Analysis of Seismic Swarm VS20210904.1 Southwest of Volcano, Hawaii
Seismic swarm VS20210904.1 was recorded 7 km southwest of Volcano, Hawaii, on the Big Island. The sequence began at 14:10 on 3 September 2021 and concluded at 01:53 on 7 September 2021, spanning 83 hours and 43 minutes. During this interval, 37 earthquakes were detected, with magnitudes ranging from 0.2 to 2.7 and focal depths predominantly between 0 and 5 km.
The events clustered in time, with the highest activity on 3 and 4 September. Notable pairs included a magnitude 2.5 event at 1 km depth paired with a magnitude 1.6 at the surface on 3 September, and a magnitude 2.7 at 4 km depth paired with a magnitude 1.8 at 3 km on the same day. Later events on 5 and 6 September showed slightly deeper occurrences up to 5 km, while the final event on 7 September reached magnitude 1.7 at 1 km depth. These shallow, low-magnitude earthquakes are characteristic of volcanic environments where fluid migration or minor stress adjustments occur within the volcanic edifice.
The location lies on the southern flank of Kilauea volcano, part of the Hawaiian hotspot chain. Kilauea remains one of Earth’s most active volcanoes, with its geology shaped by continuous magma supply from a mantle plume beneath the Pacific plate. The area experiences frequent shallow seismicity linked to magma movement, dike intrusions, and gravitational spreading of the volcanic flank. Depths recorded in this swarm align with the shallow crustal levels where such processes commonly take place.
Since 1 January 2000, 74 swarms have been documented in the region. Annual counts vary considerably, with peaks in 2020 (13 swarms), 2018 (9 swarms), and 2003 (8 swarms). Lower activity years include 2001, 2005, 2008, and 2013, each recording only one swarm. The six swarms recorded in 2021 through early September continue this established pattern of episodic unrest.
Such swarms provide valuable data on the dynamic behavior of Kilauea’s south flank. Continued monitoring supports improved understanding of volcanic processes and contributes to hazard assessment for nearby communities and infrastructure.