Seismic Swarm PS20150511.1: Activity in the Izu Islands Region

Seismic swarm PS20150511.1 occurred in the Izu Islands region of Japan between 16:54 on 10 May 2015 and 01:12 on 11 May 2015. Over 8 hours and 17 minutes, five earthquakes were recorded. The sequence began with a magnitude 5.3 event at 9 km depth, followed by a magnitude 5.1 shock at 10 km depth. The largest event reached magnitude 6.0 at 6 km depth later that evening. Two additional events of magnitudes 5.0 and 4.5 occurred at depths of 21 km and 10 km, respectively.

Swarm sequences such as this lack a dominant mainshock-aftershock pattern and instead feature closely spaced events of similar size. In tectonically active arcs, these clusters often arise from fluid migration or localized stress changes within the crust. The shallow focal depths observed here align with typical activity along volcanic island chains where magmatic processes influence seismicity.

The Izu Islands form part of the Izu-Bonin volcanic arc, created by subduction of the Pacific Plate beneath the Philippine Sea Plate. This convergent margin generates frequent earthquakes and volcanism. The arc extends southward from the Izu Peninsula on Honshu and includes islands such as Izu-Oshima, Miyakejima, and Hachijojima. Historical records document repeated seismic swarms and eruptions, reflecting ongoing plate convergence at rates of approximately 5–7 cm per year.

Geological mapping shows that the islands consist primarily of basaltic to andesitic volcanic rocks overlying an oceanic crust foundation. The 2015 swarm took place within a corridor of known microseismicity between the islands, where both tectonic and volcanic signals coexist. Depths between 6 km and 21 km correspond to the brittle-ductile transition zone in this setting, where small stress perturbations can trigger multiple events without producing surface rupture.

No damage or tsunami was associated with the 2015 activity, consistent with the moderate magnitudes and offshore locations. Monitoring networks operated by the Japan Meteorological Agency continue to track similar swarms, which serve as indicators of potential volcanic unrest or regional strain accumulation.

References

USGS Earthquake Catalog (events 2015)
Japan Meteorological Agency seismic reports
Geological Survey of Japan, AIST – Izu-Bonin arc tectonics overview