Seismic Swarm S20210927.3: Analysis of Activity Near Volcano, Hawaii

Seismic swarm S20210927.3 occurred 6 km south of Volcano, Hawaii, on the Big Island. The sequence began at 00:39 on 27 September 2021 and concluded at 06:53 on 1 October 2021, spanning 102 hours and 13 minutes. During this period, 168 earthquakes were recorded. This event aligns with patterns of seismicity common in the region, driven by the dynamic volcanic system of Kīlauea.

The swarm's first 100 events showed predominantly shallow depths, with most occurring between 0 and 3 km. Magnitudes ranged from 0.3 to 2.8, indicating low-to-moderate energy release typical of volcanic swarms rather than tectonic mainshock-aftershock sequences. Early activity on 27 September featured several events above magnitude 2.0 at depths of 0–2 km, including a 2.5 magnitude quake at 1 km depth. Subsequent days displayed clustering, with notable bursts around 29 September featuring multiple events near 2.0–2.8 magnitude at shallow levels. Depths remained consistently low, rarely exceeding 6 km, consistent with magma or fluid migration in the upper volcanic edifice. No events exceeded magnitude 3.0 in the initial phase, underscoring the swarm's non-destructive character.

The location places the swarm within the Kīlauea volcano's southern flank, part of Hawaii's active volcanic zone. Kīlauea, one of Earth's most active volcanoes, sits above a mantle hotspot that has shaped the Hawaiian Islands over millions of years. The region's geology features basaltic lava flows, rift zones, and caldera structures. Historical eruptions, including the prolonged 1983–2018 Puʻuʻōʻō episode and the 2018 lower East Rift Zone event, demonstrate ongoing magma supply from depth.

Seismic swarms in this area often precede or accompany volcanic unrest, resulting from pressure changes in the magma plumbing system. Since 2000, 82 swarms have been documented near Volcano, with notable increases in frequency during 2018 (8 swarms), 2020 (14 swarms), and 2021 (10 swarms). These events reflect persistent volcanic processes rather than isolated anomalies.

Monitoring by the Hawaiian Volcano Observatory continues to track such activity through seismic networks, GPS, and gas sensors. The 2021 swarm contributed to understanding short-term unrest patterns without leading to surface eruption.

References

USGS Hawaiian Volcano Observatory reports on Kīlauea seismicity.
Hawaii Volcanoes National Park geological summaries.
SeismoSight internal swarm classification data.