Seismic Swarm VS20210624.1: Activity Near Volcano, Hawaii

A seismic swarm designated VS20210624.1 occurred 5 km southwest of Volcano, Hawaii, from 08:17 on 23 June 2021 to 08:29 on 26 June 2021. Over this 72-hour and 12-minute interval, 45 earthquakes were recorded. Event magnitudes ranged from 0.4 to 2.7, with the majority occurring at shallow depths between 0 and 5 km. Two deeper events reached 16 km and 29 km on 24 June.

The swarm began with a cluster of events on 23 June, including multiple magnitude 1.7–2.1 shocks at depths of 0–1 km. Activity continued through 24 June with a steady sequence of low-magnitude events, several at or near the surface. On 25 June, three events of magnitude 1.7–2.1 were noted, followed by the largest shocks of the sequence—a magnitude 2.6 at 7 km depth and a magnitude 2.7 at 5 km depth—on 26 June, marking the swarm’s conclusion.

This episode aligns with the frequent seismic swarms documented in the region since 2000. A total of 70 swarms have been identified in the area during this period. Yearly counts include three in 2000, one in 2001, eight in 2003, four in 2004, one in 2005, two in 2006, four in 2007, one in 2008, four in 2011, four in 2012, one in 2013, two in 2014, three in 2015, two in 2016, two in 2017, nine in 2018, four in 2019, thirteen in 2020, and two in 2021 prior to the current swarm.

The Volcano area lies on the eastern flank of Kilauea volcano within the Island of Hawaii. Kilauea is one of the world’s most active shield volcanoes, built by successive lava flows from a hotspot beneath the Pacific plate. Its summit caldera and rift zones are subject to persistent volcanic-tectonic interactions. Shallow earthquake swarms commonly reflect magma intrusion, pressure changes within the volcanic edifice, or movement along rift-zone faults. Depths recorded in VS20210624.1 are consistent with activity in the upper volcanic crust and shallow magma pathways typical of Kilauea’s south flank and southwest rift zone.

Historical monitoring by the U.S. Geological Survey Hawaiian Volcano Observatory shows that such swarms often precede or accompany changes in eruptive behavior, ground deformation, or gas emissions, although many remain aseismic in terms of surface eruption. The dense clustering of events at depths less than 5 km underscores the shallow nature of stress release in this tectonically active volcanic environment.

References
U.S. Geological Survey, Hawaiian Volcano Observatory – Kilauea activity summaries and seismic catalogs.
SeismoSight internal swarm classification database (VS20210624.1 parameters).