Seismic Swarm S20210706.1 Near Volcano, Hawaii

On July 6, 2021, a seismic swarm designated S20210706.1 was recorded 7 km south of Volcano, Hawaii. The sequence began at 00:23 HST and concluded at 22:10 HST, encompassing 30 earthquakes over 21 hours and 47 minutes. Magnitudes ranged from 0.3 to 2.6, with the largest event measuring 2.6 at 00:39 HST. Most events occurred at shallow depths between 0 and 3 km, though two reached 6 km and 9 km.

The swarm exhibited typical characteristics of clustered, low-magnitude seismicity. Early events included a 1.9-magnitude quake at 00:23 and a 2.2-magnitude event at 02:53. Activity continued intermittently through the morning and afternoon, featuring multiple events around 09:00–09:30 HST. Later shocks included a 1.7-magnitude quake at 13:53 and a final 2.4-magnitude event at 22:10. Depths remained predominantly crustal and consistent with volcanic or tectonic stress release in the region.

The location lies on the Big Island of Hawaii within the active volcanic zone associated with Kilauea volcano. Hawaii’s geology is shaped by the Hawaiian hotspot, where magma upwelling from the mantle has built the island chain over millions of years. Kilauea, one of the world’s most active volcanoes, frequently produces earthquakes through magma intrusion, flank movement along the Hilina fault system, and volcanic tremor. Shallow swarms in this area often precede or accompany eruptive phases or dike propagation.

Since 2000, 68 seismic swarms have been documented in the vicinity. Annual counts show variability, with notable increases in 2003 (8 swarms), 2018 (9 swarms), and 2020 (12 swarms). These episodes reflect ongoing tectonic and magmatic processes beneath the volcano’s south flank. Swarms in 2018 coincided with the major lower East Rift Zone eruption, underscoring their role as indicators of subsurface unrest.

Such events provide valuable data for monitoring volcanic hazards. Although individual quakes in S20210706.1 remained below damaging thresholds, repeated swarms highlight the persistent seismic risk in this dynamic environment. Continued observation supports improved forecasting of potential eruptive activity.

References

USGS Hawaiian Volcano Observatory reports on Kilauea seismicity.
Hawaii State Earthquake Catalog (2000–2021).
USGS Professional Paper on Hawaiian hotspot geology.