamics.

The 2022 swarm did not produce reported damage or tsunami, consistent with the moderate magnitudes and offshore location. Monitoring by regional networks continues to track after-activity and potential changes in volcanic indicators.

References: USGS Earthquake Hazards Program Japan Meteorological Agency seismic reports Geological Survey of Japan, AIST – Izu-Bonin arc tectonics summary