Analysis of Seismic Swarm VS20210823.1 Near Volcano, Hawaii

Seismic swarm VS20210823.1 was recorded 9 km west-southwest of Volcano, Hawaii, on the Big Island. The sequence began at 00:12 on 23 August 2021 and concluded at 07:39 on 30 August 2021, spanning 175 hours and 26 minutes. A total of 859 earthquakes were detected during this period.

The initial 100 events exhibited predominantly low magnitudes, with values ranging from 0.2 to 2.6. The strongest event reached magnitude 2.6 at a depth of 1 km on 23 August 2021 at 15:53:43. Depths remained shallow, concentrated between 0 and 5 km, consistent with activity driven by near-surface processes. Multiple events clustered around magnitudes 0.2–0.7, with occasional peaks near 1.8–2.2, reflecting typical swarm characteristics of frequent, small-magnitude occurrences rather than isolated large shocks.

This swarm aligns with patterns observed in the region since 2000.01.01, during which 73 swarms have occurred. Annual counts show variability, including 3 in 2000, 8 in 2003, 9 in 2018, 13 in 2020, and 5 in 2021 up to the date of this event. Such recurrence underscores ongoing tectonic and volcanic influences in the area.

The location lies within the volcanic terrain associated with Kīlauea, Hawaii’s most active shield volcano. Formed by a mantle hotspot, the Hawaiian Islands feature frequent seismic swarms linked to magma intrusion, dike propagation, and rift-zone adjustments. Shallow depths observed here match the structural setting of the East Rift Zone and adjacent flanks, where brittle failure occurs in response to pressurized magma bodies at depths typically under 5 km. Historical activity includes the major 2018 lower East Rift Zone eruption, which involved extensive seismicity and ground deformation, establishing a baseline for interpreting later swarms as potential indicators of renewed magmatic movement.

No surface eruption accompanied swarm VS20210823.1, suggesting it represented a contained intrusive episode. Continuous monitoring by the Hawaiian Volcano Observatory supports ongoing assessment of such sequences for changes in rate or magnitude that could signal escalation.

References

SeismoSight internal classification records for swarm VS20210823.1.
USGS Hawaiian Volcano Observatory reports on Kīlauea seismic activity.
USGS Earthquake Catalog for regional event verification.